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When we design a bodybuilding program, we choose exercises that are intended to increase the size of specific muscles. Each exercise is allocated to workouts within the program such that each muscle is targeted with the required amount of training volume.

However, most exercises require more than one muscle to be active at the same time. This is most obvious in multi-joint barbell exercises, like the squat, which requires the hands to hold the barbell, the upper body and the trunk muscles to stabilize it, and the lower body muscles to lift it.

And yet, the squat develops some of those muscles far more effectively than others. Clearly, even though many muscles are working, only some of them are being stimulated during each rep.

When is a muscle stimulated by an exercise, and when it is just working?
Bodybuilders have often found that some exercises are better for some muscle groups, and other exercises are better for others, even when they all involve the same muscle groups.

For example, many multi-joint, lower body exercises (including the squat, deadlift, lunge, split squat, and hip thrust) involve combined hip and knee extension, and therefore require the lumbar erectors, gluteus maximus, adductor magnus, hamstrings, and quadriceps muscles to produce force.

However, each of these exercises (and their variations) distribute the load between joints and between muscles slightly differently, and this means that some muscles are stimulated very strongly, while others are stimulated far less strongly.

To identify which muscles are being strongly stimulated and which are not, when multiple muscles are involved in an exercise or exercise variation, we must first identify the limiting joint, and secondly identify the limiting muscle group. For single-joint exercises, that is a straightforward task. For multi-joint exercises, it is quite a bit harder.

Let’s take a closer look at lower body multi-joint exercises, as an example.

What determines the limiting joint in an exercise?
In the lifting (concentric) phase of any multi-joint, lower body exercise, the lumbar, hip, knee, and ankle joints each produce turning forces (torques) that interact with one another. Together, they result in a vertical ground reaction force that causes the weight and the body to travel upwards.

The placement of the load, the starting and finishing positions of the joints and the body segments, the range of motion through which each joint moves, and the way in which the body segments interact with one another during the movement all influence the sizes of each joint torque, and therefore their relative contributions to the vertical ground reaction force.

Changing any of these factors can alter which of the joints is the limiting factor for successful performance of the exercise. This in turn alters which muscle group is stimulated most strongly.

For example, the squat and the deadlift both involve the lumbar, hip and knee joints extending, and the ankle joint plantarflexes. However, compared to the squat, the deadlift involves moving through a larger range of motion at the hip, but a smaller range of motion at the knee. Also, the relative positions of the hip and knee mean that the external moment arm length at the hip is longer in the deadlift, while the external moment arm length at the knee is shorter. Consequently, the deadlift is limited by the ability of the hip muscles to produce a joint torque at the hip. In this respect, it differs from the squat, which is limited by the ability of the single-joint quadriceps to produce a joint torque at the knee.