## Highlights - This approach to yoga suggests that everything essential for our health and happiness is already present in our systems. We merely need to identify and resolve some of the obstacles that obstruct those natural forces from operating “like a farmer who cuts a dam to allow water to flow into the field where it is needed.” ([Location 269](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=269)) - Rather than view asana practice as a way of imposing order on the human system, we encourage you to use the poses as a way of uncovering the intrinsic harmony that is already there. ([Location 276](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=276)) - Alfred Korzybski, who helped found the field of general semantics, used the phrase the map is not the territory to articulate the idea that a description of an object is not the same as the object itself. ([Location 319](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=319)) - When our joint space is not balanced through the full range of movement and force is not distributed across the articulating surfaces, wear and tear on the hyaline cartilage occurs. Like other tissues in our body, our hyaline cartilage constantly remodels itself and can repair minor wear and tear without long-term consequences. (There are other tissues in our body, such as muscles, that remodel at a faster rate than our hyaline cartilage.) If the imbalance in our joint space is consistent and continuous over a long period of time, our hyaline cartilage cannot repair itself and can eventually become damaged or worn away. If our hyaline cartilage is worn away, the ends of our bones rub against each other. This friction eventually stimulates our bones to grow unevenly, which causes more friction and stress on the bones. This cycle of friction and growth can become quite painful and is one cause of osteoarthritis. ([Location 496](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=496)) - Muscles create movement, joints enable movement, and connective tissue translates movement from tissue to tissue. Bones absorb and transmit movement, and nerves coordinate and organize the whole gorgeous dance. ([Location 767](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=767)) - Muscles work together in complex ways. There’s not a right muscle for any joint action, but a whole assortment of muscles that might participate in a movement. There are many ways to do it well, and the best combination of muscles for one person might be unsuitable for another person. ([Location 769](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=769)) - Muscles are divided into three basic types: skeletal, cardiac, and smooth. Skeletal muscle is generally attached to bones and creates movement at your joints. It has alternating bands of light and dark fibers that give the tissue its striated appearance. Skeletal muscle is controlled by the somatic portion of your nervous system, which makes many of its functions voluntary, or under our conscious control. Cardiac muscle is in your heart, and smooth muscle is in your blood vessels, airways, and visceral organs. Cardiac tissue is also striated but is stimulated by your autonomic nervous system and hormones from your endocrine system. Smooth muscle is not striated and, like cardiac muscle, is stimulated by your autonomic nervous system and your endocrine system. ([Location 784](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=784)) - Myofibrils are made up of thick and thin filaments that lie alongside each other and overlap and are divided into units called sarcomeres. These thick and thin filaments are twisted strands of molecules that create contractions. ([Location 804](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=804)) - The words concentric, eccentric, and isometric are used to describe muscle actions (figure 3.5). These terms actually describe the effects of the relationship between your muscle and the resistance it meets. In a concentric contraction, your muscle fibers contract and generate more force than the resistance that is present. This causes the ends of your muscle to slide toward each other, and the muscle shortens. In an eccentric contraction, your muscle fibers contract and generate less force than the resistance that is present so that the ends of your muscle slide apart, and your muscle lengthens. Your muscle is active as it lengthens, so this is not the same as relaxing your muscle. ([Location 834](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=834)) - In an isometric contraction, your muscle fibers contract and generate the same amount of force as the resistance that is present so that the ends of your muscle neither move apart nor move together, and the length of your muscle does not change. ([Location 843](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=843)) - The places where muscles attach to bones are often classified as being the origin and the insertion of our muscles. The origin is the attachment that is closer to our torso or the center of our body, and the insertion is the attachment that is farther from our center, closer to our fingers, toes, skull, or coccyx. The underlying implication is that the origin is the fixed point and the insertion is the point that moves; however, this is only true for some of our movements. Any time we move our torso through space, we reverse the so-called origin and insertion points. ([Location 909](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=909)) - The starting place is a specific joint, the focal joint, and a specific joint action. For every joint action there are muscles that create the movement and muscles that oppose the movement. The muscles that create the joint action are called your agonists, or prime movers, and the muscles that create the opposite joint action are called your antagonists. ([Location 935](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=935)) - The muscles that support and modulate the action of the agonist or antagonist muscles are called synergistic muscles. Your synergistic muscles also act to minimize excess movement at a joint or in one part of your body to support movement in another part. When synergists act in this way, they are also called fixators. Alternatively, the term synergistic is used to describe a whole group of muscles that work together to create an action. Synergistic muscles are essential for maintaining balanced joint space and for the health of your joint. ([Location 950](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=950)) - The deeper the layer of muscle or muscle tissue, the shorter it is (with a few exceptions).4 The shortest, deepest layers of muscle that cross one joint are called monoarticular, or one-joint muscles. These one-joint muscles perform specific actions and support articulation and discrimination at each joint. They are essential for the integrity and alignment of individual joints. ([Location 967](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=967)) - These longer muscles are called multi-joint muscles if they cross two or more joints. The multi-joint muscles connect all the parts of your limbs, and they integrate your limbs into your torso. They give us the ability to negotiate large shifts of weight and movement of our whole body through space. In the diaphragm, they coordinate sophisticated shape changes in our torso. ([Location 973](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=973)) - Bones support weight; muscles move bones.   There is an enormous difference between how your muscles work when they are moving your bones into place to take weight and how they work when they are attempting to hold the weight themselves. When your muscles take on a weight-bearing function, they can overwork and become rigid and fixed. If your bones bear weight instead, your muscles can stay constantly moving and can continuously make microadjustments to create efficient movement and dynamic stillness along an entire pathway of weight. ([Location 997](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=997)) - Muscles work best when they can calibrate tone.   One basic definition of the word tone is “readiness to respond.” A tissue that has high tone needs less stimulation before a response is elicited because the tissue is more prepared to respond. On the other hand, a tissue with a lower tone needs more stimulation before a response happens. Although it is related, this is not the same thing as sensitivity. A tissue can be sensitive and have low tone. It might register a stimulus at a very fine level but not react until it receives a great deal of that stimulus. Alternatively, a tissue can have high tone and low sensitivity, where it is ready to respond but not actually responding because it isn’t picking up stimulus. All tissues need to be able to change tone in response to changes in the environment, both internal and external. The important thing is not the absolute state of tone but your tissue’s ability to adapt. ([Location 1006](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1006)) - Muscles pull, they don’t push.   In a concentric contraction, the pulling power of your muscle is greater than the resistance. In an eccentric contraction, the pulling power of your muscle is less than the resistance. In an isometric contraction, the pulling power of your muscle is exactly the same as the resistance. In all of these cases the muscle is firing, and the molecules in your myofibrils are ratcheting together to pull. Your muscle is never actively pushing the fibers in a way that slides them apart—that happens because the resistance is greater than the pulling force being generated. So, how is it that we can push something away? Any joint movement includes a part that is lengthening and a part that is shortening. Whether or not your joint is flexing, extending, or rotating, some muscles are lengthening and some are shortening. Your shortening muscles are concentrically contracting; your lengthening muscles are in various degrees of relaxation or are eccentrically contracting. ([Location 1029](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1029)) - In most situations, flexibility is not determined by the actual physical length of your muscle or of your muscle fibers that compose that muscle. The resting length of your muscle, its tone, and the amount it will lengthen are all the result of the communication between your spindle, your central nervous system, and the extrafusal fibers in your muscle. This communication creates patterns in your nervous system based on previous experiences regarding what is appropriate, safe, and functional. ([Location 1042](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1042)) - The strength of a muscle is dependent on its physical properties, including the actual number of muscle fibers. Muscle strength is also a product of the way that your nervous system recruits fibers and organizes your surrounding muscles and kinetic chains. When your nervous system is inefficient in the way it recruits and organizes muscles, it diminishes the functional strength of a muscle by creating a situation in which your muscle has to exert effort to overcome resistance from other muscles in the body. Increasing flexibility and strength is a process of reeducating your nervous system through conscious attention and practice as much as it is about stretching and repetitions. ([Location 1046](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1046)) - In the endocrine system, cells create molecular signals that travel in the bloodstream to carry messages throughout the body. In the immune system, immune cells themselves (and the signaling molecules they create) travel through the body, communicating with different tissues and organs. In the nervous system, the cells have grown into networks that create specific and targeted communication over great distances as well as complex multilayered communication in central areas. ([Location 1133](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1133)) - Synapses are described as having plasticity, which means their signals are strengthened or weakened in response to how much they are used. This plasticity can happen quickly (milliseconds to minutes) or over a more extended time (minutes to hours) and creates positive and negative feedback loops that can encourage or inhibit patterns of activity. The plasticity of our synapses plays an essential role in our ability to learn, change, adapt, and create memories. It is also key to understanding why our responses to the same activity can change over time as well as vary from person to person. ([Location 1158](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1158)) - Glial cells (figure 4.2) are divided into six groups: Astrocytes are star-shaped cells that connect to each other and to neurons, blood vessels, and membranes in the brain and spinal cord. They play many roles in creating and modulating signaling networks in the CNS, including stimulating synapse creation, influencing (stimulating and inhibiting) synaptic activity, providing nourishment to neurons, and regulating blood flow in the brain. Oligodendrocytes wrap around neurons in the brain and spinal cord (the CNS). This wrapping (called myelin) supports the neurons and facilitates the propagation of neuronal signals over long distances. Microglia closely monitor the activity of the brain and spinal cord and provide immune functions such as removing dead and damaged cells and triggering inflammatory responses in the CNS when needed. They are closely related to white blood cells in the PNS. Ependymal cells are cells in the CNS that line the inner surfaces of cavities in the brain and the central canal of the spinal cord to produce and regulate cerebrospinal fluid (and other functions). Neurolemmocytes (Schwann cells) in the peripheral nervous system (PNS) wrap around the long parts (axons) of neurons (like the oligodendrocytes in the CNS). In some cases, these wrappings form myelin sheaths that help neuronal signals travel more quickly, efficiently, and predictably. In the PNS, neurolemmocytes also play a role in healing nerve damage. ([Location 1168](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1168)) - Satellite cells are also located in the PNS. They cover the surfaces of neuronal cell bodies in ganglia (groups of cell bodies). They protect and provide nutrients to the neurons and play roles in modulating the communication that happens in the ganglia (like astrocytes in the CNS). ([Location 1186](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1186)) - There are three types of neurons: Sensory neurons carry messages about sensory stimuli from our tissues and sensory organs to the brain and spinal cord (CNS). These are also called afferent neurons because the direction the message travels is toward the CNS. Motor neurons carry messages about what actions to take. These messages travel from the brain and spinal cord to the effector organs (mainly muscles and glands). These are also called efferent neurons because the direction the message travels is away from the CNS. Interneurons carry messages between neurons; there are more interneurons than sensory and motor neurons, and they are all located in the central nervous system. Interneurons and glial cells create neural circuits and neural networks between sensory neurons, motor neurons, other interneurons, and other glial cells to support complex processes such as memory and learning. ([Location 1194](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1194)) - All the cells of our nervous system work together to pick up sensory input, process it, and generate a motoric response. Each motor response creates new experiences to sense, which leads to more processing and more responses. This cycle is called a sensory–motor loop; it happens constantly and continually in order to make the precise adjustments needed for us to survive, adapt, learn, and grow throughout our lives. ([Location 1209](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1209)) - Exteroceptors4 are sensory neurons that are stimulated by input that comes from outside your body. These include the following: Photoreceptors in the eyes Mechanoreceptors in the ears and vestibular mechanism Chemoreceptors in the nose and mouth Thermoreceptors in the skin Mechanoreceptors for touch and pressure in the skin ([Location 1222](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1222)) - Interoceptors are sensory neurons that are stimulated by input that comes from inside your body, specifically the tissues not involved in volitional movement. These include mechanoreceptors, chemoreceptors, and thermoreceptors in our visceral organs and blood vessels. Proprioceptors are sensory neurons that are stimulated by input created by our own volitional movement or potentially volitional movement, including movements that might be done unconsciously, such as balancing. Proprioceptive input comes from the mechanoreceptors in our skeletal muscles and tendons and the joint capsule and collateral ligaments. ([Location 1229](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1229)) - Thermoreceptors are stimulated by changes in temperature and include sensory neurons with receptors in our skin and blood vessels. Chemoreceptors are stimulated by chemical changes both outside and inside our bodies. These include sensory neurons with receptors for the following: Odor (in the nose) Taste (in the mouth and lining of the gut) CO2 levels (in the walls of the blood vessels in the aorta and carotid arteries) Hormones (in the brain stem) Photoreceptors are stimulated by light and include receptors in our eyes. Nociceptors (pain receptors) are stimulated by intense and potentially damaging stimuli (pain) that might be mechanically, chemically, or thermally induced. It’s proposed that these sensory neurons can be found in many different tissues. ([Location 1251](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1251)) - The somatic nervous system (SNS) creates responses in our musculoskeletal system, specifically in the striated muscles that we use to move, breathe, and act in the world. The SNS is what we use to do actions such as stepping one leg back or opening our mouth or flinging our arms out when we lose our balance. The autonomic nervous system (ANS) creates responses in the smooth muscle of our visceral organs, the cardiac muscle of our heart, our blood vessels,7 adipose tissue, and glands. The ANS increases or decreases activity in our glands and organs in patterns that are called sympathetic responses and parasympathetic responses, and most of our glands and organs receive messages from both sympathetic and parasympathetic motor neurons (figure 4.4). Sympathetic responses increase our alertness and readiness to respond to events in the outside world by increasing our heart rate and blood flow to our skeletal muscles and brain or by slowing activity in our digestive system. Sympathetic responses are often characterized as a whole-body, fight-or-flight response, but this is only an extreme expression of sympathetic activity. In nonthreatening day-to-day activities, sympathetic responses can be modulated, discrete, and localized. Parasympathetic responses increase internal activities related to digestion, homeostasis, growth, and healing by increasing peristalsis, glandular activity, and blood flow to our digestive system or by slowing the heart rate. Often characterized as being about resting and digesting, our parasympathetic responses are not simply the absence of activity but are also active motor responses oriented to the internal environment rather than the outer world.8 ([Location 1306](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1306)) - The enteric nervous system (ENS) is sometimes called our second brain because it has a network of enteric ganglia where sensory neurons, glial cells, and motor neurons communicate and where sensory input leads to processing and motor responses without traveling through the CNS. The ENS creates responses specifically in the smooth muscles, glands, and endocrine cells of the digestive tract and allows parts of our digestive system to work without interaction with the brain and spinal cord. ([Location 1329](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1329)) - We have a variety of ways to look at the nervous system: By location: central nervous system, peripheral nervous system, or enteric nervous system By function: sensing, processing, and motoring By results: somatic, autonomic (including sympathetic and parasympathetic), or enteric ([Location 1352](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1352)) - In the language of yoga, the stable boundary is sthira, and the open space is sukha. In Sanskrit, sthira can mean firm, hard, solid, compact, strong, unfluctuating, durable, lasting, or permanent. Sukha is composed of two roots: su meaning good and kha meaning space. It means easy, pleasant, agreeable, gentle, and mild. It also refers to a state of well-being, free of obstacles. All successful living things must balance containment and permeability, rigidity and plasticity, persistence and adaptability, space and boundaries. Successful human-made structures also exhibit a balance of sthira and sukha. For example, a suspension bridge is flexible enough to survive wind and earthquakes, but stable enough to support its load-bearing surfaces. ([Location 1451](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1451)) - PRANA, AGNI, AND APANA ([Location 2121](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2121)) - The Sanskrit term prana is derived from pra-, a prefix meaning “before,” and an, a verb meaning “to breathe, to blow, and to live.” Prana refers to what nourishes a living thing, but it has also come to mean the action that brings in the nourishment. Within this chapter, the term will refer to the functional life processes of a single entity. When capitalized, Prana is a more universal term that can be used to designate the manifestation of all generative life force. ([Location 2122](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2122)) - The raw materials that enter a living thing must be processed and metabolized, and that faculty is the domain of agni,1 fire. Within our body, agni is associated with the digestive fire, and in general with our ability to metabolize and assimilate anything that can nourish us on any level. ([Location 2126](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2126)) - Fire produces ash, and metabolism produces waste. Apana, which is derived from apa, meaning “away,” “off,” or “down,” refers to the waste that’s being eliminated as well as the action of elimination. ([Location 2129](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2129)) - Essentially, prana is about what brings raw materials into the system, agni is about turning those raw materials into nourishment, and apana is about eliminating whatever is unneeded. ([Location 2131](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2131)) - Our abdominal cavity changes shape like a flexible, fluid-filled structure such as a water balloon. When you squeeze one end of a water balloon, the other end bulges (figure 6.6). That is because water is noncompressible. ([Location 2234](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2234)) - In contrast to our abdominal cavity, our thoracic cavity changes both shape and volume; it behaves as a flexible gas-filled container, similar to an accordion bellows. When you squeeze an accordion, you create a reduction in the volume of the bellows, and air is forced out. When you pull the bellows open, its volume increases and air is pulled in ([Location 2242](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2242)) - Let’s now imagine our thoracic and abdominal cavities as an accordion stacked on top of a water balloon. This image gives a sense of the relationship of the two cavities in breathing; movement in one will necessarily result in movement in the other. ([Location 2252](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2252)) - Volume and pressure are inversely related; when volume increases, pressure decreases, and when volume decreases, pressure increases. Because air always flows toward areas of lower pressure, increasing the volume inside your thoracic cavity will decrease pressure and cause air to flow into it. This is an inhalation. It is important to note that despite how it feels when you inhale, you do not actually pull air into your body. On the contrary, air is pushed into your body by the sea of atmospheric pressure5 that always surrounds you. This means that the actual force that moves air into your lungs is outside of your body. The energy you expend in breathing produces a shape change that lowers the pressure in your chest cavity and permits the air to be pushed into your body by the weight of the planet’s atmosphere. In other words, you create the space, and the universe fills it. ([Location 2260](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2260)) - Breathing, the process of taking air into and expelling it from our lungs, is caused by a three-dimensional shape change in our thoracic and abdominal cavities. ([Location 2300](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2300)) - Our diaphragm is the principal muscle that causes three-dimensional shape change in our thoracic and abdominal cavities. ([Location 2316](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2316)) - The lower edges of our diaphragm’s fibers attach at four distinct regions7 (see figure 6.12): Sternal—The back of the xiphoid process at the bottom of our sternum Costal—The inner costal cartilage surfaces of ribs 6 through 10 Arcuate—The arcuate ligament8 that runs from the 10th rib’s cartilage to our lumbar spine, attaching along the way to the floating ribs (11 and 12) and the transverse process and body of L1 Lumbar—The crura (Latin for legs) at the front of our lumbar spine, L3 on right and L2 on left ([Location 2345](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2345)) - unlike other muscles, our diaphragm is associated with our thoracic and abdominal organs via their surrounding connective tissues. This is what is meant by the term organic relations. ([Location 2372](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2372)) - Pleura, which surrounds our lungs Pericardium, which surrounds our heart Peritoneum, which surrounds most of our abdominal organs ([Location 2383](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2383)) - A useful description of asana that Amy uses is “a container for an experience.” Asana is a form that we inhabit for a moment, a shape that we move into and out of, a place where we might choose to pause and pay attention differently in the continuously flowing movement of life. From this perspective, an asana is not an exercise for strengthening or stretching a particular muscle or muscle group, although it might have that effect. ([Location 3038](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=3038)) - There really aren’t dangerous asana (or safe asana), just dangerous (or safe) ways of doing them. Any asana can be done safely or dangerously depending on how it’s taught, how it’s modified, the student’s experience and skill, and the movement potential of each person. ([Location 3070](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=3070)) - Alignment is relational. Alignment is a relational term, not an absolute concept that exists without reference to something else. It describes a relationship with something, so we have to ask “Alignment with what?” ([Location 3077](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=3077)) - No single alignment instruction will work for every person’s body. Standardized alignment instructions are often offered as ways to safely do an asana. One single instruction cannot cover all the ways that people can move into and out of an asana, and what is a helpful instruction for one person might be what injures someone else if it’s based on the assumption that our bodies are all the same. ([Location 3088](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=3088)) - The physiological lock in your knee helps the bones line up with each other for the clearest pathway of weight. Hyperextending your knee inhibits the physiological lock, potentially puts more pressure on the ligaments of your knee, allows standing without much muscular activity in your legs, and usually encourages an anterior tilt of your pelvis. Although engaging the muscles around your knee might protect it or make it more stable, engaging the muscles around your knee as much as possible might create problematic patterns of overuse in those leg muscles. ([Location 3650](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=3650))  ## Highlights - This approach to yoga suggests that everything essential for our health and happiness is already present in our systems. We merely need to identify and resolve some of the obstacles that obstruct those natural forces from operating “like a farmer who cuts a dam to allow water to flow into the field where it is needed.” ([Location 269](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=269)) - Rather than view asana practice as a way of imposing order on the human system, we encourage you to use the poses as a way of uncovering the intrinsic harmony that is already there. ([Location 276](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=276)) - Alfred Korzybski, who helped found the field of general semantics, used the phrase the map is not the territory to articulate the idea that a description of an object is not the same as the object itself. ([Location 319](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=319)) - When our joint space is not balanced through the full range of movement and force is not distributed across the articulating surfaces, wear and tear on the hyaline cartilage occurs. Like other tissues in our body, our hyaline cartilage constantly remodels itself and can repair minor wear and tear without long-term consequences. (There are other tissues in our body, such as muscles, that remodel at a faster rate than our hyaline cartilage.) If the imbalance in our joint space is consistent and continuous over a long period of time, our hyaline cartilage cannot repair itself and can eventually become damaged or worn away. If our hyaline cartilage is worn away, the ends of our bones rub against each other. This friction eventually stimulates our bones to grow unevenly, which causes more friction and stress on the bones. This cycle of friction and growth can become quite painful and is one cause of osteoarthritis. ([Location 496](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=496)) - Muscles create movement, joints enable movement, and connective tissue translates movement from tissue to tissue. Bones absorb and transmit movement, and nerves coordinate and organize the whole gorgeous dance. ([Location 767](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=767)) - Muscles work together in complex ways. There’s not a right muscle for any joint action, but a whole assortment of muscles that might participate in a movement. There are many ways to do it well, and the best combination of muscles for one person might be unsuitable for another person. ([Location 769](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=769)) - Muscles are divided into three basic types: skeletal, cardiac, and smooth. Skeletal muscle is generally attached to bones and creates movement at your joints. It has alternating bands of light and dark fibers that give the tissue its striated appearance. Skeletal muscle is controlled by the somatic portion of your nervous system, which makes many of its functions voluntary, or under our conscious control. Cardiac muscle is in your heart, and smooth muscle is in your blood vessels, airways, and visceral organs. Cardiac tissue is also striated but is stimulated by your autonomic nervous system and hormones from your endocrine system. Smooth muscle is not striated and, like cardiac muscle, is stimulated by your autonomic nervous system and your endocrine system. ([Location 784](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=784)) - Myofibrils are made up of thick and thin filaments that lie alongside each other and overlap and are divided into units called sarcomeres. These thick and thin filaments are twisted strands of molecules that create contractions. ([Location 804](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=804)) - The words concentric, eccentric, and isometric are used to describe muscle actions (figure 3.5). These terms actually describe the effects of the relationship between your muscle and the resistance it meets. In a concentric contraction, your muscle fibers contract and generate more force than the resistance that is present. This causes the ends of your muscle to slide toward each other, and the muscle shortens. In an eccentric contraction, your muscle fibers contract and generate less force than the resistance that is present so that the ends of your muscle slide apart, and your muscle lengthens. Your muscle is active as it lengthens, so this is not the same as relaxing your muscle. ([Location 834](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=834)) - In an isometric contraction, your muscle fibers contract and generate the same amount of force as the resistance that is present so that the ends of your muscle neither move apart nor move together, and the length of your muscle does not change. ([Location 843](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=843)) - The places where muscles attach to bones are often classified as being the origin and the insertion of our muscles. The origin is the attachment that is closer to our torso or the center of our body, and the insertion is the attachment that is farther from our center, closer to our fingers, toes, skull, or coccyx. The underlying implication is that the origin is the fixed point and the insertion is the point that moves; however, this is only true for some of our movements. Any time we move our torso through space, we reverse the so-called origin and insertion points. ([Location 909](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=909)) - The starting place is a specific joint, the focal joint, and a specific joint action. For every joint action there are muscles that create the movement and muscles that oppose the movement. The muscles that create the joint action are called your agonists, or prime movers, and the muscles that create the opposite joint action are called your antagonists. ([Location 935](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=935)) - The muscles that support and modulate the action of the agonist or antagonist muscles are called synergistic muscles. Your synergistic muscles also act to minimize excess movement at a joint or in one part of your body to support movement in another part. When synergists act in this way, they are also called fixators. Alternatively, the term synergistic is used to describe a whole group of muscles that work together to create an action. Synergistic muscles are essential for maintaining balanced joint space and for the health of your joint. ([Location 950](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=950)) - The deeper the layer of muscle or muscle tissue, the shorter it is (with a few exceptions).4 The shortest, deepest layers of muscle that cross one joint are called monoarticular, or one-joint muscles. These one-joint muscles perform specific actions and support articulation and discrimination at each joint. They are essential for the integrity and alignment of individual joints. ([Location 967](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=967)) - These longer muscles are called multi-joint muscles if they cross two or more joints. The multi-joint muscles connect all the parts of your limbs, and they integrate your limbs into your torso. They give us the ability to negotiate large shifts of weight and movement of our whole body through space. In the diaphragm, they coordinate sophisticated shape changes in our torso. ([Location 973](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=973)) - Bones support weight; muscles move bones.   There is an enormous difference between how your muscles work when they are moving your bones into place to take weight and how they work when they are attempting to hold the weight themselves. When your muscles take on a weight-bearing function, they can overwork and become rigid and fixed. If your bones bear weight instead, your muscles can stay constantly moving and can continuously make microadjustments to create efficient movement and dynamic stillness along an entire pathway of weight. ([Location 997](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=997)) - Muscles work best when they can calibrate tone.   One basic definition of the word tone is “readiness to respond.” A tissue that has high tone needs less stimulation before a response is elicited because the tissue is more prepared to respond. On the other hand, a tissue with a lower tone needs more stimulation before a response happens. Although it is related, this is not the same thing as sensitivity. A tissue can be sensitive and have low tone. It might register a stimulus at a very fine level but not react until it receives a great deal of that stimulus. Alternatively, a tissue can have high tone and low sensitivity, where it is ready to respond but not actually responding because it isn’t picking up stimulus. All tissues need to be able to change tone in response to changes in the environment, both internal and external. The important thing is not the absolute state of tone but your tissue’s ability to adapt. ([Location 1006](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1006)) - Muscles pull, they don’t push.   In a concentric contraction, the pulling power of your muscle is greater than the resistance. In an eccentric contraction, the pulling power of your muscle is less than the resistance. In an isometric contraction, the pulling power of your muscle is exactly the same as the resistance. In all of these cases the muscle is firing, and the molecules in your myofibrils are ratcheting together to pull. Your muscle is never actively pushing the fibers in a way that slides them apart—that happens because the resistance is greater than the pulling force being generated. So, how is it that we can push something away? Any joint movement includes a part that is lengthening and a part that is shortening. Whether or not your joint is flexing, extending, or rotating, some muscles are lengthening and some are shortening. Your shortening muscles are concentrically contracting; your lengthening muscles are in various degrees of relaxation or are eccentrically contracting. ([Location 1029](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1029)) - In most situations, flexibility is not determined by the actual physical length of your muscle or of your muscle fibers that compose that muscle. The resting length of your muscle, its tone, and the amount it will lengthen are all the result of the communication between your spindle, your central nervous system, and the extrafusal fibers in your muscle. This communication creates patterns in your nervous system based on previous experiences regarding what is appropriate, safe, and functional. ([Location 1042](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1042)) - The strength of a muscle is dependent on its physical properties, including the actual number of muscle fibers. Muscle strength is also a product of the way that your nervous system recruits fibers and organizes your surrounding muscles and kinetic chains. When your nervous system is inefficient in the way it recruits and organizes muscles, it diminishes the functional strength of a muscle by creating a situation in which your muscle has to exert effort to overcome resistance from other muscles in the body. Increasing flexibility and strength is a process of reeducating your nervous system through conscious attention and practice as much as it is about stretching and repetitions. ([Location 1046](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1046)) - In the endocrine system, cells create molecular signals that travel in the bloodstream to carry messages throughout the body. In the immune system, immune cells themselves (and the signaling molecules they create) travel through the body, communicating with different tissues and organs. In the nervous system, the cells have grown into networks that create specific and targeted communication over great distances as well as complex multilayered communication in central areas. ([Location 1133](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1133)) - Synapses are described as having plasticity, which means their signals are strengthened or weakened in response to how much they are used. This plasticity can happen quickly (milliseconds to minutes) or over a more extended time (minutes to hours) and creates positive and negative feedback loops that can encourage or inhibit patterns of activity. The plasticity of our synapses plays an essential role in our ability to learn, change, adapt, and create memories. It is also key to understanding why our responses to the same activity can change over time as well as vary from person to person. ([Location 1158](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1158)) - Glial cells (figure 4.2) are divided into six groups: Astrocytes are star-shaped cells that connect to each other and to neurons, blood vessels, and membranes in the brain and spinal cord. They play many roles in creating and modulating signaling networks in the CNS, including stimulating synapse creation, influencing (stimulating and inhibiting) synaptic activity, providing nourishment to neurons, and regulating blood flow in the brain. Oligodendrocytes wrap around neurons in the brain and spinal cord (the CNS). This wrapping (called myelin) supports the neurons and facilitates the propagation of neuronal signals over long distances. Microglia closely monitor the activity of the brain and spinal cord and provide immune functions such as removing dead and damaged cells and triggering inflammatory responses in the CNS when needed. They are closely related to white blood cells in the PNS. Ependymal cells are cells in the CNS that line the inner surfaces of cavities in the brain and the central canal of the spinal cord to produce and regulate cerebrospinal fluid (and other functions). Neurolemmocytes (Schwann cells) in the peripheral nervous system (PNS) wrap around the long parts (axons) of neurons (like the oligodendrocytes in the CNS). In some cases, these wrappings form myelin sheaths that help neuronal signals travel more quickly, efficiently, and predictably. In the PNS, neurolemmocytes also play a role in healing nerve damage. ([Location 1168](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1168)) - Satellite cells are also located in the PNS. They cover the surfaces of neuronal cell bodies in ganglia (groups of cell bodies). They protect and provide nutrients to the neurons and play roles in modulating the communication that happens in the ganglia (like astrocytes in the CNS). ([Location 1186](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1186)) - There are three types of neurons: Sensory neurons carry messages about sensory stimuli from our tissues and sensory organs to the brain and spinal cord (CNS). These are also called afferent neurons because the direction the message travels is toward the CNS. Motor neurons carry messages about what actions to take. These messages travel from the brain and spinal cord to the effector organs (mainly muscles and glands). These are also called efferent neurons because the direction the message travels is away from the CNS. Interneurons carry messages between neurons; there are more interneurons than sensory and motor neurons, and they are all located in the central nervous system. Interneurons and glial cells create neural circuits and neural networks between sensory neurons, motor neurons, other interneurons, and other glial cells to support complex processes such as memory and learning. ([Location 1194](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1194)) - All the cells of our nervous system work together to pick up sensory input, process it, and generate a motoric response. Each motor response creates new experiences to sense, which leads to more processing and more responses. This cycle is called a sensory–motor loop; it happens constantly and continually in order to make the precise adjustments needed for us to survive, adapt, learn, and grow throughout our lives. ([Location 1209](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1209)) - Exteroceptors4 are sensory neurons that are stimulated by input that comes from outside your body. These include the following: Photoreceptors in the eyes Mechanoreceptors in the ears and vestibular mechanism Chemoreceptors in the nose and mouth Thermoreceptors in the skin Mechanoreceptors for touch and pressure in the skin ([Location 1222](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1222)) - Interoceptors are sensory neurons that are stimulated by input that comes from inside your body, specifically the tissues not involved in volitional movement. These include mechanoreceptors, chemoreceptors, and thermoreceptors in our visceral organs and blood vessels. Proprioceptors are sensory neurons that are stimulated by input created by our own volitional movement or potentially volitional movement, including movements that might be done unconsciously, such as balancing. Proprioceptive input comes from the mechanoreceptors in our skeletal muscles and tendons and the joint capsule and collateral ligaments. ([Location 1229](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1229)) - Thermoreceptors are stimulated by changes in temperature and include sensory neurons with receptors in our skin and blood vessels. Chemoreceptors are stimulated by chemical changes both outside and inside our bodies. These include sensory neurons with receptors for the following: Odor (in the nose) Taste (in the mouth and lining of the gut) CO2 levels (in the walls of the blood vessels in the aorta and carotid arteries) Hormones (in the brain stem) Photoreceptors are stimulated by light and include receptors in our eyes. Nociceptors (pain receptors) are stimulated by intense and potentially damaging stimuli (pain) that might be mechanically, chemically, or thermally induced. It’s proposed that these sensory neurons can be found in many different tissues. ([Location 1251](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1251)) - The somatic nervous system (SNS) creates responses in our musculoskeletal system, specifically in the striated muscles that we use to move, breathe, and act in the world. The SNS is what we use to do actions such as stepping one leg back or opening our mouth or flinging our arms out when we lose our balance. The autonomic nervous system (ANS) creates responses in the smooth muscle of our visceral organs, the cardiac muscle of our heart, our blood vessels,7 adipose tissue, and glands. The ANS increases or decreases activity in our glands and organs in patterns that are called sympathetic responses and parasympathetic responses, and most of our glands and organs receive messages from both sympathetic and parasympathetic motor neurons (figure 4.4). Sympathetic responses increase our alertness and readiness to respond to events in the outside world by increasing our heart rate and blood flow to our skeletal muscles and brain or by slowing activity in our digestive system. Sympathetic responses are often characterized as a whole-body, fight-or-flight response, but this is only an extreme expression of sympathetic activity. In nonthreatening day-to-day activities, sympathetic responses can be modulated, discrete, and localized. Parasympathetic responses increase internal activities related to digestion, homeostasis, growth, and healing by increasing peristalsis, glandular activity, and blood flow to our digestive system or by slowing the heart rate. Often characterized as being about resting and digesting, our parasympathetic responses are not simply the absence of activity but are also active motor responses oriented to the internal environment rather than the outer world.8 ([Location 1306](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1306)) - The enteric nervous system (ENS) is sometimes called our second brain because it has a network of enteric ganglia where sensory neurons, glial cells, and motor neurons communicate and where sensory input leads to processing and motor responses without traveling through the CNS. The ENS creates responses specifically in the smooth muscles, glands, and endocrine cells of the digestive tract and allows parts of our digestive system to work without interaction with the brain and spinal cord. ([Location 1329](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1329)) - We have a variety of ways to look at the nervous system: By location: central nervous system, peripheral nervous system, or enteric nervous system By function: sensing, processing, and motoring By results: somatic, autonomic (including sympathetic and parasympathetic), or enteric ([Location 1352](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1352)) - In the language of yoga, the stable boundary is sthira, and the open space is sukha. In Sanskrit, sthira can mean firm, hard, solid, compact, strong, unfluctuating, durable, lasting, or permanent. Sukha is composed of two roots: su meaning good and kha meaning space. It means easy, pleasant, agreeable, gentle, and mild. It also refers to a state of well-being, free of obstacles. All successful living things must balance containment and permeability, rigidity and plasticity, persistence and adaptability, space and boundaries. Successful human-made structures also exhibit a balance of sthira and sukha. For example, a suspension bridge is flexible enough to survive wind and earthquakes, but stable enough to support its load-bearing surfaces. ([Location 1451](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=1451)) - PRANA, AGNI, AND APANA ([Location 2121](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2121)) - The Sanskrit term prana is derived from pra-, a prefix meaning “before,” and an, a verb meaning “to breathe, to blow, and to live.” Prana refers to what nourishes a living thing, but it has also come to mean the action that brings in the nourishment. Within this chapter, the term will refer to the functional life processes of a single entity. When capitalized, Prana is a more universal term that can be used to designate the manifestation of all generative life force. ([Location 2122](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2122)) - The raw materials that enter a living thing must be processed and metabolized, and that faculty is the domain of agni,1 fire. Within our body, agni is associated with the digestive fire, and in general with our ability to metabolize and assimilate anything that can nourish us on any level. ([Location 2126](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2126)) - Fire produces ash, and metabolism produces waste. Apana, which is derived from apa, meaning “away,” “off,” or “down,” refers to the waste that’s being eliminated as well as the action of elimination. ([Location 2129](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2129)) - Essentially, prana is about what brings raw materials into the system, agni is about turning those raw materials into nourishment, and apana is about eliminating whatever is unneeded. ([Location 2131](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2131)) - Our abdominal cavity changes shape like a flexible, fluid-filled structure such as a water balloon. When you squeeze one end of a water balloon, the other end bulges (figure 6.6). That is because water is noncompressible. ([Location 2234](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2234)) - In contrast to our abdominal cavity, our thoracic cavity changes both shape and volume; it behaves as a flexible gas-filled container, similar to an accordion bellows. When you squeeze an accordion, you create a reduction in the volume of the bellows, and air is forced out. When you pull the bellows open, its volume increases and air is pulled in ([Location 2242](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2242)) - Let’s now imagine our thoracic and abdominal cavities as an accordion stacked on top of a water balloon. This image gives a sense of the relationship of the two cavities in breathing; movement in one will necessarily result in movement in the other. ([Location 2252](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2252)) - Volume and pressure are inversely related; when volume increases, pressure decreases, and when volume decreases, pressure increases. Because air always flows toward areas of lower pressure, increasing the volume inside your thoracic cavity will decrease pressure and cause air to flow into it. This is an inhalation. It is important to note that despite how it feels when you inhale, you do not actually pull air into your body. On the contrary, air is pushed into your body by the sea of atmospheric pressure5 that always surrounds you. This means that the actual force that moves air into your lungs is outside of your body. The energy you expend in breathing produces a shape change that lowers the pressure in your chest cavity and permits the air to be pushed into your body by the weight of the planet’s atmosphere. In other words, you create the space, and the universe fills it. ([Location 2260](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2260)) - Breathing, the process of taking air into and expelling it from our lungs, is caused by a three-dimensional shape change in our thoracic and abdominal cavities. ([Location 2300](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2300)) - Our diaphragm is the principal muscle that causes three-dimensional shape change in our thoracic and abdominal cavities. ([Location 2316](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2316)) - The lower edges of our diaphragm’s fibers attach at four distinct regions7 (see figure 6.12): Sternal—The back of the xiphoid process at the bottom of our sternum Costal—The inner costal cartilage surfaces of ribs 6 through 10 Arcuate—The arcuate ligament8 that runs from the 10th rib’s cartilage to our lumbar spine, attaching along the way to the floating ribs (11 and 12) and the transverse process and body of L1 Lumbar—The crura (Latin for legs) at the front of our lumbar spine, L3 on right and L2 on left ([Location 2345](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2345)) - unlike other muscles, our diaphragm is associated with our thoracic and abdominal organs via their surrounding connective tissues. This is what is meant by the term organic relations. ([Location 2372](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2372)) - Pleura, which surrounds our lungs Pericardium, which surrounds our heart Peritoneum, which surrounds most of our abdominal organs ([Location 2383](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=2383)) - A useful description of asana that Amy uses is “a container for an experience.” Asana is a form that we inhabit for a moment, a shape that we move into and out of, a place where we might choose to pause and pay attention differently in the continuously flowing movement of life. From this perspective, an asana is not an exercise for strengthening or stretching a particular muscle or muscle group, although it might have that effect. ([Location 3038](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=3038)) - There really aren’t dangerous asana (or safe asana), just dangerous (or safe) ways of doing them. Any asana can be done safely or dangerously depending on how it’s taught, how it’s modified, the student’s experience and skill, and the movement potential of each person. ([Location 3070](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=3070)) - Alignment is relational. Alignment is a relational term, not an absolute concept that exists without reference to something else. It describes a relationship with something, so we have to ask “Alignment with what?” ([Location 3077](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=3077)) - No single alignment instruction will work for every person’s body. Standardized alignment instructions are often offered as ways to safely do an asana. One single instruction cannot cover all the ways that people can move into and out of an asana, and what is a helpful instruction for one person might be what injures someone else if it’s based on the assumption that our bodies are all the same. ([Location 3088](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=3088)) - The physiological lock in your knee helps the bones line up with each other for the clearest pathway of weight. Hyperextending your knee inhibits the physiological lock, potentially puts more pressure on the ligaments of your knee, allows standing without much muscular activity in your legs, and usually encourages an anterior tilt of your pelvis. Although engaging the muscles around your knee might protect it or make it more stable, engaging the muscles around your knee as much as possible might create problematic patterns of overuse in those leg muscles. ([Location 3650](https://readwise.io/to_kindle?action=open&asin=B0998616FH&location=3650))