8.2 Kinesiology and Biomechanics

Kinesiology & Biomechanics

Kinesiology is the scientific study of human movement, while biomechanics applies the principles of mechanical physics to biological systems. For physical educators, biomechanical analysis provides the foundation for evaluating skill execution, designing corrective movement cues, preventing musculoskeletal injury, and enhancing motor performance across diverse physical activities.

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Anatomical Levers and Mechanical Advantage

A lever is a rigid bar (bone) that rotates around a fixed point called a fulcrum (joint) when force (effort, produced by muscle contraction) is applied to overcome a resistance (load, such as body weight or an external object).

$\text{Mechanical Advantage (MA)} = \frac{\text{Effort Arm Length}}{\text{Load Arm Length}}$

Human anatomical leverage is categorized into three distinct lever classes based on the relative positions of the fulcrum, effort, and load:

1. First-Class Levers (Effort – Fulcrum – Load)

• Configuration: The fulcrum sits between the effort force and the load ($E - F - L$).
• Function: Designed for balance, equilibrium, and changing the direction of force.
• Anatomical Example: The atlanto-occipital joint in the neck. The neck extensor muscles (effort) pull down on the posterior cranium, pivoting on the cervical joint (fulcrum) to keep the head (load) upright.
• Mechanical Example: Triceps extending the elbow overhead or a seesaw.

2. Second-Class Levers (Fulcrum – Load – Effort)

• Configuration: The load sits between the fulcrum and the effort force ($F - L - E$).
• Function: Mechanical Advantage is always greater than 1 ($MA > 1$). Maximizes force production; a small effort can move a large resistance, but sacrifices speed and range of motion.
• Anatomical Example: Plantar flexion at the ankle joint during heel raises. The ball of the foot acts as the fulcrum, body weight acting through the talus serves as the load, and contraction of the gastrocnemius/soleus pulling on the calcaneus provides the effort force.
• Mechanical Example: A wheelbarrow.

3. Third-Class Levers (Fulcrum – Effort – Load)

• Configuration: The effort force sits between the fulcrum and the load ($F - E - L$).
• Function: Mechanical Advantage is always less than 1 ($MA < 1$). Most common lever class in the human musculoskeletal system. Requires high muscular force, but maximizes linear speed, velocity, and range of motion at the distal end of the lever segment.
• Anatomical Example: Elbow flexion by the biceps brachii. The elbow joint is the fulcrum, the biceps insertion on the radial tuberosity is the effort, and the forearm/hand weight is the load.
• Mechanical Example: Swinging a baseball bat, tennis racket, or broom.

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Center of Gravity, Base of Support, and Stability

The blueprint names center of gravity alongside summation of forces, force-speed relations, and torque, and these four concepts explain most of what a physical educator needs to analyze in a movement.

• Center of gravity (center of mass) is the point at which the body's mass is evenly distributed in all directions. In anatomical standing position it sits roughly anterior to the second sacral vertebra, but it moves whenever segments move -- raising the arms raises it, and in a high-jump arch it can pass outside the body entirely, which is how a jumper clears a bar their center of gravity travels beneath.
• Base of support is the area enclosed by the body's points of contact with the surface.
• Line of gravity is the vertical line from the center of gravity to the ground.

The four stability principles

Stability and mobility trade off: a stable position resists disturbance but is slow to move from, which is why a starting block position is deliberately unstable -- the athlete puts the line of gravity outside the base so gravity begins the acceleration.

Force-speed (force-velocity) relations

Within a muscle, force and velocity of shortening are inversely related: the faster a concentric contraction shortens, the less force it can generate, and maximum force occurs at zero velocity (isometric) or during lengthening. The practical implications:

• A heavier implement is moved more slowly; a lighter one faster
• Training for maximal strength uses heavy loads at low velocity; training for power uses moderate loads moved rapidly
• Power = force x velocity, so power is maximized at an intermediate load rather than at maximal or minimal load -- which is why explosive training uses moderate resistance moved fast

Summation of forces and the kinetic chain

Maximum force is produced when the body's segments contract sequentially from largest and most proximal to smallest and most distal, each adding velocity to the one before it, and when all segments contribute in the same direction at the right moment. A throw that begins at the arm forfeits the contribution of legs, hips, and trunk -- typically most of the available force -- and loads the shoulder to compensate.

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Newton's Laws of Motion in Sport and Physical Activity

Sir Isaac Newton's three laws of motion govern how physical forces influence movement in athletic environments.

1. First Law of Motion (Law of Inertia)

An object at rest tends to stay at rest, and an object in motion tends to stay in motion at a constant velocity unless acted upon by an external net force.

• Inertia: The resistance of an object to any change in its velocity, directly proportional to its mass.
• Physical Education Application: A heavier basketball player requires more force to change direction than a lighter player. In track, a runner coming off the blocks must exert force against the ground to overcome resting inertia.

2. Second Law of Motion (Law of Acceleration / Force)

The acceleration ($a$) of an object is directly proportional to the net external force ($F$) acting on it and inversely proportional to its mass ($m$).

$F = m \times a \quad \Longleftrightarrow \quad a = \frac{F}{m}$

• Physical Education Application: If a soccer player kicks a ball with double the force, the ball accelerates twice as fast (assuming constant ball mass). In shot put, throwing a lighter 4kg shot with maximum force yields greater release velocity than throwing an 8kg shot.

3. Third Law of Motion (Law of Action-Reaction)

For every action force exerted, there is an equal and opposite reaction force exerted.

• Ground Reaction Force (GRF): When a runner drives their foot down and backward into the track (action force), the track exerts an equal and opposite force up and forward against the runner's foot (reaction force), propelling the athlete forward.
• Physical Education Application: Swimmers push water backward with their hands to move their bodies forward through the pool.

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Anatomical Planes and Axes of Movement

Human movement analysis relies on three cardinal anatomical planes that divide the body, each rotating around a perpendicular axis.

          Sagittal Plane                   Frontal Plane                  Transverse Plane
       (Left / Right Halves)           (Front / Back Halves)           (Top / Bottom Halves)
                 │                               │                               │
  Flexion / Extension / Dorsiflexion  Abduction / Adduction / Inversion  Internal / External Rotation
                 │                               │                               │
        Frontal-Horizontal Axis           Sagittal-Horizontal Axis             Vertical (Longitudinal) Axis

1. Sagittal Plane (Frontal-Horizontal Axis):

   • Divides the body vertically into left and right halves.
   • Permissible Movements: Flexion, extension, hyperextension, dorsiflexion, plantarflexion.
   • Activity Examples: Running, bicep curls, squats, forward somersaults.

2. Frontal / Coronal Plane (Sagittal-Horizontal Axis):

   • Divides the body vertically into anterior (front) and posterior (back) halves.
   • Permissible Movements: Abduction, adduction, lateral flexion of trunk/neck, inversion, eversion.
   • Activity Examples: Jumping jacks, side lunges, cartwheels, lateral side-stepping.

3. Transverse / Horizontal Plane (Vertical / Longitudinal Axis):

   • Divides the body horizontally into superior (upper) and inferior (lower) halves.
   • Permissible Movements: Internal (medial) rotation, external (lateral) rotation, horizontal abduction/adduction, pronation, supination, spinal rotation.
   • Activity Examples: Golf swing, baseball bat swing, discus spin, seated torso twist.

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Muscular Analysis of Movement Mechanics

During dynamic physical performance, skeletal muscles fulfill four distinct functional role classifications:

• Agonist (Prime Mover): The primary muscle directly responsible for generating torque to produce a specific joint movement (e.g., quadriceps during leg extension, biceps brachii during elbow flexion).
• Antagonist: The muscle located on the opposite side of the joint that opposes the action of the agonist. It must relax or lengthen to permit smooth movement, or contract eccentrically to decelerate rapid movement (e.g., hamstrings during leg extension, triceps during elbow flexion).
• Synergist: Muscles that assist the prime mover by contributing extra force or neutralizing unwanted secondary movements (e.g., brachialis and brachioradialis assisting the biceps brachii).
• Fixator / Stabilizer: Muscles that contract statically (isometrically) to anchor a bone or body segment, establishing a firm foundation against which prime movers can contract effectively (e.g., core abdominals and scapular stabilizers during an overhead shoulder press).

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Mechanics of Force Production and Absorption

Biomechanical effectiveness depends on applying force efficiently while absorbing impact forces safely.

Force Production (Kinetic Chain and Impulse)

• Kinetic Chain Sequencing: Maximal velocity (e.g., overhand throwing or kicking) requires sequential summation of forces from larger, slower proximal muscle groups (legs, hips, trunk) to smaller, faster distal segments (shoulder, arm, wrist).
• Impulse-Momentum Relationship: Impulse is the product of applied force ($F$) and time duration ($t$). Overcoming momentum or generating maximum release velocity requires maximizing force application time:

$\text{Impulse} = F \times \Delta t = \Delta \text{Momentum}$

Force Absorption

• To safely absorb impact (e.g., landing from a jump or catching a hard pass), athletes must increase the time duration ($\Delta t$) or displacement over which impact force is absorbed:

$F = \frac{\Delta \text{Momentum}}{\Delta t}$

• Practical Technique: Soft landing mechanics (bending knees and hips upon landing) lengthen $\Delta t$, dramatically reducing peak ground reaction force ($F$) on articular cartilage and ligaments, thereby preventing ACL tears and overuse injuries.
• Trap: Assuming the center of gravity always lies inside the body. In an arched high-jump clearance it passes outside it.
• Trap: Believing maximum power occurs at maximum load. Force and velocity are inversely related, so power peaks at an intermediate load.