Kinesiology and Biomechanics
Levers and torque
Every human joint is a lever system: a fulcrum (joint), an effort (muscle force), and a load (resistance). Class I puts the fulcrum in the middle; Class II puts the load in the middle; Class III puts the effort in the middle. Most joints in the body are Class III, which is mechanically inefficient but favors speed and range. Torque = force × moment arm, so a resistance held farther from the joint dramatically increases demand.
Planes, axes, and muscle actions
Sagittal plane → flexion/extension (axis runs side-to-side). Frontal plane → abduction/adduction and lateral flexion (axis runs front-to-back). Transverse plane → rotation and horizontal movements (axis runs vertically). Concentric action shortens the muscle while producing force; eccentric action lengthens it under tension and yields the highest force output; isometric action produces force without length change.
Mechanical relationships
Length-tension: a muscle generates the most force at its resting length, where actin–myosin overlap is optimal. Force-velocity: concentric force falls as shortening speed rises, while eccentric actions can exceed both.
Traps and a rule of thumb
Questions routinely mislabel the plane a movement occurs in, or claim the body is mostly Class I levers. Read the action first, then assign the plane. Rule of thumb: “Fulcrum middle = first, load middle = second, effort middle = third - and the body is mostly third.” Worked example: in a biceps curl, the elbow is the fulcrum, the biceps is the effort, and the forearm-plus-dumbbell is the load, so it is a Class III lever; the dumbbell being far from the elbow is precisely why the exercise feels harder at the bottom of the range.