A client moves the barbell a few centimetres forward and suddenly the lower back is working far harder. Another changes foot position and feels the glutes instead of the quads. These are not coincidences. They are joint moments in action.
What is a joint moment?
A joint moment is the rotational effect acting on a joint. Two sets of moments act at every joint: External moment: created by outside forces such as gravity, a barbell or a resistance band, acting at a distance from the joint.
Internal moment: created by the muscles, tendons and ligaments around the joint to resist or control the external moment.
During a slow, controlled lift, the internal moment roughly matches the external moment.
So the external moment is a good indicator of how much the muscles around the joint must work.
Why position changes the moment
A joint moment depends on two things: the size of the force and the distance of that force from the joint (the moment arm). Body position changes the moment arm without changing the load.
This means two clients can lift the same weight and place completely different demands on their hips, knees and ankles.
Example 1: Squat depth and stance
At the bottom of a squat, the hips and knees are both heavily flexed. The distance between the barbell line and each joint determines the moment at that joint.
- A narrow stance with an upright trunk usually increases the knee moment because the knees travel further forward.
- A wider stance with the hips pushed back usually increases the hip moment and reduces the knee moment.
- Raising the heels (heel-elevated shoes or plates) usually helps an upright trunk and shifts demand toward the knee and quadriceps.
None of these is a mistake. They are choices that shift demand to match the training goal.
Example 2: Hip hinge and deadlift
In a deadlift, the barbell stays close to the legs. The hip and lower back moments are large because the bar is far in front of the hips horizontally compared with the knees. The more the bar drifts forward, the larger both moments become.
A Romanian deadlift (RDL) with a soft knee bend keeps the shin nearly vertical, which keeps the knee moment small and puts most of the demand on the hips and hamstrings.
Example 3: Lunge variations
- A forward lunge with a long step puts more of the demand on the hip of the front leg.
- A lunge with a forward trunk lean increases the hip moment and glute demand.
- A short step increases knee demand because the front knee travels further over the toes.
Example 4: Shoulder exercises
In a front raise or lateral raise, the moment at the shoulder is largest when the arm is horizontal. Bending the elbow brings the weight closer to the shoulder and reduces the moment. Straightening the arm increases it. This is why a straight-arm raise feels harder than a bent-arm one with the same dumbbell.
Example 5: Machine and cable exercises
Machines use cams and cables to change the resistance moment across the range. A well- designed machine matches the moment to the muscle's strength curve. A poorly aligned one may leave part of the range too easy or too hard. The joint axis should line up with the machine's pivot for the loading to behave as intended.
A simple method to analyse any exercise
- Identify the direction of the external force. For free weights it is straight down.
- Locate the joint you are interested in.
- Estimate the horizontal distance between the line of force and the joint.
- Larger distance means larger moment and higher muscle demand.
- Change position, stance or load placement and check how the distance changes.
Practical coaching points
Use position changes to shift stress from one joint to another when a client has a sensitive joint.
Choose exercises based on the joint you want to train, not only the muscle name.
Remember that the largest moment is not always the most dangerous. It depends on tissue capacity, control and how well load is progressed.
Final thoughts
Exercise position is a programming tool. When you understand joint moments, you can adjust a movement for the client instead of forcing the client into the movement.