How important are the hamstrings during squats?

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How important are the hamstrings during squats?

In many ways, the increasing popularity of powerlifting methods for improving strength has led to a much more technical approach to strength and conditioning. Most obviously, prominent powerlifting groups have popularized extremely detailed linear and undulating periodization models as well as accommodating resistance for improving rate of force development. Such developments are obviously very welcome and have clearly improved strength- and power-related athletic ability in a number of team sports. However, not everything that is associated with the use of powerlifting methods necessarily stands up to investigation.

One popular belief in some powerlifting groups is that the hamstrings are very active during the conventional back squat. Unfortunately, the research suggests that this is not the case. In this article, Chris Beardsley (@SandCResearch) reviews two studies that show just how active the hamstrings are in comparison with other leg muscles in the squat.

The study: Hamstring Activation During Lower Body Resistance Training Exercises, By Ebben, in International Journal of Sports Physiology and Performance, 2009

(Too much detail? Skip to the practical implications)


What’s the background?

One of the main reasons that sports scientists are so interested in the hamstrings is that they are one of the most frequently injured muscle groups, representing up to around 25% of all athletic injuries depending on the type of sport. Some researchers have proposed that such injuries might arise because of hamstring weakness and indeed some studies have found that weaker hamstrings or lower hamstring-to-quadriceps strength ratios might predispose athletes to hamstring strain injury. Anterior cruciate ligament (ACL) injuries are also very common, particularly among female athletes. Again, researchers have suggested that athletes might be predisposed to ACL injury where they have a low hamstring-to-quadriceps strength ratio. For these reasons, researchers have explored which resistance training exercises might lead to the greatest levels of hamstring activation, as it is generally believed that exercises involving higher levels of muscular activity lead to greater hypertrophy.


What did the researchers do?

The researchers wanted to identify the exercises that maximally activate the hamstrings.  They also wanted to determine the activation ratios between the quadriceps and hamstrings muscles during those exercises. For subjects, they recruited 34 athletes (21 men and 13 women) who were in the NCAA Division-I or Division-III. First of all, the subjects performed a maximum voluntary isometric contraction (MVIC) at 60 degrees of knee flexion for both the hamstrings and the quadriceps.  Next, the subjects tested their 6RM on the squat, seated leg curl, stiff leg dead lift, single leg stiff leg dead lift, good morning, and Russian curl. The Russian curl is a natural glute-ham raise with added weight where possible. After carrying out these tests, the subjects rested for c. 72 hours, before returning to perform two full range-of-motion (ROM) repetitions using their 6RM loads with 5 minutes rest between exercises.  While the subjects performed the exercises, the researchers recorded EMG activity using surface electrodes.


What happened?

Hamstring activation

The researchers found significant differences in hamstring activation between exercises.  The following chart shows the different activity produced by each exercise.

Hamstrings squats

The researchers found that the Russian curl was the most effective exercise for activating the hamstrings, while the squat was the least effective.

Hamstring to quadriceps ratio

The researchers found that the various exercises produced very different ratios of hamstring-to-quadriceps activity, as the following chart shows:

Hamstrings-to-quadriceps ratio

What is slightly misleading about the above chart is that you might assume that there is therefore a 1:1 ratio of hamstring-to-quadriceps muscle activity during the squat. This is not the case. In fact the ratio was 0.37:1.0, so the conventional back squat actually produced 2.7 times more quadriceps activity than hamstring activity. In other words, the conventional back squat is most definitely a quadriceps exercise.


Exploring the findings of other, similar studies

Without going into too much detail regarding the methods of other studies, it is probably worth just noting that a number of other researchers have made very similar findings. For example, McCaw (1999) found that the activity of each of the quadriceps muscles was very much higher than that of the hamstrings and gluteal muscles during the conventional back squat, as is shown in the chart below:

McCaw hamstrings squat

Similarly, Paoli (2009) also found that the normal stance width back squat did not significantly recruit the hamstrings in comparison with the quadriceps.  The conventional parallel squat seems to be a knee-dominant exercise that is limited by the strength of the quadriceps and not by the strength of the hamstrings.

Paoli hamstrings squat

The above two studies are not ideal on their own as a picture of the relative activity of the hamstrings and quadriceps muscles, as they did not normalize the EMG activity data to MVIC. Some muscle groups are covered more by fat than others and this can lead to greater impedance when testing those muscles, which leads to lower EMG values. Hence, looking at the voltage (mV) alone doesn’t really give us sufficient information to draw strong conclusions about the relative EMG activity of the various muscles. So we must see these studies as supportive rather than conclusive. On the other hand, Wright (1999) compared the medial (semitendinosus) and lateral (biceps femoris) activity during leg curls, stiff-leg deadlifts and squats and normalized the data to MVIC. The following chart shows the details:

Wright hamstrings

These researchers found that the squat produced markedly less medial and lateral hamstrings activity than either of the other two exercises. Unfortunately, they did not also measure quadriceps EMG activity.


What did the researchers conclude?

The researchers concluded that the Russian curl and seated leg curl are the most effective hamstring exercises and that the conventional back squat is not an effective hamstrings exercise.


What are the limitations?

Obviously, the key limitations are that technique and equipment are going to play some role in affecting the degree of hamstrings muscular activity during the squat. It is reasonable to expect that athletes who sit back more into a squat and use a more hip-dominant squatting pattern might generate more hamstrings activity than those who squat more upright. However, whether this actually occurs is uncertain. I will look at a couple of studies next week that investigate this area a bit more closely. Also, it is reasonable to assume that using powerlifting gear might also lead to greater hamstrings activity during a squat because of the restrictions that it places on the freedom of movement and the greater ability that it allows for lifters to sit back. This is certainly an area that needs more research and I am not aware of any studies that have actually explored this issue as yet.


What are the practical implications?

The squat is not a suitable exercise for developing the hamstrings. Therefore, where hamstring development is required for sports performance or for injury prevention, other exercises should be introduced into a program, such as the Nordic curl or Romanian deadlift.

Where the squat is programed extensively for building strength and size, care should be taken that the hamstrings are not neglected as a result. Many programs that are built around the squat program the deadlift in much lower volumes and frequencies out of necessity. However, this could lead to muscular imbalances and a predisposition to either hamstring strains or ACL injury.

It is likely that training the squat alone will not increase performance in the deadlift, as the deadlift involves a very marked contribution from the hamstrings. Therefore, powerlifters who are unable to deadlift often should include alternative hamstring exercises in their routines, such as good mornings.

While the squat is probably still the king of leg exercises additional hamstring exercises are certainly needed to maximize all-over thigh hypertrophy.

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