Kinesiology and Biomechanics
Planes and joint actions
Anatomical position is the reference frame: palms forward, feet forward. Flexion and extension occur in the sagittal plane about a frontal axis; abduction and adduction occur in the frontal plane about a sagittal axis; rotation occurs in the transverse plane about a longitudinal axis. Questions often describe an action and ask for the plane, or describe a plane and ask for the axis - the axis is always perpendicular to the plane.
Muscle roles
The agonist is the prime mover; the antagonist opposes it; synergists assist; stabilizers fix a joint or segment; neutralizers cancel an unwanted action. Reciprocal inhibition lets the antagonist relax while the agonist contracts. Active insufficiency occurs when a two-joint muscle shortens so much it cannot generate force; passive insufficiency occurs when it is stretched so far it cannot lengthen further - hamstrings in a seated versus standing stretch is a classic discriminator.
Levers and torque
Torque equals force times moment arm, the perpendicular distance from the line of force to the axis. Most joints are third-class levers, favoring speed and range over force; a standing calf raise is second class and has mechanical advantage. Lengthening the moment arm increases torque demand, so a lateral raise is harder at 90 degrees than at 45 degrees with the same dumbbell.
Chains and stability
Closed-chain exercises fix the distal segment (squat, push-up) and are more joint-cooperative; open-chain exercises free it (leg extension, bench press). Stability increases with a larger base of support and a lower center of gravity.
Rule of thumb: to make an exercise harder without adding load, add moment arm or shrink the base of support.