What is Joint Torque?

What is Joint Torque?

Joint torque is the turning effort produced around a joint, and it is the quantity that really governs whether you can move a load, not muscle force on its own. Your joints are pivots and your bones are levers, so muscles do not push a weight in a straight line; they rotate a bone about a joint axis. The measure of that rotation is torque. Torque is the product of two things: how much force the muscle pulls with, and the moment arm, which is the perpendicular distance from the joint axis to the line along which the force acts. Multiply those together and you have the torque in newton-metres. The same idea explains why a long wrench loosens a stubborn bolt more easily than a short one: more distance from the pivot means more torque from the same hand force. In the body, the resistance also creates a torque about each joint, and to move the weight your muscle must produce a joint torque larger than the torque the load demands. Because the moment arm of both the muscle and the resistance changes as the joint rotates through its range, the torque demand is never constant across a rep. That is why almost every exercise has a point where it feels hardest, the sticking point, and easier zones on either side. When coaches talk about a lift being hard in the hole of a squat or at the bottom of a curl, they are describing points of peak joint torque demand. Understanding torque turns vague feelings about which exercises are hard into a clear, predictable framework you can use to choose exercises, set up cables, and troubleshoot weak points.

How it works

Joint torque works through the lever mechanics of the musculoskeletal system. Consider a barbell curl. The elbow is the joint axis, the forearm is the lever, and the biceps inserts a short distance from the elbow. The muscle force acts almost along the bone, so its moment arm about the elbow is small, only a few centimetres, and it changes with elbow angle. The load in your hand, however, sits far from the elbow, so its moment arm is large. To lift the weight, the biceps must generate a joint torque at the elbow that exceeds the torque the dumbbell creates. Because the muscle has such a short moment arm, it has to produce a force many times larger than the weight itself, which is the general rule for the human body: muscles trade large force for speed and range of motion. As the forearm rotates, the moment arm of the load changes with the sine of the joint angle, peaking when the forearm is horizontal, which is exactly where a standing curl feels hardest. The squat is the same principle at two joints. Fry and colleagues measured hip and knee torques during the barbell squat and showed how bar position and how far the knees travel forward shift the balance of torque between the knee and the hip. Keeping the shins vertical reduces knee torque but increases hip torque, and letting the knees drift forward does the reverse. Escamilla's knee-biomechanics work on the squat showed how the internal joint forces and torques rise and fall with squat depth and load. This is why cueing matters: a low-bar squat with more forward torso lean loads the hips more, and a high-bar or front squat with a taller torso loads the knees and quads more, purely because those setups change the moment arms of the load about each joint. Muscle force itself also depends on the length-tension relationship and the joint angle, so the torque a muscle can produce is not constant either. The interaction of the torque a muscle can supply and the torque a load demands, across the whole range of motion, is what biomechanists call the strength curve, and it is the deep reason different exercises and tools feel the way they do.

The formula

Torque = force x moment arm

Joint torque equals the muscle (or load) force multiplied by its moment arm, the perpendicular distance from the joint axis to the line of force. A larger force or a longer moment arm both increase torque. Units are newton-metres.

How to apply it

• Change the moment arm of the load: Where the resistance sits relative to a joint decides the torque it demands. A front squat holds the bar forward and loads the knees; a low-bar squat sits it back and loads the hips. Pick the variation whose torque demand matches the muscle you want to train.
• Train through the high-torque range: The hardest part of a lift is where joint torque demand peaks. Pause reps and partials at that angle, such as a pause in the hole of a squat, build strength exactly where the torque requirement is greatest, which is usually the sticking point.
• Use tempo to keep torque honest: Bouncing or using momentum lets you cheat past the peak-torque angle. Controlled eccentrics and paused reps force the muscle to actually produce the required joint torque at the hardest angle rather than throwing the weight through it.
• Match tool to strength curve: Free weights peak the load's torque in the mid-range under gravity; cables and bands let you place peak torque elsewhere. Choose the tool whose torque profile lines up with where you want the muscle worked hardest.
• Balance torque across joints: In compound lifts the total torque is shared between joints. Cues that shift torso lean or knee travel redistribute torque between hip and knee. Use this to bias a weak muscle or to spare a cranky joint without dropping the load much.
• Respect that muscle torque varies with angle: A muscle cannot produce the same torque everywhere; force falls at very short and very long lengths. Programming full-range work trains the muscle to produce torque across its whole operating range, not just the easy middle.

Types

Worked example

This shows how the load torque about the knee changes across a barbell squat as depth changes the horizontal distance from the knee to the bar's line of force. The muscle must supply at least this much torque to keep moving. Values are illustrative to show the pattern, not exact readings.

As you descend, the horizontal distance from the knee to the bar grows, so the load's torque about the knee rises. Peak knee torque near the bottom is why the drive out of the hole is the hardest part of the squat.

High-bar squat vs low-bar squat torque

Neither bar position is better in general. Moving the bar changes the moment arms of the load about the hip and knee, redistributing torque. Choose the one whose torque demand matches your goal, your leverages, and any joint you need to spare.

By goal

• Strength athletes: Know where peak joint torque falls in your competition lifts and train there. Pause squats and deadlifts from the sticking-point height build the specific torque your muscles must produce at the hardest joint angle.
• Hypertrophy lifters: Use exercise selection to place high joint torque where a muscle is lengthened, which drives strong growth. Then use cables or machines to keep torque on the muscle in ranges free weights offload.
• Rehab and joint care: If a joint hurts at a certain angle, choose exercises and cues that lower the load torque at that angle, for example shifting torso lean or using a cable, while keeping the muscle trained. Rebuild torque tolerance gradually.

Common misconceptions

• "Joint torque is the same as muscle force." It is not. Torque is muscle force multiplied by its moment arm. A very strong muscle with a short moment arm may produce less joint torque at some angles than a weaker one with better leverage. Force and torque are related but distinct quantities.
• "A lift is equally hard through the whole range." Almost never. Because the moment arm of the load changes as the joint rotates, the torque demand rises and falls across the range. Every lift has a peak-torque angle, the sticking point, and easier zones on either side of it.
• "Keeping shins vertical in a squat removes stress." It does not remove stress, it moves it. Vertical shins lower knee torque but raise hip torque, so the hips and lower back take more. Torque is redistributed between joints by your positioning, not made to disappear.
• "Bigger muscles automatically mean more joint torque." Size helps because it usually means more force, but torque also depends on moment arm, which is set by your skeleton and insertion points. Two lifters with equal muscle can differ in joint torque because their leverages differ.

Related terms

Joint Torque FAQ

What is joint torque in simple terms?

It is the turning force a muscle produces to rotate a bone about a joint. Because your joints are pivots and bones are levers, muscles create torque, not straight-line force, and torque is what actually moves a load in every lift you do.

What is the formula for joint torque?

Torque equals force multiplied by moment arm. The force is how hard the muscle pulls, and the moment arm is the perpendicular distance from the joint axis to the line of force. A longer moment arm or a bigger force both increase the torque, measured in newton-metres.

Why is joint torque different from muscle force?

Force is the straight-line pull of the muscle; torque is that force multiplied by its moment arm about the joint. Because moment arms change with joint angle, the same muscle force can produce very different torque at different points in a movement.

Why does a squat feel hardest at the bottom?

Near the bottom, the horizontal distance from your knees and hips to the bar's line of force is largest, so the load's torque about those joints peaks. Your muscles must produce their greatest torque there, which is why the drive out of the hole is the sticking point.

How do high-bar and low-bar squats change joint torque?

Moving the bar changes the moment arms of the load about the hip and knee. A low-bar squat with more forward lean raises hip torque and lowers knee torque; a high-bar squat does the reverse, loading the knees and quads more. Torque is redistributed, not removed.

What units is joint torque measured in?

Newton-metres, written N.m. One newton-metre is the torque from a one-newton force acting at a moment arm of one metre. Biomechanics labs report joint torques in newton-metres, often normalised to bodyweight so athletes can be compared fairly.

Can I change the joint torque an exercise demands?

Yes, by changing the moment arm of the load. Adjusting bar position, torso lean, cable height, or your stance all move the line of force relative to a joint, which changes the torque that joint must produce even if the weight stays the same.

Does joint torque explain sticking points?

Largely, yes. A sticking point is the joint angle where the torque the load demands comes closest to the maximum torque your muscles can supply. Training paused reps and partials at that angle builds the specific torque you lack there.

References

1. Fry AC, Smith JC, Schilling BK. Effect of knee position on hip and knee torques during the barbell squat. J Strength Cond Res, 2003. PubMed 14636100
2. Escamilla RF. Knee biomechanics of the dynamic squat exercise. Med Sci Sports Exerc, 2001. PubMed 11194098
3. Escamilla RF, et al. Biomechanics of the knee during closed kinetic chain and open kinetic chain exercises. Med Sci Sports Exerc, 1998. PubMed 9565938
4. Variation in the patellar tendon moment arm identified with an improved measurement framework. J Orthop Res, 2022. PubMed 34191354
5. Physiology, Muscle. StatPearls, NCBI Bookshelf