2.3 Biomechanics, Levers, Force, Torque, and Planes of Motion
Key Takeaways
- Biomechanics explains how force, torque, leverage, and motion affect exercise difficulty and safety.
- Third-class levers are the most common lever arrangement in the human body and favor speed and range of motion.
- The sagittal, frontal, and transverse planes organize movement analysis and exercise selection.
- Changing load position or body position can change torque even when the external weight stays the same.
Biomechanics for Exercise Reasoning
Biomechanics is the study of forces and their effects on the body. For the NASM-CPT exam, it is not advanced physics for its own sake. It is the reason a dumbbell held farther from the shoulder feels harder, a wider stance changes movement demand, and a rotational exercise belongs in the transverse plane.
Force is a push or pull that can create motion, stop motion, or change motion. Gravity, muscle contraction, ground reaction force, bands, cables, and external loads all create force. A trainer uses force concepts when choosing load, angle, range of motion, tempo, and stability demand.
Torque is rotational force around an axis. It depends on the amount of force and the distance from the axis, often called the moment arm. Holding a weight farther from a joint usually increases torque at that joint, even if the dumbbell weight is unchanged.
| Concept | Plain-language meaning | Training example |
|---|---|---|
| Force | Push or pull | Pushing the floor during a squat |
| Torque | Rotational demand around a joint | Dumbbell lateral raise at shoulder height |
| Moment arm | Distance between force line and axis | Weight farther from the elbow during a curl |
| First-class lever | Fulcrum between force and resistance | Head balanced on the neck |
| Second-class lever | Resistance between fulcrum and effort | Calf raise at the ankle |
| Third-class lever | Effort between fulcrum and resistance | Biceps curl at the elbow |
Third-class levers are most common in the human body. They usually sacrifice mechanical advantage for speed and range of motion. That is why small changes in load position can feel large during free-weight exercises.
Planes of motion are another frequent test target. The sagittal plane divides left and right and includes flexion and extension, such as squats and biceps curls. The frontal plane divides front and back and includes abduction and adduction, such as lateral lunges and jumping jacks. The transverse plane divides top and bottom and includes rotation, such as cable chops.
Exercises are often multiplanar, but the exam usually asks for the dominant motion. A walking lunge is mostly sagittal because hip and knee flexion and extension dominate. A side lunge is mostly frontal. A medicine ball rotational throw is mostly transverse.
Biomechanics also explains regression and progression. A push-up from the floor creates more demand than an incline push-up because body position changes the effective load. A cable press can create different lines of resistance than a dumbbell press.
Exam trap: do not classify planes by the equipment used. A cable can train any plane. Classify by the joint motion and body movement. Also remember that increasing instability is not always a better progression if the goal is force production or technique control.
Which lever class is most common in the human body and has the effort between the fulcrum and resistance?
A dumbbell feels harder to hold as the arm moves farther from the shoulder. Which concept best explains the increased demand?
Which movement is primarily in the transverse plane?