When assessing the extent to which a particular muscle contributes to a particular exercise, load is an important variable that is often under-discussed. For example, the bench press clearly trains both your pecs and your triceps. However, if you’re benching 80% of your 1RM, that doesn’t necessarily mean your pecs and triceps are both producing force at 80% of their maximum capacities. For male lifters, it appears that pec usage ramps up in the bench press before triceps usage – in other words, when benching with 80% of 1RM, the pecs may be exerting 90% of their maximal force, while the triceps are only exerting 70% of their maximal force. So, as loads increase from 80% to 100% of 1RM, pec usage only increases by a little bit, while triceps usage increases considerably. The opposite pattern has been observed in female lifters – triceps usage may be prioritized, with pec usage ramping up as female lifters approach maximal loads.
I’ve previously argued that the same general pattern is observed in the squat: quad usage is prioritized, with hip extensor usage increasing as lifters approach maximal loads. However, the direct evidence for this contention was limited. There was an EMG study finding that vastus medialis and lateralis EMG barely changed with increasing loads from 50% to 90% of 1RM (2), and there was a 2012 study by Bryanton and colleagues (3) quantifying relative muscular effort in the squat with loads from 50-90% of 1RM in experienced female squatters (it’s not worth explaining the “relative muscular effort” metric in-depth here. Just know that, within a particular population, relative muscular effort will scale 1:1 with observed joint moments at a particular joint angle). Bryanton and colleagues found that barbell load had a large effect on relative muscular effort of the hip extensors, but virtually no effect on relative muscular effort of the knee extensors. Thus, their data suggest that quad usage in the squat is near-maximal at pretty low loads, with hip extensor usage picking up the slack as loads increase. You can see these results in Figures 1 and 2.
A recent study by Larsen and colleagues (1) confirms and extends Bryanton’s findings. 12 healthy, experienced male lifters completed the present study. To be included, subjects needed to squat at least 150% of their body mass (the average squat 1RM was 149kg, at a body mass of 83.5kg). Subjects completed single reps at 90% and 100% of their squat 1RM, and attempted a rep at 102% of their 1RM. For our purposes here, the most important outcomes were measurements of knee and hip extension moments. Joint moments were assessed via force plate data, and data concerning joint positioning from a 3D motion capture system. Knee and hip joint moments were assessed at four points in the squat: the start of the concentric phase (v0), the point of maximal velocity before the sticking region (Vmax1), the point of peak deceleration during the sticking region (dmax1), and the point of minimum velocity at the end of the sticking region (vmin). You can see these points of interest illustrated in Figure 3.
Overall, this study confirms and extends Bryanton’s findings. Bryanton’s study used female lifters, while the present study used male lifters. Bryanton’s study assessed loads from 50-90% of 1RM, while the present study assessed loads from 90-102% of 1RM. Thus, this pair of studies provides pretty strong evidence to suggest that relative muscular contributions to the squat are strongly impacted by loading. With lower loads, the squat is a much more quad-dominant exercise, with contributions of the hip extensors increasing as loads increase. I strongly suspect that this pattern is also present when considering proximity to failure – reps that are further from failure are more reliant on the quads, whereas the hip extensors contribute more and more as a set approaches failure – but there’s less direct evidence to support this suspicion.
I think that this research helps explain why we commonly observe robust quad growth in squat studies where lifters train quite far from failure. If we assume that muscular effort for all prime movers increases basically linearly as loads increase or as proximity to failure increases, we’d assume that training with a particular number of reps in reserve should have similar effects for a particular muscle, regardless of the exercise being performed. However, if that isn’t a correct assumption (and I contend that it isn’t), you may be able to effectively train a particular muscle with more reps in reserve for some exercises, but need to push closer to failure when performing other exercises. In concrete terms, your quads seem to get an adequate stimulus from squats performed further from failure, but you may need to train closer to failure when performing single-joint exercises (like knee extensions; 4).
Note: This article was published in partnership with MASS Research Review. Full versions of Research Spotlight breakdowns are originally published in MASS Research Review. Subscribe to MASS to get a monthly publication with breakdowns of recent exercise and nutrition studies.
- Larsen S, Kristiansen E, Nygaard Falch H, Estifanos Haugen M, Fimland MS, van den Tillaar R. Effects of barbell load on kinematics, kinetics, and myoelectric activity in back squats. Sports Biomech. 2022 Jun 10:1-15. doi: 10.1080/14763141.2022.2085164. Epub ahead of print. PMID: 35686617.
- van den Tillaar R, Andersen V, Saeterbakken AH. Comparison of muscle activation and kinematics during free-weight back squats with different loads. PLoS One. 2019 May 16;14(5):e0217044. doi: 10.1371/journal.pone.0217044. PMID: 31095625; PMCID: PMC6521994.
- Bryanton MA, Kennedy MD, Carey JP, Chiu LZ. Effect of squat depth and barbell load on relative muscular effort in squatting. J Strength Cond Res. 2012 Oct;26(10):2820-8. doi: 10.1519/JSC.0b013e31826791a7. PMID: 22797000.
- Goto K, Ishii N, Kizuka T, Takamatsu K. The impact of metabolic stress on hormonal responses and muscular adaptations. Med Sci Sports Exerc. 2005 Jun;37(6):955-63. PMID: 15947720.