A lot of coaches skim anatomy. Then they give everyone the same exercises and wonder why somebody gets hurt. The rule is simple: before you understand a muscle, you have to understand the skeleton. The skeleton decides how a person can move at all: how long their levers are, how their joints are shaped, how their spine sits. If you can read that from the way a client stands, before they ever touch a barbell, you're ahead of most of the field.
The skeleton has five jobs. It holds the body upright (structure), it makes movement possible (it works as a set of levers the muscles pull on), it protects the organs, it makes blood cells, and it stores minerals (mainly calcium and phosphorus) to keep the body in balance.
Pay attention to this part. On its own, the skeleton produces no force. It's passive. Think of the stiff frame of a slingshot. The force comes from muscle and connective tissue, which tense against that stiff frame and then release (force = mass × acceleration). The skeleton just supplies the levers.
And here's what matters most. Everyone's skeleton is different. Different lever lengths, different bone shapes, different joint angles. Those differences feed straight into performance, health, and how a person looks. That's why there's no single "correct squat for everyone."
An adult has 206 bones, split into two groups:
Axial skeleton, 80 bones: head, neck, and trunk. The central axis of the body. It protects the nervous system and gives the muscles something to attach to. Breathing is driven from the rib cage too, so a problem in the upper back shows up in the breath as well.
Appendicular skeleton, 126 bones: the arms and legs, plus the pelvic and shoulder girdles that connect them to the axis. It's built "looser" than the axis, which is why it has a much bigger range of motion.
The sagittal plane is the vertical plane that splits the body into a left and a right half. Bending the spine forward and back happens in this plane. This is the most important part of the whole lesson.
Every person has their own "neutral" spine. It's the balance point between the muscles at the front and the muscles at the back. But careful: neutral doesn't mean "perfectly straight." In neutral, the skeleton works most efficiently and transfers force best. That's why, when you're lying under a barbell, there's usually a small gap between your lower back and the bench (unless you actually want flexion).
Where that neutral sits is different for everyone. It comes down to the genetics of the bones. Your job is to find what's normal for this person, and above all what's safe. When the spine drifts away from it, the surrounding muscles have to work nonstop just to hold the body upright, and that's where fatigue and pain begin. The pelvis sits underneath all of this like a pendulum. It tilts forward and back to compensate for the spine, so you don't fold like a plank when you hinge.
Pelvic position, the curves of the spine, and how the limbs sit hint at mechanical problems before the first exercise. You look for them in the intake and the basic assessment, and you pick exercises and cues based on what you see.
Exercise cues have to fit the person in front of you. A one-size-fits-all cue like "chest up" can line one person up nicely and push another from too much flexion straight into hyperextension.
Too much mobility is often as big a problem as too little. Hypermobility comes from four things: collagen and protein structure (genetics), the shape of the bone ends, muscle tone (the muscle's resting resistance), and proprioception (your sense of where a joint is and how it's moving).
Long: the limbs, fingers, and toes. They carry weight and act as levers (femur, humerus, tibia). The end is the epiphysis (covered in articular cartilage that cushions impact), and the shaft is the diaphysis.
Short: cube-shaped, about as long as they are wide. They give stability and allow only a little movement (wrist, ankle).
Flat: they protect organs like shields and give muscles a large area to attach to (pelvis, ribs, shoulder blades, sternum). Handy for cueing and for posture.
Irregular: complex shapes that don't fit any other group. They protect organs and serve as attachment points (vertebrae, pelvic bones).
Sesamoid: small bones embedded in tendons where there's a lot of stress and friction. They protect the tendon (the largest one is the kneecap, or patella).
Synarthrodial (fibrous): immovable, held together by connective tissue (the sutures of the skull).
Amphiarthrodial (cartilaginous): allow a small amount of movement in several directions (between the vertebrae, the sacroiliac joint).
Diarthrodial (synovial): a large range of motion, and the most common type (the joints of the arms and legs). They have hyaline cartilage (a near-frictionless surface), a joint capsule filled with synovial fluid (the lubricant), and they're held together by ligaments, connective tissue, and muscle.
The skeleton is passive. It supplies the levers, and the muscles produce the force. Every skeleton is different, so there's no universal technique. A neutral spine isn't a straight spine. And you read a client's sagittal balance before they pick up a weight.
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