Clinical Biomechanics: Kinetics & Muscle Function

Flashcards covering key concepts in kinetics, the kinetic chain, muscle anatomy, types of muscle contractions, and lever systems in human movement.

Kinetics

Forces that cause movement.

Kinetic Chain

Helps analyze human movement patterns; provides a rationale for exercise and rehabilitation emphasizing the entire body; involves sequentially activated body segments where separate anatomical units work together mechanically.

Summation of Forces

In kinetic chains, large forces are sequentially built upon one another up the chain, like a whip action, to maximize efficiency.

Open Kinetic Chain

A type of kinetic chain where the distal limb can move freely (open end of the chain); often non-weight bearing and used to isolate specific muscle groups for strengthening.

Closed Kinetic Chain

A type of kinetic chain where the distal limb is planted (closed end of the chain); often weight bearing or pressing against an immovable object; typically uses multiple muscle groups working together, promoting stabilization, proprioception, and balance.

Myofilaments

Composed of thin (actin) and thick (myosin) filaments, which are the contractile proteins within muscle fibers.

Sarcoplasm

The cytoplasm of a muscle cell.

Myofibril

Long contractile organelles within muscle fibers, composed of subunits called sarcomeres.

Epimysium

The outermost layer of connective tissue, deep fascia, that covers the entire muscle.

Endomysium

The thin layer of connective tissue that surrounds individual muscle fibers.

Sarcolemma

The cell membrane of a muscle fiber.

Fasciculus

A bundle of muscle fibers within a muscle.

Perimysium

The connective tissue sheath that surrounds a fasciculus (a bundle of muscle fibers).

Sarcomere

The functional unit of force generation in a muscle; extends from Z line to Z line and contains organized actin (thin) and myosin (thick) myofilaments; explains muscle contractions via the Sliding Filament Theory.

Actin

The thin myofilament, a contractile protein involved in muscle contraction.

Myosin

The thick myofilament, a contractile protein that pulls on actin during muscle contraction.

Sliding Filament Theory

The mechanism by which muscle contraction occurs, involving the sliding of actin and myosin myofilaments past each other.

Titin

A large protein in the myofibril (sometimes referred to as the 3rd myofilament) with spring-like characteristics, allowing for stable and elastic properties of the sarcomere.

Mass (m)

The amount of matter that makes up an object (quantity).

Center of Mass (CoM or Center of Gravity)

The point at which the body's mass is evenly distributed.

Momentum (p)

The quantity of motion of an object, equal to the product of its mass and velocity.

Force

The action of one body on another, represented as a vector, factoring in magnitude, location, direction, duration, frequency, and rate.

Absolute Muscle Strength

The maximum force a muscle can generate, influenced by factors like hypertrophy of muscle fibers, neural adaptations, joint angle, muscle fiber length, arrangement (pennation), fiber types, force-velocity relationship, lever arm, and mechanical advantage.

Hypertrophy

An increase in the size of muscle fibers.

Neural Adaptations

Changes in the nervous system, such as increased efficiency of recruitment and lower stimulus threshold, that can improve muscle strength.

Isotonic Contraction

A type of muscle contraction where the muscle changes length while generating force; includes concentric and eccentric contractions.

Concentric Contraction

A type of isotonic contraction where the muscle generates a force greater than the force it opposes, causing the muscle to shorten and producing positive work and angular momentum at a joint.

Eccentric Contraction

A type of isotonic contraction where the muscle generates a force less than the force it opposes, causing the muscle to lengthen while controlling movement, producing negative work, and slowing angular momentum; requires less metabolic energy but can produce high force.

Isometric Contraction

A type of muscle contraction where the muscle generates a force equal to the force it opposes, resulting in no change in muscle length but a change in tension, and no angular momentum around the joint, thus maintaining a static joint position.

Joint Angle (in force production)

The angle of pull of a muscle and the angle of the joint at which the muscle action occurs, both of which affect the amount of force produced.

Length-Tension Relationship

The principle that maximal force is produced by a muscle near its normal resting length.

Pennation

Describes muscle fibers that attach obliquely to a central tendon, forming an angle between the fiber direction and the line of pull.

Angle of Pennation

The angle created between the fiber direction of a muscle and its line of pull; if 0 degrees, all force transmits through the tendon; if greater than 0 degrees, force transmitted to the joint is decreased.

Line of Pull

The direction of force generated by a muscle, running from tendon to tendon.

Pennate Muscles

Muscles with obliquely arranged fibers that, despite individual fiber disadvantages, can exert greater total force due to their higher volume of shorter, more numerous muscle fibers.

Large cross-section

_____ muscles provide greater force with less motion

Longer tendons

____ allow rapid motion over a distance

Force Velocity Relationship

The relationship between the rate that a muscle contracts and the force it can provide; slow contraction velocity generally allows for a greater ability to create myosin-actin binding sites and thus higher force.

Lever

A simple machine designed to increase or decrease mechanical advantage.

First Class Lever

A lever system where the fulcrum (axis) is located between the force and the resistance (e.g., a teeter-totter, or the splenius extending the head).

Second Class Lever

A lever system where the resistance is located between the fulcrum and the force (e.g., a wheelbarrow, or gastrocnemius in plantarflexion); generally the most efficient and has the greatest mechanical advantage.

Third Class Lever

A lever system where the force is located between the fulcrum and the resistance (e.g., using a shovel); this type is the most common in the human body.

Fulcrum (Lever System)

The axis, joint, contact point, or pivot around which a lever rotates.

Force (Lever System)

The effort, often provided by muscle contraction, in a lever system.

Resistance (Lever System)

The load, mass, or object the lever system is attempting to move, which can be an external weight or body weight.

Lever Arm

The bone on which the muscle is contracting within a lever system.

Mechanical Advantage (MA)

The ratio of the effort arm to the load arm; a larger effort arm compared to the load arm indicates greater mechanical advantage (an easier job); calculated as Effort arm / Load arm.

Effort (Force) Arm

The distance between the joint (fulcrum) and the muscle insertion site.

Load (Resistance) Arm

The distance between the joint (fulcrum) and the load.

Concentric contraction

Positive work- angular momentum at a joint

Concentric contraction

Tendon lengthening through tensile strain

Eccentric contraction

Negative work, slows the velocity of the angular momentum

Eccentric contraction

Requires less metabolic energy due to passively induced motion of the joint muscle

Isometric contraction

No angular momentum around the joint