Biomechanics and Movement Analysis
The Biomechanics and Movement Analysis domain tests how forces, torques, levers, and movement patterns produce human motion. Questions often require identifying a lever class, calculating torque, or interpreting gait and muscle action.
Core rules
- Kinematics describes motion (displacement, velocity, acceleration); kinetics explains its causes (force, torque, momentum).
- Torque = force × perpendicular moment arm. The moment arm is the perpendicular distance from the axis of rotation to the line of action of the force, not the length of the bone or muscle.
- Levers: first class (axis between force and load; e.g., neck extension), second class (load between axis and force; e.g., calf raise), third class (force between axis and load; most human joints, e.g., biceps curl). Third-class levers favor speed and range over force.
- Mechanical work = force × displacement; power = work ÷ time. Muscular power depends on force and velocity; the force–velocity relationship shows that shortening velocity rises as load falls.
- The length–tension relationship: active tension is greatest near optimal sarcomere length.
- Gait cycle: stance phase is about 60% and swing about 40% of a normal walking cycle; double support occurs in walking but not running.
- EMG indicates muscle activation, not force directly.
Worked example
A client holds a 10 kg dumbbell in a biceps curl. The weight acts 0.30 m horizontally from the elbow axis. Force = mass × gravity = 10 × 9.81 = 98.1 N. Torque = 98.1 × 0.30 = 29.4 N·m. If the elbow flexes so the horizontal moment arm shortens to 0.15 m, torque from the same load falls to 98.1 × 0.15 = 14.7 N·m. The muscle must generate less torque at that joint angle, even though the mass is unchanged.
Traps
- Using the whole segment length as the moment arm.
- Confusing torque with force; torque depends on the axis and line of action.
- Claiming most human joints are second-class levers; most are third class.
- Mixing work and power units.
- Assuming EMG amplitude equals force across all conditions.
- Reversing stance and swing percentages in gait.