There is a popular notion that your muscle strength and function needs to be “balanced” — symmetrical. This is built on the assumption that muscle strength gets out of alignment, like car wheels. Many therapists, coaches, and trainers go to considerable lengths to diagnose and treat these imbalances. It harmonizes nicely with other popular concepts, such as the importance of good posture, core strength, and muscles that are supposedly not “activating” correctly.
Muscle “balance” is one of the most common justifications for prescribing strength training to people in rehab for muscle strains (tears), especially athletes. Strains of the hamstrings are a menace in sport, and notoriously prone to re-ripping. Even a little bit of statistically meaningful prevention of injury and re-injury would be valuable.
Seems like that’s a possibility that is probably worth studying scientific-like.
In the rest of the post, I’ll look into “H:Q” ratios, the balance of strength between the hamstrings and the quads: a bunch of evidence that says it doesn’t matter, one good recent science experiment that says it does, plus an interesting detail about fatigue. I will finish with some recommendations: should you bother tinkering with your H:Q ratio? Do you need to “balance” your thigh strength?
But only for members! Most posts are still free, but this one is a members-only post: unlock for three or four more minutes of reading. Members also get access to all past premium posts, and a growing list of exclusive content scattered around PainScience.com. Just yesterday I added a big new members-only area to the stretching article, for a total of 11 large and 3 small member zones around the site. Plus 8 large audio articles. Read more about membership.
Front-to-back symmetry: “muscle balance” between the quads and hammies
The claim: the strength of your quadriceps and hamstrings has to be just-so and matched, or else you are much more likely to rip a ‘string. The “hamstring-to-quadriceps (H:Q) strength ratio” is frequently and eagerly measured in the elite sporting world.
And it sounds great. So technical! If you want to impress athletes with your kinesiology savvy, you cannot do much better than talking about their H:Q ratio.
But three solid citations say you’re blowing smoke. A poor H:Q ratio is not a well-established predictor of injury. In fact, it’s well-established that it’s not a risk factor. van Dyk 2016 (and then again in 2017 and Green 2018 reviewed studies of basically every kind of strength ratio that has ever been tested, and all three papers are muscle balance party poopers. From Green et al:
Isokinetic strength asymmetries or imbalances were not associated with future risk of hamstring injury. Similarly, there was no significant relationship with any strength ratio tested, despite previous reports and isolated study findings. This is consistent with recently published results, including risk examined against specified strength ratio cut-offs, showing a lack of predictive validity for detecting risk of hamstring muscle injury.
But the studies do point in both directions. Don’t they always?
A good science experiment with contrarian results
The three papers I just cited are fine papers in lovely journals, including both the British and American Journal of Sports Medicine and all three are (very) wet blankets on muscle balancing. But another good study, Lee et al. in Journal of Science & Medicine in Sport (not quite the same league, but still a good journal), lands on exactly the opposite conclusion.
This is just one study, mind, and not a review of many studies like the previously cited papers. But it is worthy of consideration because it is a good study, a sizeable prospective trial — that is, they measured H:Q in athletes before they were injured, and then tracked them through a season. This is the right way to determine causality.
Professional football players with significant lower isokinetic hamstring strength, lower hamstring-to-quadriceps strength ratio… were linked to an increased risk of acute hamstring strain injury.
And not just a little bit increased either: according to this data, you are three times more likely to tear a hamstring with a low H:Q ratio, and five times more likely if you are a skookum ratio but are just weak.
And it is still just one study, at odds with excellent reviews of lots of other studies — which the authors discuss. They have their reasons for thinking that their results might be better than anyone else’s, and they aren't crazy. But they could still be wrong, their results could still be a bit of an illusion (as so many results are). This stuff is complicated and so the results need to be replicated.
The role of fatigue
Lee et al. delivers an eyebrow-raising result that emphasizes that we probably still have more to learn about this. For example: what if we factored in fatigue? This is quite interesting…
The H:Q ratio is usually measured in a non-fatigued state, which might not be the best time to check. A 2018 experiment by Pinto et al. showed that the ratio may get worse when we’re tired: “H:Q ratio measurement during a fatiguing test provides different outcomes to the traditional H:Q ratio.”
Interesting and sensible: maybe those ratios-while-fatigued are better indicators of injury danger than conventional ratios. But no one has checked that yet.
What should you do if you are accused of having unbalanced muscles?
My opinion is that tinkering with your H:Q ratio is probably just another supposedly “advanced” method of injury prevention that hasn’t really panned out — a good example of the “correct exercise trap,” a rehab exercise that can’t actually fix a problem that doesn’t exist to begin with. It’s probably a great example of the “The Corrective Exercise Trap”.
But that result from Lee et al. tames my cynicism a bit. Especially the part where a low ratio was actually much less of a risk factor than just straight up weakness. Having weak thighs might be an exception to the general rule that these biomechanical metrics are mostly bogus.
So don’t sweat “balance.” To prevent muscle strain and re-strain, I think you can mostly just focus on being strong, period. There’s an excellent chance the ratio doesn’t matter nearly as much as the overall strength and fitness of the muscles, and in any case it’s not practical for most people to either measure or optimize their H:Q.
We don’t need the H:Q ratio or any other muscle balance concept to justify strengthening. It’s not like you are going to regret being stronger!
If you’ve already torn any thigh muscles, front or back, the injured side is probably going to be weaker, and you’ll have no way to know what your pre-injury ratio was like anyway. So cautiously train the injured side until it can roughly match the uninjured side… and then work on making both sides stronger.