Biomechanical Principles

Levers: How Muscles Move Bones

A lever is a rigid bar (bone) that rotates around a fixed point (the fulcrum, i.e., the joint axis) when a force (muscle contraction) acts against a resistance (an external load or a body segment's own weight). The human body relies almost entirely on levers to convert muscular force into movement, and the exam expects an EP-C to classify a given movement by lever class and explain the mechanical trade-off each class produces.

• First-class lever — the fulcrum sits between the force and the resistance, similar to a seesaw. Extending the elbow against resistance with the triceps brachii is a first-class arrangement: the elbow (fulcrum) sits between the triceps insertion (force) and the hand (resistance).
• Second-class lever — the resistance sits between the fulcrum and the force, similar to a wheelbarrow. A standing calf raise is the classic example: the ball of the foot is the fulcrum, body weight is the resistance positioned in the middle, and the gastrocnemius/soleus apply force at the heel, farther from the fulcrum than the resistance. Because the force arm is longer than the resistance arm, second-class levers provide a mechanical advantage greater than 1 — they favor force production over speed. This arrangement is the least common in the human body.
• Third-class lever — the force sits between the fulcrum and the resistance, similar to using a shovel. A biceps curl is third-class: the elbow is the fulcrum, the biceps brachii applies force near the elbow on the radius, and the hand holding the weight is the resistance, farther from the fulcrum. Because the resistance arm is longer than the force arm, third-class levers operate at a mechanical disadvantage (the muscle must generate more force than the resistance) but they produce greater range of motion and speed at the distal end for a given amount of muscle shortening. Third-class levers are, by far, the most common configuration in the human musculoskeletal system.

Center of Gravity, Base of Support, Balance, and Stability

The center of gravity (COG) is the point at which a body's mass is balanced in all directions; in a person standing in anatomical position it lies roughly at the level of the second sacral vertebra, though it shifts continuously as limbs move, posture changes, or external loads are added or removed. The base of support (BOS) is the area bounded by all points of contact between the body and the supporting surface — for example, the area between and beneath the feet during standing.

Stability is governed by the relationship between the two:

• A body is most stable when its COG is centered within the BOS.
• Lowering the COG (e.g., bending the knees and hips) increases stability.
• Widening the BOS (e.g., widening the stance, or staggering the feet in the direction of an anticipated force) increases stability.
• Moving the COG closer to the edge of the BOS, narrowing the BOS, or raising the COG all decrease stability.

Balance is the ability to control the body's COG over its BOS and is trained and assessed in two forms: static balance (maintaining COG over a stationary BOS, e.g., a single-leg stance) and dynamic balance (maintaining control of COG over a BOS that is changing, e.g., walking on an uneven surface or performing a lateral bound). These principles directly inform coaching cues — widening a stance and lowering the hips before a heavy lift, a change of direction drill, or balance training for an older adult all apply the same COG/BOS logic.

Posture, Spinal Alignment, and Their Impact on Mobility

Neutral posture preserves the spine's three natural curves: cervical lordosis (a forward/anterior curve in the neck), thoracic kyphosis (a backward/posterior curve in the mid-back), and lumbar lordosis (a forward/anterior curve in the low back). These curves distribute load and absorb shock along the entire spinal column far more effectively than a straight column could.

Deviations from neutral alignment are common postural findings an EP-C should recognize:

• Excessive kyphosis — an exaggerated thoracic curve, often associated with a forward head and rounded shoulders
• Excessive lordosis — an exaggerated lumbar curve, often paired with an anterior pelvic tilt
• Scoliosis — an abnormal lateral (side-to-side) curvature of the spine
• Forward head posture — the head displaced anterior to the shoulders, increasing load on the cervical extensors

Chronic postural deviations alter the resting length and length-tension relationship of surrounding muscles: some become chronically shortened and tight while their opposing muscles become lengthened and weak. This imbalance restricts joint range of motion, changes movement mechanics during exercise, and increases injury risk — which is why posture and spinal alignment are assessed before designing an exercise program and are revisited whenever a client reports new pain or movement restriction.

Basic Biomechanical Principles of Human Movement

Several additional principles underlie safe, efficient movement and are woven throughout exercise prescription:

• Newton's laws of motion apply directly to exercise: a body at rest stays at rest until an external force acts on it (inertia), the acceleration of a mass is proportional to the force applied to it, and every muscular force applied to the ground or a resistance produces an equal and opposite reactive force.
• Summation of forces / the kinetic chain — efficient, powerful movement (e.g., a throw or a jump) sequences force generation from the larger, more proximal segments (hips, trunk) to the smaller, more distal segments (arm, hand), transferring energy along the kinetic chain.
• Mobility–stability alternation (joint-by-joint concept) — moving up the kinetic chain, joints alternate between a primary need for mobility (ankle, hip, thoracic spine, shoulder) and a primary need for stability (knee, lumbar spine, scapula). Programming that respects this alternation reduces compensation and injury risk.

Together, lever mechanics, COG/BOS relationships, postural alignment, and these general biomechanical principles give the EP-C the vocabulary and reasoning needed to assess movement quality, select safe exercise technique, and explain to a client why a given cue or modification improves performance or reduces risk.