Clinical Kinesiology & Biomechanics: Linear Kinetics

Kinetics: Branch of biomechanics describing forces acting on bodies, causing observed kinematics.
Musculoskeletal system: Generates forces for human body movement.
Divided into: Linear and Angular Kinetics.

Force: Vector quantity with magnitude, direction, and point of application.
Scalars: Quantities with magnitude only (e.g., mass, length, speed).
Vectors: Quantities with both direction and magnitude (e.g., displacement, velocity, acceleration).
Muscle Forces: Represented as vectors; point of application at attachment, direction towards muscle center.
Resolution of a Vector: Replacing a single vector by two or more component vectors using trigonometry.
Clinical Connection - Muscle Forces: Generate moments (angular motion) and forces (linear motion, can be stabilizing or destabilizing).
Internal Forces: Produced within the body; active (stimulated muscles) or passive (stretched connective tissues).
External Forces: From outside the body (gravity, external loads, physical contact).
Force Types:
- Tensile: Collinear, opposite, pull apart; tends to make tissue longer/thinner (e.g., ligament strain).
- Compression: Collinear, similar, push together; tends to make tissue shorter/thicker (e.g., compression fracture).
- Shear: Coplanar, opposite, non-collinear; causes surfaces to slide; poorly tolerated (e.g., pelvic shear fracture).
- Bending: Unilateral compression coupled with opposite side tension.
- Torsion: Parallel rotational forces in opposite directions.
- Combined: Tissue often subject to combinations.

Definition: Average load on the plane of material ( $Stress = Force / area$ ).
Tension Stress: Force pulling tissue apart over area (e.g., Achilles tendon: $300 N / 2 cm^2 = 150 N/cm^2$ ).
Compression Stress (Pressure): Force pushing tissues together over area (e.g., ground reaction force on feet).
Shear Stress: Force causing surfaces to slide (e.g., pubic symphysis during walking).

First Law (Inertia): An object at rest remains at rest, or an object in motion remains in motion at a constant velocity, unless acted upon by an unbalanced external force. For static equilibrium, all external forces sum to zero ( $\Sigma F<em>X = 0, \Sigma F</em>Y = 0, \Sigma M_Z = 0$ ).
Second Law (Acceleration): Force equals mass times acceleration ( $F = m \times a$ ).
- Larger acceleration requires decreased mass or increased force.
- Decreasing mass can increase performance and decrease injury (e.g., obesity and musculoskeletal injury).
Third Law (Action-Reaction): For every action, there is an equal and opposite reaction (e.g., Ground Reaction Forces (GRFs) during walking).

Peak Force: Highest magnitude of the force-time curve.
Rate of Force Development (RFD): Positive slope of the force-time curve.
Impulse: Area under the force-time curve, equal to change in momentum.
- Propulsive impulse: Increases momentum.
- Braking impulse: Decreases momentum.
Injury Reduction: Increasing the time over which a force is applied decreases the resultant force (e.g., soft landing from a jump).

Definition: Point about which an object's mass is evenly distributed.
Segmental CoM: Each body segment has its own CoM.
Human Body CoM: Approximately anterior to the second sacral vertebra (S2) in anatomical position.
CoM Movement: Altered by body segment rearrangement (e.g., trunk flexion shifts CoM significantly).
Stability:
- Line of action of body weight must intersect the base of support.
- Larger base of support and lower CoM increase stability.
- Crutches or walking sticks increase base of support and aid stability.

Concurrent Force System: Forces acting at the same point but in different directions.

Stress-Strain Relationship:
- Toe Region: Initial non-linear phase as collagen fibers straighten.
- Elastic Region: Linear relationship, tissue returns to original length upon force removal (e.g., ACL strain 3-4%).
- Yield Point: Point where tissue begins plastic deformation).
- Plastic Deformation: Permanent deformation after yield point.
- Ultimate Failure Point: Tissue partially or completely separates (tendons fail at 8-13% elongation).
Viscoelasticity: Combination of elasticity and viscosity.
- Creep: Progressive strain of material under constant load over time (e.g., stretching shortened tissue). Higher temperature increases creep rate.
- Rate-dependent Properties: Stress-strain curve sensitive to loading rate; faster loading increases stiffness (e.g., articular cartilage during running).

Joint Instability: Excessive/uncontrolled range of motion or small, abnormal movements causing pain/dislocation.
Stable Joint: Provides appropriate responses to stimuli without excessive stress.
Passive Subsystem: Contact forces between joint surfaces and tensile forces of ligaments provide stability.
Active Subsystem: Muscle-tendon complex (MTC) activation increases joint stiffness via compression and resisting perturbations.
Static Stability: Ability to maintain a reference position under perturbation.
Dynamic Stability: Ability to maintain a given trajectory under perturbation; focuses on robustness (disturbance tolerance) and performance (deviation from trajectory and return speed).