Biomechanics and Kinesiology
Biomechanics questions are applied physics: they ask why a lift feels hardest at a particular joint angle or why a client loses balance.
Levers. A lever has a fulcrum, effort, and load. First-class levers place the fulcrum between effort and load (triceps during elbow extension). Second-class levers place the load between fulcrum and effort (a calf raise is commonly cited). Third-class levers place effort between fulcrum and load and are by far the most common in the body (biceps curl). If asked which class is most common, the answer is third.
Torque and moment arms. Torque = force x perpendicular distance from the joint axis. A muscle's moment arm changes through range of motion, so resistance torque peaks where the moment arm is longest, usually mid-range. That explains sticking points and why machines use cams.
Length-tension and force-velocity. Active tension is greatest near resting length. Concentric force falls as velocity rises, while eccentric actions can exceed concentric force, which is why negatives feel easier to control but cause more soreness.
Planes and axes. Sagittal plane for flexion/extension, frontal plane for abduction/adduction, transverse plane for rotation.
Stability. Stability improves with a larger base of support, a lower center of gravity, and a center of gravity projected inside the base.
Rule of thumb: to make an exercise harder without adding weight, lengthen the moment arm by moving the load farther from the joint.