The muscles

The twenty-two muscles

• Pectoralis major
  The pectoralis major covers the front of the chest and draws the arm toward the midline. It is the main driver of horizontal pushing and contributes to internal rotation of the shoulder.
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• Deltoids
  The deltoid caps the shoulder with three heads pointing in different directions. It raises the arm in every direction, and its shape explains why the same shoulder changes function depending on the angle of the movement.
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• Triceps brachii
  The triceps brachii fills the whole back of the upper arm and extends the elbow. Its long head also crosses the shoulder, which changes its length depending on the position of the arm.
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• Serratus anterior
  Serratus anterior links the ribs to the inner border of the shoulder blade and holds it flat against the ribcage. Without it the blade wings out and the arm cannot fully rise.
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• Latissimus dorsi
  The latissimus dorsi is the broadest muscle of the back. It links the pelvis and the spine to the humerus, and draws the arm down and backward in every pulling movement.
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• Trapezius
  The trapezius links the skull and the spine to the shoulder girdle. Its three portions orient the shoulder blade, a condition without which the arm cannot rise fully.
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• Biceps brachii
  The biceps brachii sits on the front of the upper arm and crosses both the shoulder and the elbow. It flexes the elbow, but its most specific function is supination of the forearm.
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• The rhomboids
  Two oblique sheets between the spine and the inner border of the shoulder blade. They draw the blade towards the spine — the exact reverse of what serratus anterior does.
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• Quadriceps
  The quadriceps brings together four muscles on the front of the thigh that share a common tendon. It extends the knee, and one of its heads also crosses the hip.
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• Hamstrings
  The hamstrings occupy the back of the thigh. They extend the hip and flex the knee, and their length depends on the combined position of these two joints.
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• Glutes
  The three gluteal muscles cover the back and the side of the pelvis. The gluteus maximus extends the hip, while the gluteus medius and minimus steady the pelvis as soon as the body is supported on one leg.
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• Calves
  The calf brings together the gastrocnemius and the soleus, which share the calcaneal tendon. They push the foot downward, and one of the two also depends on the position of the knee.
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• Tibialis anterior
  Tibialis anterior runs down the front of the shin and lifts the foot. Quiet under load, it works on every step to keep the toes from dragging on the ground.
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• Hip adductors
  Five muscles fill the inner thigh and draw the leg back towards the midline. Most of their work is stabilising, on every step taken on one leg.
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• Abdominal wall
  The abdominal wall brings together four stacked muscles whose fibres cross. It flexes and rotates the trunk, but its most constant role is to resist movement and to manage internal pressure.
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• Erector spinae
  The erector spinae form three muscular columns running along the spine. They straighten it, control its forward bending and hold the trunk during hip hinge movements.
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• The diaphragm
  A muscular dome separating the chest from the abdomen. It is the engine of quiet breathing and, at the same time, the ceiling of the cavity we pressurise to stiffen the trunk — two jobs competing for one muscle.
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• Rotator cuff
  Four short muscles link the shoulder blade to the upper arm and hold the head of the humerus against its socket. They orient the shoulder more than they move it.
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• Iliopsoas
  The iliopsoas is the only muscle that links the lumbar spine directly to the femur. It flexes the hip and acts on the spine, and it is never visible from outside.
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• Gluteus medius
  It pushes nothing forward. Its work is sideways: stopping the pelvis dropping on the unsupported side, on every step — roughly half the time you spend walking.
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• Tensor fasciae latae
  A short muscle that attaches not to a bone but to a fibrous band. That band runs down to the tibia and carries its pull along the whole length of the thigh.
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• Forearm muscles
  The forearm muscles drive the wrist and fingers from a distance, through long tendons. They carry grip strength, which often gives out before the large muscles do.
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