Exercise Science: Anatomy, Physiology, Biomechanics, and Kinesiology
Anatomy and muscle action
The sarcomere - actin and myosin filaments bounded by Z-lines - is the contractile unit; cross-bridge cycling (sliding filament theory) shortens it. Motor units are recruited by the size principle: small, low-threshold (Type I) units fire first, and larger Type II units join as force demand rises. Know the three actions - concentric (shortening), eccentric (lengthening, and capable of the greatest force), and isometric (no length change) - plus muscle roles: agonist, antagonist, synergist, and stabilizer.
Biomechanics
Torque = force × moment arm (the perpendicular distance from the joint axis). A joint angle that lengthens a moment arm increases torque. Most human joints behave as third-class levers (effort between fulcrum and load), favoring speed and range over force; the standing calf raise at the ankle is the classic second-class exception. Movement occurs in three planes - sagittal (flexion/extension), frontal (abduction/adduction), and transverse (rotation) - around the matching axis.
Physiology
Energy comes from ATP-PCr (roughly 0–10 seconds), fast glycolysis (about 10 seconds to 2 minutes), and oxidative phosphorylation (endurance work). Acute aerobic responses include rising heart rate, stroke volume, and cardiac output, with a widening a-vO2 difference; systolic pressure rises while diastolic stays near baseline. Proprioceptors matter: muscle spindles sense stretch and trigger the stretch reflex, while Golgi tendon organs sense tension and inhibit it.
Trap: eccentric actions produce the highest force at the lowest metabolic cost, and questions love to pair high force with low fatigue. Confusing spindle (stretch) with GTO (tension) is another repeat offender.
Worked example: A 200 N load sits 5 cm (0.05 m) from the elbow. Torque = 200 × 0.05 = 10 Nm. Move the load to 0.10 m and torque doubles to 20 Nm - the muscle works twice as hard for the same weight.