Kinematics and kinetics are the two halves of biomechanics. They are often confused, but the difference is simple: kinematics describes the movement, and kinetics explains the forces behind it.
Kinematics: describing motion
Kinematics looks at motion without asking what caused it. It answers questions such as:
- Where is the body or joint? (position)
- How far did it move? (displacement and range of motion)
- How fast is it moving? (velocity)
- How quickly is the speed changing? (acceleration)
- How much is a joint bending? (joint angle) Kinematic measures are usually collected with video analysis, motion capture, inertial sensors or even a simple phone app.
Gym examples of kinematics:
- Squat depth, such as the hip dropping below knee level.
- Knee flexion angle at the bottom of a lunge.
- Bar speed during a bench press.
- Stride length and cadence in running.
- Trunk lean during a deadlift.
Kinetics: explaining the cause
Kinetics studies the forces and moments that produce or change motion. It answers:
- How large is the force the ground applies to the body? (ground reaction force)
- How much turning effect acts on the joint? (joint moment)
- How much force are the muscles producing?
- How much work or power is being generated?
Kinetic measures usually need force plates, load cells or calculations based on body measurements and motion data.
Gym examples of kinetics:
- Peak ground reaction force when landing from a jump.
- Knee extensor moment during a squat.
- Force applied to a bar during a deadlift.
- Power output during a vertical jump.
Side-by-side comparison
Kinematics Kinetics Focus How the body moves Why the body moves
Why the difference matters
Two clients can show exactly the same squat on video. The kinematics look identical. Yet the joint moments can differ if one client holds the bar further forward or has a different limb length. So looking at movement alone does not tell you the whole story.
In the same way, you could measure a large force without knowing how the body produced it. A client may reach a high ground reaction force by landing stiffly or by landing softly. The kinematics (how the knees and hips bend) tell you which.
How trainers can use both
- Start with kinematics. Watch the movement. Note depth, angles, speed and symmetry. This is what you can see without equipment.
- Ask the kinetic question. Where is the load relative to each joint? Which joint is under the most turning demand?
- Link the two. When a joint angle changes, ask what happens to the moment arm and the muscle demand.
- Coach with reasons. Instead of saying "sit back more", explain that it moves demand from the knee to the hip.
Quick memory trick
- Kinematics = the movement you can see. Angles, speed and positions on a video.
- Kinetics = the forces you cannot see. The loads and turning effects that make the movement happen.
Kinematics Kinetics Key measures Position, angle, velocity, acceleration Force, torque, power, pressure Question answered What does the movement look like?
What does the movement demand? Common tools Video, motion capture, sensors Force plates, load cells, calculations Gym example Knee angle at the bottom of a squat Force the knee muscles produce at that angle
Final thoughts
A good trainer needs both lenses. Kinematics shows what is happening, and kinetics shows what it costs the body. Using them together gives better coaching, better programming and safer training.