13.1 Kinesiology, Biomechanics & Functional Anatomy

Kinesiology, Biomechanics & Functional Anatomy

Kinesiology and biomechanics are Domain A applied-science foundations. Understanding forces, moments, levers, and muscle physiology lets the physiatrist prescribe exercise, analyze gait, design orthoses, and prevent injury.

Forces, Moments, and Joint Reaction Force

• Force (Newton): a push or pull; vector with magnitude and direction.
• Moment (torque): force x perpendicular distance from the axis of rotation; the rotary component that moves joints.
• Joint reaction force: the net internal force at a joint surface, often several times body weight because muscles must counter external moments.

External Moment = (External Force) x (Moment Arm)
   │
   └──►  Internal Muscle Force Must Balance It
            │
            └──►  Joint Reaction Force (frequently 2-4x body weight at the hip/knee)

Because muscle moment arms are short relative to external loads, muscle forces and joint reaction forces are large. This explains why the hip joint reaction force during gait is ~2.5-3x body weight and why a contralateral cane (long external moment arm) reduces required abductor force and joint reaction force.

Muscle Contraction Types

Eccentric loading underpins both tendinopathy rehab (controlled lengthening) and the higher rate of muscle strain injury (e.g., hamstring during sprint deceleration).

Length-Tension & Force-Velocity Relationships

• Length-tension: peak active force at near-resting sarcomere length; too short or too stretched reduces cross-bridge overlap and force (active component); passive tension rises at long lengths.
• Force-velocity: concentric force decreases as shortening velocity increases; eccentric force increases with velocity up to a plateau; isometric is the reference. These guide exercise tempo and loading choices.

Lever Systems

The predominance of third-class levers explains why muscles generate large forces (small moment arm) to overcome external loads at long moment arms—the musculoskeletal system is built for speed and range of motion, not mechanical advantage.

Functional Anatomy Applications

• Glenohumeral mobility-stability trade-off (rotator cuff and scapular stabilizers).
• Lumbopelvic load transfer (core stability, multifidus, transversus abdominis).
• Kinetic chains: open vs closed kinetic chain exercise change joint forces and muscle activation (e.g., closed-chain favored early after ACL reconstruction to reduce anterior tibial translation).

This foundation recurs throughout orthotic, prosthetic, and gait prescriptions in the guide.

Kinetic Chains: Open vs Closed

Closed-chain exercise reduces shear at the knee (e.g., preferred early after ACL reconstruction, reducing anterior tibial translation) and recruits co-contraction for stability; open-chain isolates muscles and is useful for targeted strengthening. Selection depends on healing stage, joint stability, and goal.

Core Stability & Lumbopelvic Mechanics

Trunk stability is generated by coordinated activation of transversus abdominis, multifidus, diaphragm, and pelvic floor—"core stability" transfers loads between upper and lower extremities while protecting the spine. Impairment (deconditioning, post-surgical, low back pain) leads to aberrant movement and injury. Rehabilitation emphasizes motor control and endurance before pure strength, integrating breathing and bracing into functional tasks.

Biomechanics of Common Rehab Interventions

• Stretching: sustained stretch elicits viscoelastic deformation and (over time) tissue remodeling; static stretching > 30 s improves ROM; PNF (contract-relax) uses autogenic inhibition.
• Resistance: progressive overload increases strength through neural adaptation (early) and hypertrophy (later); tendon, bone, and ligament remodel to load.
• Proprioception/balance: sensory reweighting reduces falls; relevant to ankle instability, vestibular, and geriatric rehab.
• Gait and transfer training: applies moment and lever principles to minimize joint loading and energy cost.

Worked Example: Cane Biomechanics

A 70 kg patient with right hip OA uses a cane in the left hand generating 100 N downward force at a 0.5 m moment arm from the right hip; body weight (~700 N) acts at ~0.1 m from the hip center. The cane's counter-moment reduces the required abductor force and the joint reaction force substantially—illustrating how a simple assistive device leverages biomechanics. Candidates should reason qualitatively about moments and forces rather than memorize numbers, applying the lever and joint-reaction-force principles to orthotic, prosthetic, and assistive-device questions.