What you’re getting yourself into
7-14 minute read time. If you’d rather watch than read, there are both a video and a graphic covering the same information at the end of the article.
1. In general, the body utilizes single-joint muscles before two-joint muscles really kick in. This makes movement more efficient.
2. At the bottom of the squat, overactive hamstrings make the movement unnecessarily difficult, so squatting in a manner that maximizes hamstrings involvement in the hole is probably unwise.
3. If you find yourself getting pitched forward in the squat, strengthening your glutes helps your hamstrings relax a bit, taking some strain off your quads and making the bottom half of the squat easier.
4. The place you want your hamstrings to kick in is at the sticking point, when demands on the quads have decreased a bit, and the added hip extension torque can help you keep the bar moving.
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The study I have for you guys today is one I’ve been waiting on for quite a while. Last January, I wrote this article about 1) why the squat is an overrated hamstrings exercise and 2) why you wouldn’t want your hamstrings firing too hard when squatting anyways. It was met with mixed reactions. The first assertion wasn’t very controversial, but the second was. Unfortunately, the second assertion – excessive hamstrings activation not being a particularly good thing for squatting performance – was the one I couldn’t back up with much more than a little biomechanical reasoning. I wasn’t overly familiar with lower body biomechanics literature at the time, and even if I was, there was only indirect evidence – no data directly addressing the issue.
This new study, Quadriceps effort during squat exercise depends on hip extensor muscle strategy (Bryanton, 2015), changes all that.
It’s a follow-up to Bryanton’s 2012 study I’ve referenced in almost every squat article on this site. If you start reading this thinking “I have no idea what the hell is going on right now,” then check out these articles first as a refresher. One Two Three Four Five Six Seven Eight
First, a little background.
In the original study, the researchers measured the participants maximal knee and hip extension torques at a bunch of different joint angles, then got them to work up to squats at 90% of their 1rm, and finally compared the net joint torques required to complete the lifts to the maximum torque the participants were capable of producing at those joints. The resultant number (as a percentage) is termed Relative Muscular Effort (RME).
Just to use arbitrary numbers, if someone’s maximum hip extension torque at 90 degrees of hip flexion was measured to be 500Nm, and video analysis revealed that at 90 degrees of hip flexion, that person needed to produce 400Nm of hip extension torque to lift the bar, that would mean Relative Muscular Effort was 80%.
A curious finding in that study was that at the bottom of the squat – the most challenging part for the quads – RME was only about 60%, even with near-max loads. Obviously, a heavy squat doesn’t represent a mere 60% effort for the quads. The reason that number is so low is that it represents the minimum amount of torque necessary to move the weight, without accounting for the contraction of the hamstrings, which cause knee flexion torque, counteracting the quads’ knee extension torque.
The purpose of this study was to see how much the hamstrings affected quadriceps effort. To investigate, they took the data from the original study, and used two different models that were based on different assumptions about how the hip extensors work. Model 1 assumed that the glutes and hamstrings would both work equally hard to extend the hips. Model 2 assumed that the glutes did as much as they possibly could to produce hip extension torque, and the hamstrings would only kick in once the glutes were “maxed out.”
Just to illustrate, let’s say you have to produce 40% of your maximal hip extension torque. Model 1 would assume your glutes and hamstrings both contracted at 40% of their maximal capacity. Model 2 would assume that your glutes did all the work, and that your hamstrings wouldn’t kick in until you added more weight to the bar and your glutes were maxed out (at ~50% of maximal hip extension demands).
To be very clear about this point – neither of these models are “true.” They represent upper and lower bounds of how much the hamstrings could possibly be doing. Model 1 (equal glute and hamstrings contributions) would be the upper bound, and Model 2 (glutes do as much as possible, and hamstrings do as little as possible) would be the lower bound. The purpose of the study was to see which one was likely to be closer to what’s actually happening in the squat, based on how they’d impact quadriceps effort.
Now, after all that build-up and background, here’s what they found:
The black line is the one from the original study, only taking into account the maximum strength of the quadriceps at those joint angles and the net torque required to move the bar. It does not taking into account knee flexion torque from the hamstrings at all.
The green line is the one from Model 2, assuming the glutes do all they can before the hamstrings start kicking in. As you can see, with a 60% load, it’s not too different from the original study’s data. That’s because with such a light weight, a maximal contraction from the glutes could handle almost all the hip extension needs, with the hamstrings contributing minimally, meaning a very small increase in demands on the quads.
The red line is the one from Model 1, assuming the glutes and hamstrings are sharing the load equally. As you can see, at the bottom of the squat (knee flexion exceeding 105 degrees), it would require an unrealistically strong (impossible) contraction from the quads to produce enough knee extension torque to both counteract the hamstrings and extend the knees. Keep in mind that this is just at 60%1rm squat.
Here at 90%, the hamstrings are kicking in more and more, even in Model 2, as you can see the divergence between the black and green lines. However, the peak demand on the quadriceps at full depth is about 87% of maximal, which is just about what you’d expect for a 90%1rm squat.
Model 1, on the other hand, would require the quads to contract with 20% more force than they’re capable of. Obviously unrealistic.
This study further fleshes out the concept I presented here – the vasti (three of the four quad muscles) and the glutes are really what do the bulk of the work in the squat, while the hamstrings and rectus femoris primarily function to distribute forces throughout the system.
This idea should make sense intuitively. When the vasti and glutes contract, there is no “cost.” They cause knee and hip extension without also inhibiting extension of another joint. Contraction of the rectus femoris and hamstrings, on the other hand, does carry a “cost.” The hamstrings help extend the hips, while also resisting extension of the knees, and vice versa for the rectus femoris. It makes sense that the single-joint muscles would do all they could before the two-joint muscles kick in.
Apparently, the central nervous system (which is coordinating the movement) agrees with me. This principle (prioritizing single-joint muscles over two-joint muscles) has already been observed in the squat, and it seems to be a pretty generalizable principle, as it’s also been observed in the triceps: the two shorter heads that only cause elbow extension do the bulk of the work, with the long head (also a shoulder extensor) only kicking in as-needed.
This study lends even more support to the position that it’s not wise to purposefully squat in a way that increases hamstrings activation in the hole; in fact, it’s probably smart to purposefully minimize how much your hamstrings are doing as you start the ascent.
There are two basic ways to get your hamstrings more involved at the bottom of a squat:
- Focus on “sitting back” more.
- Purposefully lean farther forward to increase hip flexion.
When you do either of those things (which really accomplish the same basic purpose), you make the movement harder on your hip extensors without actually making it any easier on your quads. Although your knees don’t track as far forward (meaning lower external torque), the additional hamstrings contraction means the demands on the quads will still be pretty similar (this study gives strong theoretical support for that idea, and it’s already been shown to be the case for the sit-to-stand task, which is pretty similar biomechanically).
As I said in my last squat article, you should try to remain as upright as possible, and strive for what would look like a more quad-dominant squat: The quads should do what they can, and the hips should do what they must.
Squatting in that manner – not putting any additional, unneeded tension on the hamstrings – ensures that the quads and glutes can do as much of the work of squatting as possible. The hamstrings serve their proper role of coordinating the movement and kick in to add the additional hip extension torque the glutes can’t manage, without unnecessarily resisting knee extension any more than they have to. They’ll still do what they need to do, but you should try to squat in a manner that ensures they aren’t kicking in stronger (or sooner) than they need to. If they do, the movement will still be just as hard for your quads, even if your knees aren’t tracking as far forward, along with being more challenging for your hip extensors than it needs to be.
Your quads and glutes have a nice synergistic relationship. The stronger your quads are, the more of the load you can shift to them, making the movement easier on your hip extensors as a whole. Conversely (and more interestingly), the stronger your glutes are, the easier the movement becomes on your quads. Since your glutes essentially (not 100%. Remember, all models have drawbacks, but in this case, Model 2 seems to more accurately describe the situation) do everything they can before your hamstrings kick in, the more of your hip extension needs they can meet, the more you can lift before your hamstrings need to start contracting hard to meet your hip extension needs while simultaneously fighting against your quads at the knee.
Another fun implication: If you do have super strong hamstrings, the best way to get the most out of them in the squat is to get your quads stronger. The stronger your quads are, the better they can anchor the knee, and the stronger hamstrings contraction they can resist. This is essentially what your body is doing when you follow the “hips forward” cue I recommend for squats (discussed near the end of this article).
At the sticking point, if you can extend your hips ever-so-slightly while maintaining a constant knee flexion angle (your knees and hips drift forward, while your torso straightens up slightly), it’s easy to finish the lift. To make that happen, I purposefully engage my hamstrings to give me an extra hip extension boost, but to make that happen (especially since the knees are drifting forward, increasing the necessary knee extension torque), my quads have to be strong enough to keep my knees from flexing again.
That last bit may be a little confusing. After I just said not to emphasize the hamstrings in the lift, I specifically talked about using them to make it through the sticking point.
The key is timing.
At the bottom of the squat, knee extension demands are at their max. More hamstrings involvement makes it even harder to get moving out of the hole. If the demands on your quads exceed what they can manage, you shift more of the load to your posterior chain, winding up in a “good morning squat” position, which makes the rest of the movement way harder.
However, midway up at the sticking point of the lift, demands on the quads are considerably lower. That gives you more leeway to get your hamstrings more involved in the lift to produce the required hip extension torque necessary to make it through the sticking point. Your quads still need to be strong enough to resist that increased hamstrings contraction, along with your knees shifting forward as your hips extend, though.
Tying it all together
- Your glutes are what your body primarily relies upon to extend your hips in the squat, because a powerful glute contraction carries only benefits with no “costs,” whereas a powerful hamstrings contraction helps extend the hips while also resisting knee extension.
- The primary culprit for the infamous “good-morning squat” is weak quads. However, if you have strong quads but still find yourself in that position, the next place to look is your glutes. If you have weak glutes, your hamstrings have to do more to aid in hip extension, making the movement harder on your quads. The stronger your glutes are, the less your hamstrings have to kick in with any absolute load, making the movement easier overall.
- Don’t purposefully squat in a manner than increases hamstrings activation in the hole – you get the downside of the techniques that bias hamstrings involvement (greater hip extension demands) with no real upside (the movement is still just as hard for your quads due to increased hamstrings co-contraction).
- If you have strong hamstrings, the best way to get them more involved in the squat is to strengthen your quads, so that they can anchor the knee more strongly, allowing your hamstrings to help you through the sticking point by increasing hip extension torque using the “hips forward” cue.
A simpler way to express this idea for people who are mathematically inclined:
For every x amount your quads get stronger, your squat will likely increase by roughly x. Perhaps slightly greater because the quads can do a bit more of the work with any absolute load (meaning you’ll hit your failure point for hip extension a bit later).
For every x amount your glutes get stronger, your squat will likely increase by a value greater than x, because not only can you produce more total hip extension torque, but you’ll naturally be less reliant on your hamstrings (with any absolute load), meaning net knee extension torque also increases a bit (because the hamstrings are imposing a bit less knee flexion torque). Stronger glutes benefit you both at the hip and the knee.
For every x amount your hamstrings get stronger, your squat will likely increase by a value less than x, because the increase in maximal hip extension torque is offset to a degree by an increase in knee flexion torque, countering knee extension. Of course, the hamstrings are stronger hip extensors than they are knee flexors, so your squat will still go up, but of the three, focusing on your hamstrings for the purpose of improving your squat gives you the lowest return on investment.
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