Biomechanics and Kinesiology
Biomechanics and kinesiology test how forces produce movement, how joints move, and how to analyse gait and exercise safely.
Core rules and concepts
- Kinematics describes motion (displacement, velocity, acceleration); kinetics explains forces and torques.
- Torque = force × perpendicular moment arm. Muscle force depends on length–tension and force–velocity relationships.
- Lever classes: first (fulcrum between force and load), second (load between fulcrum and force), third (force between fulcrum and load; most limbs).
- Arthrokinematics: roll, glide, spin. Concave–convex rule: convex moving surface glides opposite to bone motion; concave surface glides in the same direction.
- Open versus closed kinetic chain changes joint loading and muscle recruitment.
- Gait cycle: stance and swing; stance is about 60% of the cycle.
Worked example
A patient holds a 5 kg mass 0.30 m from the elbow. Weight = 5 × 9.81 = 49.05 N. Torque about the elbow = 49.05 × 0.30 = 14.7 Nm. The biceps must produce an equal opposing moment. If its moment arm is 0.04 m, required force = 14.7 ÷ 0.04 = about 368 N. Reasoning: the external torque is fixed by load and distance, but the muscle force needed depends on its small moment arm, which is why the elbow joint and biceps tendon experience large forces when holding a load away from the body.
Traps that cost marks
- Confusing mass in kilograms with weight in newtons; multiply by 9.81.
- Using the wrong moment arm; it must be perpendicular distance from the axis to the force line.
- Applying the concave–convex rule backwards.
- Assuming closed-chain exercise is always safer; it increases joint compression.