Trainers often say an exercise is "harder" at one point in the range. Force, torque and moment arm explain exactly why. Once you understand these three ideas, you can predict where an exercise feels heavy without any equipment.
Force
A force is a push or pull. It is measured in newtons (N). The weight of a dumbbell is a force pulling straight down. A 10 kg dumbbell has a weight of roughly 98 N.
Force alone does not tell you how hard a joint has to work. The direction and the distance from the joint matter just as much.
Moment arm
The moment arm is the shortest (perpendicular) distance from the joint centre to the line of action of the force. For gravity, the line of action is vertical, so the moment arm is the horizontal distance between the joint and the weight.
Short moment arm means the force has little leverage over the joint. Long moment arm means it has a lot.
Torque (joint moment)
Torque, also called moment, is the turning effect a force creates around a joint.
Torque = Force × Moment arm It is measured in newton-metres (Nm). To resist a torque from an external load, the muscles around the joint must create an equal and opposite torque. So a higher external torque means higher muscle demand.
Example 1: Biceps curl
Hold a 10 kg dumbbell (about 98 N). Assume the dumbbell is 0.30 m from the elbow when the forearm is horizontal.
Forearm hanging down: the dumbbell is almost directly below the elbow. Moment arm is close to 0 m, so the torque is close to 0 Nm.
Forearm horizontal: moment arm is about 0.30 m, so torque is about 98 × 0.30 = 29 Nm.
Forearm raised high: the dumbbell moves back toward the elbow's vertical line, and torque drops again.
The weight never changed. Only the moment arm did. This is why the middle of a curl feels hardest.
Example 2: Lateral raise
A lateral raise starts with the arm at the side, where the moment arm at the shoulder is tiny. As the arm rises toward horizontal, the moment arm grows and the shoulder muscles work much harder. This is why light weights can feel very heavy at the top of a lateral raise.
Example 3: Deadlift bar position
The barbell's line of action is vertical. If the bar drifts away from the body, the horizontal distance to the hips and lower back increases, so the torque on those joints increases.
Cueing a client to "keep the bar close" reduces the moment arm and lowers the demand on the back.
Example 4: Squat and trunk lean
In a squat, the bar sits over the midfoot. A more upright trunk (like a front squat) tends to increase the knee's moment arm and reduce the hip's. A more forward-leaning trunk (like a low-bar squat) does the opposite. Neither is wrong. They shift the demand between the knee and the hip.
Example 5: Push-up variations
Pushing from the knees moves the pivot closer to the body's centre of mass, so the arms carry less body weight and the shoulder moment drops. Raising the feet on a bench shifts more body weight onto the hands and increases it. You can regress or progress a push-up simply by changing moment arms.
How to use this in coaching
- To make an exercise easier, shorten the moment arm. Move the load closer to the joint.
- To make it harder without adding weight, lengthen the moment arm.
- When a client reports pain at one point in the range, look for where the moment arm peaks.
- Use variations to shift demand between joints, such as knee-dominant versus hip- dominant squats.
Final thoughts
Heavier is not the only way to make an exercise harder, and lighter is not always easier.
Force tells you how much, the moment arm tells you where, and torque tells you what the joint must answer.