Biomechanics and Kinesiology
Domain 3: Biomechanics and Kinesiology
The exam tests mechanical reasoning about human movement: vectors, torque, impulse, levers, projectile motion, and gait. You should be able to choose the principle that explains a performance change, not just define it.
Core rules
- Displacement is a vector; distance is scalar. Velocity = displacement/time; speed = distance/time. Acceleration is change in velocity/time.
- Torque = force × perpendicular moment arm. A longer moment arm increases torque for the same force, which is why a muscle’s moment arm changes through range.
- Impulse = force × time = change in momentum. Landing softly, following through, or using a countermovement increases time and reduces peak force or increases impulse.
- Angular impulse = torque × time = change in angular momentum. Angular velocity depends on both torque and moment of inertia.
- Projectile range depends on speed, angle, release height, and air resistance. The optimal angle is often below 45° when release height is above landing height.
- Gait cycle: stance and swing; ground reaction force, center of mass, and base of support determine stability.
Worked example
A catcher receives a pitch by letting the glove move backward. The ball’s momentum change is fixed, but extending the stop over a longer time lowers the average force on the hand: F = Δp/Δt. The same logic explains why a boxer rolls with a punch and why a jump landing with knee flexion reduces peak ground reaction force.
Traps
Moment arm is not the same as bone length; it is the perpendicular distance to the force line. Mass and weight differ. At constant velocity, acceleration is zero even though forces may act. Action–reaction forces act on different bodies and do not cancel. Do not call a larger lever “more efficient” without specifying force versus speed.