Every movement you coach is a story of forces. Some come from outside the body, such as gravity, the barbell or the ground. Others come from inside, such as muscles, tendons and ligaments. Learning how these forces travel through the body helps you understand both performance and injury.
External forces
External forces act on the body from outside.
- Gravity pulls the body and any held load straight down at all times.
- Ground reaction force (GRF) is the push the ground gives back when you push into it.
It is the reason you can walk, jump and run.
- External load includes barbells, dumbbells, machines, resistance bands and even a partner's push.
- Friction and air resistance also act, although in most gym exercises they are small.
Internal forces
Internal forces are produced or carried inside the body.
- Muscle force: muscles pull on bones through tendons. They can only pull, never push.
- Joint contact force: the compressive force between two bones at a joint.
- Ligament and tendon tension: passive tissues that stretch and resist motion.
Internal forces are often much larger than the external load. A muscle attaches close to the joint it moves, so it works at a mechanical disadvantage and must pull hard to control a load held far away.
The kinetic chain: how force travels
The body behaves like a linked chain of segments: foot, shank, thigh, pelvis, trunk, arm. A force applied at one end travels through the chain, passing through each joint.
Take a standing overhead press. The ground pushes up through the feet. That force passes through the ankles, knees, hips and spine, and finally reaches the shoulders and arms. If one link is weak or poorly positioned, force can leak or the stress can concentrate in the wrong place.
This is why trainers often look beyond the joint that hurts. A knee problem may start with ankle stiffness. A shoulder problem may start with poor trunk control.
Forces at the joints
At each joint, forces can act in three main ways:
- Compression: bones pushed together, for example the spine under a back squat.
- Tension (distraction): tissues pulled apart, for example the shoulder when you hang from a bar.
- Shear: forces sliding one surface across another, for example the tibia moving forward on the femur.
Healthy tissues tolerate these loads well when they are gradually built up. Problems tend to appear when the load is too high, too sudden or repeated too often without recovery.
How loading changes with position
The same weight can create very different internal forces depending on body position.
When a barbell is held further from the hips in a deadlift, the back muscles must work harder to resist the turning effect. When the bar stays close, the demand drops. Position changes the lever lengths, and lever lengths change muscle force.
Practical takeaways for trainers
- Always think of the whole chain, not just the target muscle.
- Notice where the load is relative to the joint, not only how heavy it is.
- Progress load gradually so tissues can adapt.
- When a client has pain, check the joints above and below it.
- Teach clients to feel the ground. Good force transfer often starts at the feet.
Final thoughts
Forces always take a path through the body. A good trainer learns to follow that path and make it efficient, safe and strong.