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.
This is why the deadlift is often chosen as an exercise to develop the low back and hip extensor muscles (lumbar erectors, gluteus maximus, adductor magnus, and hamstrings), while the squat is commonly allocated to workouts used to develop the quadriceps.
N.B. Changing joint contribution without altering the limiting joint
Changing biomechanical factors can also alter the proportional involvement of each the joints without simultaneously affecting which joint is the limiting factor.
For example, the high-bar and low-bar squats both involve hip and knee extension joint torques, but the placement of the load alters peak joint angles. When expressed relative to the vertical ground reaction force, the low-bar squat may therefore involve a proportionally larger hip extension torque, while the high-bar squat may involve a proportionally larger knee extension torque.
This means that the single-joint quadriceps could contribute proportionally more to vertical ground reaction force than the hip extensors in the high-bar squat, than in the low-bar squat.
Even so, since the single-joint quadriceps muscle group is the limiting factor in both squat variations, it is not really possible to load them more in the high-bar squat than in the low-bar squat. From a bodybuilding point of view, the only difference between these two exercise variations is that in the low-bar squat (1) a heavier weight must be lifted, and (2) the hip extensors must produce more force, and therefore experience greater mechanical loading.
N.B. Individual differences
It is also worth noting that different people tend to alter the exact movement pattern (combination of joint angle movements) that they use to perform a lift, according to the relative strength of the involved muscle groups.
For example, individuals who have stronger legs relative to their back muscles tend to perform lifting tasks with more knee involvement, while people who have stronger backs relative to their legs tend to do the same lifting tasks with more back involvement.
Whether this tendency affects the limiting joint is not completely clear, however. It may simply shift some of the load onto the stronger joint, such that a heavier load can be lifted, while keeping the limiting joint the same. Alternatively, it could genuinely alter the exercise such that the limiting factor differs between different individuals.
What determines the limiting muscle in an exercise?
Often, there are multiple muscles working at a joint. This makes it difficult to tell which muscles are being stimulated by an exercise, even when it is possible to identify which joints are the limiting factor.
The ability of each muscle at a joint to contribute to joint torque is determined partly by the amount of force it can exert, and partly by its internal moment arm length. Changes in internal moment arm lengths of each muscle group occur with changing joint angle, which means that each muscle contributes a different amount to joint torque at different joint angles.
This means that (1) the range of joint angles used in an exercise, and (2) the exercise strength curve, can affect which of the muscles at a joint is worked more, relative to the others.
In addition, some joints have muscles that cross neighboring joints. These muscles are called “two-joint” muscles.
When an exercise involves movement at both of these joints, movement at one joint can alter how much force that the two-joint muscle produces at the other joint, and this can affect the contribution of the muscles in a way that is not immediately predictable from the joint torque contributions to the vertical ground reaction force. Consequently, the way in which two-joint muscles are working at both joints in a multi-joint exercise can affect which muscle group is the limiting factor.
#1. Range of joint angles
Differences in the range of joint angles used in an exercise are most easily appreciated by comparing different ranges of motion in the same exercise, although they can also occur between totally different exercises.
For example, the adductor magnus contributes the most to hip extension torque when the hip is flexed, the gluteus maximus contributes the most when the hip is extended, and the hamstrings contribute a similar level at all joint angles. This is why partial squats involve a proportionally greater contribution from the gluteus maximus, and deeper squats involve a proportionally greater contribution from the adductor magnus.
The same effect can be observed when comparing the hip thrust and the squat. The squat requires hip extension torque to be produced over a large range of motion (including very flexed hip joint angles, where the adductor magnus is the most important contributor), but the hip thrust requires hip extension torque to be produced through a smaller range of motion that includes fairly extended joint angles, where the gluteus maximus is the most important contributor.
#2. Strength curves
Differences in the strength curves used in an exercise are most easily appreciated by comparing the effects of using different types of resistance in the same exercise.
Strength curves are determined by body position and also by the type of resistance used. Using weight as resistance usually leads to a steep strength curve where the greatest forces are required at the start of a lift, when the joint angles are flexed. In contrast, using elastic resistance usually flattens the strength curve, or even causes an increasing strength curve, where the forces required are highest at the end of the lift.
For example, when using elastic resistance in the deadlift, the activation of the erector spinae is higher at the end of the lifting phase of the exercise range of motion, but it is higher in the first part of the lifting phase when using free weights.
The same effect can be observed when comparing the hip thrust and the squat. The strength curve of the squat is steep because the external moment arm lengths of the barbell decrease over the lifting (concentric) phase of the movement.
This means that the greatest hip extension torques are required at very flexed joint angles, where the adductor magnus is the most important contributor. In contrast, the strength curve of the hip thrust is flatter, and since it only moves through a small range of motion including fairly extended joint angles, the gluteus maximus is the most important contributor.
#3. Behavior of two-joint muscles
The behavior of two-joint muscles depends primarily on the direction of the joint torques that are required to produce the lift.
For example, in the deadlift, the vertical ground reaction force is created by the hip producing a hip extension torque, and the knee producing a knee flexion torque. Since the hamstrings are both hip extensors and knee flexors, this means that the contribution to system force required from the two-joint hamstrings is very high, because they must work both to extend the hip and also to flex the knee.
In contrast, in the squat, the vertical ground reaction force force is created by the hip and knee both producing extension torques. Since the hamstrings are hip extensors and knee flexors, this means that the contribution required from the hamstrings is conflicting. When the hamstrings produce force to assist with extending the hip, they simultaneously produce force that flexes the knee, and this acts to reduce the knee extension torque. Consequently, whenever the hamstrings increase their contribution to hip extension torque, the quadriceps must simultaneously increase their contribution to knee extension torque.
This is why adding a requirement to produce hip extension torque to an existing knee extension torque increases the activation of the quadriceps. Comparing the joint torques at the hip and at the knee in combined hip and knee extension exercises will always underestimate the contribution of the quadriceps.
What does this mean for hypertrophy?
We know that muscles grow after strength training when the muscle fibers controlled by their high-threshold motor units are simultaneously recruited and shorten slowly. This is what brings about the mechanical loading that stimulates hypertrophy.
When we experience fatigue during a series of reps of a strength training exercise, motor unit recruitment of the muscle or muscle group that is the limiting factor will increase, and reach a maximum as we approach failure. However, it is unlikely that the motor unit recruitment levels of the other muscles (the ones that are not the limiting factor) will reach a maximum at the same time.
Some of those other muscles may be partially stimulated, or they may not be stimulated at all. Either way, only limiting muscles will receive the full benefit of each stimulating rep during training with any given exercise.
What does this mean for programming?
The fact that an exercise produces a greater stimulating effect on the limiting muscle, compared with the other muscles, has two implications.
Firstly, it means that we can only count the number of stimulating reps for limiting muscles during exercises. Other muscles can be stimulated, but the extent to which they are stimulated will vary from exercise to exercise, and from person to person. For example, some exercises will stimulate one muscle strongly, and other muscles to a low-to-moderate extent. Other exercises might stimulate one muscle strongly, and another muscle to a moderate-to-strong extent.
Secondly, it means that while only one muscle will receive the full stimulus of the stimulating reps in an exercise, a number of muscles will be worked (to a lower level) and will experience fatigue, and subsequently muscle damage.
In practice, this means that bodybuilding programs that are mainly made up of single-joint exercises are easier to program for muscular development (easier to program is not necessarily better). Workouts using single-joint exercises are less likely to involve muscles being worked without also being stimulated. In such workouts, it is easy to track the true stimulating volume for each muscle over the week. Also, few muscles will experience fatigue and muscle damage without also being stimulated, and this makes it easy to increase the recovery time between workouts for each muscle.
What is the takeaway?
Some exercises stimulate a muscle more than others, even when both exercises require the same muscle groups to contribute to overall force production.
Which muscle is stimulated most strongly during an exercise depends firstly on which joint is the limiting factor for the exercise, which can differ between exercises and exercise variations, and could also be affected by individual movement patterns. It also depends on the range of joint angles used and the exercise strength curve, as these affect the joint angle at which the muscles must produce the highest forces, and therefore which muscle is required to contribute to the greatest extent.
Additionally, which muscle is stimulated most strongly in an exercise also depends upon the behavior of two-joint muscles, because joint torques at neighboring joints can influence the forces required of either the two-joint muscle or the antagonist to the two-joint muscle, depending on the exercise.