Big picture
Kinesiology studies human movement, drawing on anatomy, physiology and related fields. Biomechanics applies mechanical principles to living systems. It explains how forces, body positions and equipment influence performance, while kinesiology provides the broader study of how movement occurs.
A player bends the knees before jumping, extends the legs during take-off and adjusts the body to land in balance. Describing these phases with precise movement terms helps explain technique. The purpose is to connect observable actions with structure and mechanics rather than memorise lists without examples.
Learning goals
Distinguish kinesiology and biomechanics and explain their uses.
Describe common joint movements from anatomical position.
Match movements to planes and perpendicular axes.
Explain force, torque and lever classes with examples.
Identify factors affecting balance and stability.
Analyse a sporting movement in phases using mechanical evidence.
Core ideas
1. Scope and importance
Kinesiology asks how movement is produced and controlled. Biomechanics studies its mechanical features, including position, velocity, forces and turning effects. Kinematics describes motion without concentrating on its causes; kinetics considers forces causing or changing motion.
A video showing the angle of knee flexion provides kinematic information. A force measurement during take-off contributes kinetic information. Both can support technique feedback, equipment design and understanding of injury mechanisms. They do not guarantee that one observed posture explains every injury or that one technique suits every body.
In coaching, start with the task goal. A high jump and a long jump both involve take-off, but the desired directions of motion differ. Analyse whether the movement supports the event's purpose, then consider the learner's ability and safety.
2. Reference position and movement language
Use anatomical position as the reference: upright, facing forward, arms by the sides and palms forward. A movement term describes the change at a joint, not simply the direction of the whole person. During a squat the body moves downward while the knees and hips flex.
Movement | Meaning | Example |
|---|---|---|
Flexion | Decreasing the angle between segments | Bending the elbow |
Extension | Increasing the angle between segments | Straightening the knee |
Abduction | Moving away from the body's midline | Raising an arm sideways |
Adduction | Moving towards the midline | Lowering that arm towards the side |
Rotation | Turning a segment about its long axis | Turning the trunk |
Circumduction | Sequential movement in a cone-like pattern | Circling an arm at the shoulder |
Extension need not mean moving backwards in space. Likewise, raising an arm forwards is shoulder flexion, while raising it sideways is abduction. Identify the joint and reference position before naming the action.
3. Additional joint movements
Dorsiflexion brings the top of the foot towards the shin. Plantar flexion points the foot away, as during the final part of a heel rise. Inversion turns the sole inward and eversion outward. Pronation and supination of the forearm turn the palm down/backward and up/forward respectively relative to position.
Elevation raises a structure, such as shrugging the shoulder girdle; depression lowers it. Protraction moves it forwards and retraction backwards. A throwing action combines several movements, so do not label the whole action with the name of only one joint movement.
Circumduction is not identical to axial rotation. In circumduction the distal end travels around a circle while the limb describes a cone. Rotation turns the segment around its own longitudinal axis. A shoulder can contribute both within a complex action.
4. Planes of movement
A plane is an imaginary flat surface used to describe movement. The three main planes intersect at right angles. They divide the body differently and are not physical sheets inside it.
Plane | Divides the body into | Typical movement |
|---|---|---|
Sagittal | Left and right portions | Elbow flexion and extension |
Frontal or coronal | Front and back portions | Arm abduction and adduction |
Transverse or horizontal | Upper and lower portions | Trunk rotation |
The median sagittal plane passes through the midline; sagittal planes more generally need not divide the body into equal halves. Many sports movements occur in several planes, so “primarily sagittal” is often more accurate than saying that a whole sport has only one plane.
5. Axes and their plane relationships
An axis is an imaginary line about which rotation occurs. For the simple movement being analysed, the axis is perpendicular to the plane.
Axis | Direction | Perpendicular plane | Example |
|---|---|---|---|
Transverse or mediolateral | Side to side | Sagittal | Forward roll or elbow bending |
Sagittal or anteroposterior | Front to back | Frontal | Sideways arm raising |
Longitudinal or vertical | Head to feet | Transverse | An upright body spin |
A common trap is matching the sagittal plane with the sagittal axis because the names agree. They are not the correct pair. Imagine a wheel: the flat circular movement and the axle point in perpendicular directions. Use that relationship instead of matching words.
6. Forces and changes of motion
A force is a push or pull and is measured in newtons, N. Internal forces act within the body, such as muscle forces. External forces include gravity, contact with the ground and air resistance. A body's acceleration depends on the resultant external force, not just the presence of one named force.
Newton's second law: F = ma for constant mass, where m is mass in kg and a acceleration in m/s².
Worked example: A 12 kg training object has a resultant horizontal force producing acceleration 3 m/s².
F = 12 × 3 = 36 N.
If opposing horizontal forces cancel, there is no horizontal acceleration. That does not require the object to be at rest; it can move at constant velocity. Weight is gravitational force, whereas mass is the quantity measured in kilograms.
7. Torque and moment arms
Torque, or moment of force, describes a turning effect. Torque = force × perpendicular moment arm. Its unit is newton metre, N m. The moment arm is the perpendicular distance from the axis to the force's line of action, not automatically the full bone length.
Worked example: A force of 300 N acts with a perpendicular moment arm of 0.08 m.
Torque = 300 × 0.08 = 24 N m.
A force of 30 N at 0.80 m also gives 24 N m. The same turning effect can arise from different combinations of force and distance. In a static balance problem, clockwise and anticlockwise torques must balance, as well as the resultant force being zero.
8. Levers in the body
A lever is a rigid body that turns about a fulcrum. In a simple body model, a bone is the lever, a joint is the fulcrum, muscle action supplies effort and an external load or body weight supplies resistance. The order of these three determines the class.
Lever class | Middle component | Body example | Main point |
|---|---|---|---|
First | Fulcrum | Balancing or extending the head about the upper neck | Effort and resistance lie on opposite sides |
Second | Resistance | Rising onto the toes in a simplified model | Load lies between toe fulcrum and calf effort |
Third | Effort | Biceps acting in elbow flexion | Effort lies between elbow fulcrum and hand load |
Most common limb examples are third-class systems that trade force advantage for movement range and speed at the distal end. Real biological movement is more complex than a three-point drawing, but the simplified model is useful when its assumptions are clear.
9. Mechanical advantage and balancing moments
For an ideal simple lever, mechanical advantage = effort arm ÷ resistance arm. It can also be expressed as resistance divided by effort under ideal balancing conditions. A value below one means the effort force must exceed the resistance force in that model.
Worked example: Effort arm = 0.05 m and resistance arm = 0.35 m.
Mechanical advantage = 0.05 ÷ 0.35 = 0.143 approximately.
If an 80 N force acts at 0.05 m, its moment is 80 × 0.05 = 4 N m. A 40 N force at 0.10 m produces the same moment. Do not conclude that equal forces are necessary for equilibrium; equal opposing moments are what matter for rotation.
10. Equilibrium, centre of gravity and stability
Equilibrium requires zero resultant force and zero resultant torque. Static equilibrium means remaining at rest; dynamic equilibrium is often used in school contexts for balanced motion at constant velocity. Moving balance in sport also involves continuous adjustments rather than only a perfectly static mechanical state.
The centre of gravity is the point through which the resultant gravitational force acts. In a reasonably uniform gravitational field it closely corresponds to the centre of mass. The base of support is the contact area and the area between supporting contacts.
Stability generally improves with a wider base in the relevant direction, a lower centre of gravity and the line of gravity remaining within the base. Friction and the direction of external forces also matter. A wrestler lowers the body and adjusts foot placement to resist a push; merely being heavier does not guarantee balance in every position.
11. Movement analysis in phases
Divide a task into preparation, execution and recovery. In a squat jump, preparation includes hip and knee flexion; execution includes extension and plantar flexion; recovery includes controlled flexion on landing. Much of the movement is sagittal about transverse axes, while balance adjustments can involve other planes.
Feedback should be specific and observable: “control the landing and keep the body balanced over the support area” is more useful than “use biomechanics.” Analyse a movement from more than one view when possible. A single camera angle can hide joint motion, so avoid claiming precise three-dimensional measurements from an unclear image.
A reliable approach to questions
Identify the joint, body position and task phase.
Name the movement using anatomical reference terms.
Match the plane to its perpendicular axis.
Identify forces, fulcrum, effort and resistance where relevant.
Use perpendicular moment arms in torque calculations.
Explain how the position affects the task's performance or stability.
Connections that matter
Anatomy supplies the bones, joints and muscle attachments in movement models.
Physics provides force, torque, acceleration and equilibrium principles.
Coaching uses movement analysis to make specific, observable technique changes.
Common mistakes
Pairing a sagittal plane with a sagittal axis ignores their required perpendicular relationship.
Calling arm circling pure rotation confuses circumduction with axial turning.
Using the full limb length instead of a perpendicular moment arm can give wrong torque.
Assuming equilibrium requires equal force magnitudes ignores differences in moment arms.
Revision checklist
I can distinguish kinesiology, biomechanics, kinematics and kinetics.
I can identify joint movements from anatomical position.
I can pair planes and axes correctly.
I can classify body levers and calculate moments.
I can explain factors affecting stability.
I can analyse a sporting action in phases.