How does squat depth affect how hard the various leg muscles are working?

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Does squatting deeper affect all leg muscle groups equally?

Every lifter worth their salt knows that squatting deeper is harder. But a very interesting question is whether squatting deeper is harder for all of the key muscle groups – the hip extensors (glutes and hamstrings), the knee extensors (quadriceps) and the ankle plantar-flexors (soleus and gastrocnemius). Similarly, as anyone who has ever tried to improve their squat quickly finds out, putting more weight on the bar makes it harder to lift. But does that added weight make it equally harder for all three muscle groups? In this article, Chris Beardsley (@SandCResearch) looks at a very interesting study that takes a good stab at both of these questions.

The study: Effect of Squat Depth and Barbell Load on Relative Muscular Effort in Squatting, by Bryanton, Kennedy, Carey and Chiu, in Journal of Strength and Conditioning Research Publish Ahead of Print

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What did the researchers do?

The researchers wanted to investigate the effects of both barbell load and squat depth on the hip extensor, knee extensor and ankle plantar-flexor net joint moments during squats. They calculated the net joint moments during the squat by recording joint angles using a motion capture system coupled with retroreflective markers placed on key anatomical landmarks and ground reaction forces using a force plate. They then input the data into inverse dynamics equations, which produced the net joint moments. For the experiment itself, the researchers recruited 10 strength-trained women who had a minimum of one year of experience performing the high-bar back squat and who were able to perform a deep squat with a minimum barbell load of one times bodyweight. The subjects performed three sessions: a back squat 1RM test, a session involving multiple sets of dynamic squats with different weights, and a test of MVIC.

In the session involving multiple sets of dynamic squats with different weights, the subjects squatted one set of three reps with loads of 50%, 60%, 70%, 80% and 90% 1RM, with 3 – 5 minutes of rest between each set. During this test, the researchers recorded the movements and ground reaction forces. In the session involving the MVIC, the researchers tested hip extension, knee extension and ankle plantar-flexion strength at different joint angles, including 30, 60 and 90 degree angles for both the hip and knee and 5, 15 and 25 degrees for the ankle.

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What happened?

Changes in isometric force at different joint angles

The following chart shows the differences in MVIC for the different joint angles at which the hip extension, knee extension and ankle-plantar flexion muscles were tested. The chart shows that as the hip, knee and ankle joint angles increase, the muscle torque increases.

And the interesting thing is that the increases for each muscle group are not the same. So, you can see that the knee extension torque increases hugely between a knee angle of 30 degrees and a knee angle of 60 degrees and then it increases again at 90 degrees. It’s actually producing 55% more torque at 60 degrees than it is at 30 degrees. And it’s producing an incredible 81% more torque at 90 degrees than it is at 30 degrees. So, you can see that the increases for the hip are quite a bit less than those for the knee. And the increases for the ankle are lower still. So we can see that joint angle makes the biggest difference to the knee extensors, then the hip extensors, then the ankle plantar-flexors, in percentage terms.

Effect of squat depth on net joint moments

The following chart shows the findings for the net joint moments at 90% 1RM loading, as calculated by the researchers using inverse dynamics. The chart shows that for the 90% loading, the moments of all three joints increased with squat depth. However, again, the increases were not the same for all muscle groups.

You can see that after the increase from 30 to 60 degrees knee/hip angle, the ankle plantar-flexors don’t work much harder with deeper squats. So squat depth doesn’t seem to affect the muscle torque that the ankle plantar-flexors have to produce, assuming that the weight is the same in both cases. However, in the case of the knee and hip extensors, the squat depth has a significant effect and the deeper that you squat, the greater the muscle torque that they have to produce. So deeper squats mean that you need a lot more glute, hamstring and quadriceps strength but not really much more soleus or gastrocnemius muscle strength.

Effect of squat load on net joint moments

The following chart shows the effect of squat load on the three joint moments, at 90 degrees of hip and knee angle. You can see from the chart that the ankle and hip moments increase with increasing load but the knee moment does not increase particularly, at least not compared with how much it increases with increasing squat depth.

This means that as the weight increases for the same depth of squat, the hip extensors and ankle plantar-flexors have to work a lot harder but the knee extensors don’t. The researchers note that a recent study by Hartmann (2012) found that partial squats were not effective at improving vertical jump performance, while parallel squats were effective. They also note that models have suggested that the knee extensors are more important in vertical jumping than the hip extensors, although other studies have suggested that this may depend on the actual jumping strategies of the athlete. Some athletes have been reported to use a hip-dominant jumping strategy while others have been found to use a knee-dominant strategy.

Relative muscular effort findings

The researchers also calculated the relative muscular effort (RME) of the various squat depths and loads. The RME of a muscle group, such as the hip extensors, knee extensors and ankle plantar flexors, is calculated as its net joint moment for a given set of criteria divided by the maximum joint moment during its MVIC. RMEs are only interesting for multi-joint exercises, as for single-joint exercises the ratio of MVIC to the maximum net joint moment in a dynamic exercise should be 100% or thereabouts. However, for multi-joint exercises, the weakness of a particular muscle at a particular joint range of motion can prevent the other muscles from working to their maximum potential.

In this study, the researchers found that the greatest RME for each muscle group tended to occur at the greatest depth and load. However, they also noted that the RMEs of the muscles at a squat of 90% 1RM were very low in comparison to the intensity of the exercise. They noted that at 90% of 1RM, the ankle plantar-flexors were working at a RME of 71% of MVIC, the knee extensors were only working at 56% of MVIC and the hip extensors were working at 76% of MVIC. There are several possible explanations for this. The researchers suggest that the net joint torques ignore the forces that might be required to overcome antagonist co-contractions of the hamstrings that are required during the squats but not during the MVIC. While hamstring co-activation is required for hip extension, it may also be required at the knee for stabilization.

An alternative, or perhaps supplemental, explanation could be that the lower back may be an important part of the squat exercise and these muscles were not included in the current study. However, since there was no data on the lower back in this study, this is just speculation.

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What did the researchers conclude?

Load had a more pronounced effect than squat depth on the force required from the ankle plantar-flexors

So load is mainly what dictates how hard the ankle-plantar flexors are working in comparison with their maximum possible force output. Going deeper makes little difference to how hard the ankle-plantar flexors have to work.

Depth was a more significant factor than load for the force required of the knee extensors.

So depth is mainly what dictates how hard the knee extensors (quadriceps) have to work. Adding more weight to the bar makes much less difference. This could be part of the reason that Olympic lifters have large quadriceps in comparison to other strength athletes, because they of all athletes have to squat deep and depth makes more difference to the quadriceps than load.

The force required from the hip extensors was influenced by both barbell load and squat depth.

So both depth and loading influence how hard the hip extensors have to work. Partial squats are therefore putting much more emphasis on the hip extensors and much less on the quadriceps, since depth is key for the knee extensors.

Both depth and load should be considered as variables in using squats depending on which muscle groups are to be strengthened.

So this means that the knee extensors can be strengthened most effectively by deep squats and can be performed with lighter loads. However, the hip extensors and ankle plantar-flexors can be trained using heavier loads with smaller ranges of motion, as these muscle groups are less sensitive to depth.

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Limitations

The study was limited to the study of the hip extensors, knee extensors and ankle plantar-flexors and did not cover the lower back muscles or the co-contraction activities of the hamstrings. Additionally, the study was limited in that it did not check the EMG activity of the various muscles during the various exercises, to see what percentage of MVIC activation was produced for each muscle in each case. Finally, and perhaps most importantly, the study did not compare partials using heavier weights with deeper squats with lighter weights, which would be how they would be used in real training. For example, this could be done by reference to 1RM, so a 3RM partial could be compared with a 3RM full squat and the weights used in each would be very different.

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What are the practical implications?

Athletes wanting to develop vertical jump height, who have a knee-dominant jumping strategy, should squat deeper in order to maximize the stress on the quadriceps.

Athletes wanting to develop hip extension power for sprinting and other movements could use partial squats for this purpose, although there are many other suitable hip extension exercises.

Athletes should be aware that, for all its great benefits, the squat does not use the leg muscles to their maximum capabilities, even at very high percentages of 1RM. This suggests that athletes should make use of a variety of exercises to develop the leg musculature to their full extent.

Physique competitors such as bodybuilders looking to improve their quadriceps size should employ deeper but lighter squats.

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