Every trainer has watched two clients do the same exercise and get very different results.
One feels it in the right muscle. The other feels it in the lower back. Biomechanics is the science that explains why.
What is biomechanics?
Biomechanics is the study of how forces act on the living body and how the body responds to them. The word combines "bio" (living) and "mechanics" (the physics of forces and motion). In simple terms, it applies the laws of physics to human movement.
When you squat, walk, throw or lift, your body follows the same mechanical rules as any other object. Gravity pulls down, muscles pull on bones, and the ground pushes back.
Biomechanics measures and explains all of this.
The main branches of biomechanics
Kinematics describes motion without asking what caused it. It covers position, angles, speed and acceleration. For example, how deep a squat goes or how fast the knee extends.
Kinetics studies the forces that cause or change motion. It covers muscle force, joint loading, ground reaction force and torque.
Together, these two branches answer both questions a trainer cares about: what is the movement, and what is it demanding from the body?
Why biomechanics matters for trainers
- Better exercise selection. Knowing how load, joint angle and body position change muscle demand helps you pick the right exercise for the right goal.
- Smarter technique cues. Instead of copying cues you have heard, you can explain why a cue works. "Keep the bar close" makes sense once you understand moment arms.
- Lower injury risk. Poor positions can place high stress on a joint or tissue. Understanding load direction helps you spot and fix these before they become problems.
- Individual programming. Body proportions differ. A client with a long femur squats differently from one with a short femur. Biomechanics gives you a framework to adapt exercises rather than force everyone into one "perfect" form.
Key concepts every trainer should know
- Force: a push or pull that can change movement.
- Torque (moment): the turning effect of a force around a joint.
- Moment arm: the perpendicular distance from the joint to the line of force.
- Center of mass: the balance point of the body.
- Ground reaction force: the force the ground applies back to the body.
- Levers: bones act as levers, joints as pivots, muscles as the source of force.
Each of these ideas has its own article in this series.
Biomechanics in everyday training
Take a simple example: the biceps curl. At the start, the forearm hangs down and the weight pulls straight down along the arm, so the turning effect at the elbow is low. As the forearm reaches horizontal, the weight is furthest from the elbow in the horizontal direction and the demand on the elbow muscles peaks. Same weight, different demand.
That is biomechanics at work.
Common myths
- "There is one perfect technique for everyone." Body structure varies, so good technique varies too.
- "Biomechanics is only for researchers." Basic principles can be applied in any gym with no equipment.
- "It is too much maths." Trainers need the ideas, not heavy equations.
Final thoughts
Biomechanics turns coaching from guesswork into reasoning. You do not need a lab to use it. You need to understand a few core ideas and apply them every time you watch a client move.