2.3 Biomechanics, Levers, Force, Torque, and Planes of Motion

The Three Planes of Motion

All human movement is described relative to three imaginary planes of motion, each bisecting the body and each paired with an axis of rotation. Mastering them lets a trainer classify any exercise and program balanced, multiplanar training.

Most foundational gym movements are sagittal-plane (squats, presses, curls, lunges, running). The frontal plane adds side-to-side work (lateral lunges, lateral raises), and the transverse plane adds rotation (cable chops, throwing). Because daily life and sport are multiplanar, NASM emphasizes progressing clients beyond the sagittal plane once they demonstrate control.

Force, Torque, and the Moment Arm

Force is any push or pull acting on the body, defined by Newton's second law as force = mass × acceleration. In training, external force comes from gravity acting on a load, bands, machines, or the client's own bodyweight.

Torque is rotational force — the turning effect produced when a force acts at a distance from a joint's axis. It equals force × moment arm, where the moment arm is the perpendicular distance from the axis of rotation to the line of force. This single relationship explains many exam scenarios: holding a dumbbell with the elbow extended (long moment arm) creates far more torque at the shoulder than holding the same dumbbell close to the body (short moment arm), even though the weight never changed.

Practical applications:

• Sliding the hands wider on a lat pulldown or moving a plate farther from the trunk during a core exercise increases torque and difficulty.
• Keeping the bar path close to the body in a deadlift shortens the moment arm at the spine, reducing the rotational load and protecting the low back.

Lever Systems in the Body

A lever is a rigid bar (a bone) that rotates around a fulcrum (a joint) when force is applied against a resistance. The three classes are defined by the order of the fulcrum, effort, and load:

The third-class lever is the most common arrangement in the human body. It sacrifices mechanical advantage — the muscle must produce more force than the load — but rewards the body with greater speed and range of motion at the end of the limb. This is a frequent exam fact: when asked which lever dominates human movement, the answer is third-class.

Newton's Laws Applied to Coaching

Three of Newton's laws turn biomechanics into coaching cues:

• Law of inertia (first law): a body at rest or in motion stays that way unless acted on by a force. Heavier loads have more inertia, so a slow, controlled start prevents momentum from taking over.
• Law of acceleration (second law): force equals mass times acceleration. To accelerate a load faster (power training), the client must apply more force.
• Law of reaction (third law): for every action there is an equal and opposite reaction. Pushing into the ground during a jump produces an equal ground-reaction force that propels the body upward.

Trainers use these principles to adjust tempo, load placement, and stability. Slowing the eccentric tempo increases time under tension; lengthening a moment arm raises torque; and changing the base of support alters how ground-reaction forces are managed. Recognizing why an exercise becomes more demanding — rather than only that it does — is the reasoning NASM biomechanics questions assess.

Putting Biomechanics to Work in Program Design

Biomechanics is not abstract physics — it is the toolkit a trainer uses to make an exercise easier, harder, or more specific without ever changing the weight on the bar. Several worked applications the exam expects:

• Regressing or progressing by leverage: a plank held from the knees shortens the lever between the fulcrum (shoulders) and the load (hips), reducing torque on the core; moving to a full plank lengthens that lever and raises the demand.
• Changing the plane to add specificity: an athlete who cuts and rotates in sport needs transverse-plane training (cable rotations, rotational med-ball throws), not just sagittal-plane squats and presses.
• Managing spinal torque: coaching a lifter to keep the load close to the body during a row or deadlift shortens the moment arm at the lumbar spine, lowering the rotational stress that drives low-back strain.
• Using ground-reaction force for power: plyometric and jump training teach the client to apply force rapidly into the ground so the equal-and-opposite reaction propels the body, the basis of NASM's power phase.

Stability and the Base of Support

A wider base of support and a lower center of gravity make a position more stable, which is why NASM often begins clients on stable surfaces and two-leg stances before progressing to single-leg or unstable-surface work. Narrowing the base or raising the center of gravity increases the balance demand and forces the nervous system and stabilizers to work harder. Each of these adjustments is a deliberate biomechanical choice, and the exam rewards the trainer who can explain the underlying reason — leverage, plane, force, or stability — rather than guessing.