170% More Muscle Growth by Focusing On The Stretch?
Take-aways
These results suggest that extending the hips during the leg extension increases rectus femoris, but not vastus lateralis hypertrophy.
If you’re doing leg extensions, and you want overall quadriceps growth, extend your hips or use a machine that allows you to do so.
Exercises like the reverse nordic curl or sissy squat may further increase quadriceps hypertrophy (particularly rectus femoris) compared to the leg extension.
HOT off the presses, we recently pre-printed a study on longer-muscle length training and muscle growth. Let me break it down for you.
In this study, participants trained using single-leg leg extensions. Since the rectus femoris is a hip flexor, we sought to examine whether lengthening it during the leg extension would increase its hypertrophy. We hypothesized that while the rectus femoris would see more muscle growth with the hips extended, vastus lateralis growth would be similar, as hip position doesn’t impact its muscle length.
With one leg, leg extensions were performed at 90 degrees of hip flexion, shortening the rectus femoris. With the opposite limb, leg extensions were performed at 40 degrees of hip flexion, lengthening the rectus femoris. Twenty-two untrained participants trained the leg extension twice a week for 10 weeks.
The results? We observed quite a bit more rectus femoris growth when it was trained at longer-muscle lengths, whereas vastus lateralis growth was very similar, as expected.
In relative terms, we saw around 50% more growth of the rectus femoris proximally and 170% more growth of the rectus femoris distally when training at longer-muscle lengths. In Bayesian terms, there was “extreme” evidence for the superiority of training at longer-muscle lengths for the rectus femoris.
The difference in rectus femoris hypertrophy is noteworthy, as it represents ~50-170% more growth, which is likely to be a meaningful difference to many lifters IMO. The mechanisms underlying the additional hypertrophy remain unclear, but are briefly discussed in the full-text. Interestingly, this is one of the first studies that found a large benefit in favour of longer-muscle length training at proximal measurement sites.
Since we observed similar hypertrophy of the vastus lateralis, for powerlifters limited by weight-class restrictions, where rectus femoris hypertrophy is less desirable than vastii muscle hypertrophy, doing leg extensions with greater hip flexion could be helpful.
Why?
There’s a whole host of research finding that longer-muscle length training leads to more muscle growth than shorter-muscle length training, all else being equal. This has a few interesting consequences. First, using lengthened partials - just doing the stretched half of a lift - could produce more hypertrophy than doing a full range of motion. Second, exercises that lengthen a given muscle more (e.g. a reverse nordic curl vs a leg extension) probably lead to more muscle growth. While we have few studies that compare exercises directly, the bulk of the research suggests that the stretch is one of the main things we should be looking for in good exercise selection to build muscle. Therefore, I personally suspect the reverse nordic curl and sissy squat are better exercises than the leg extension, particularly for the rectus femoris. In the leg extension, you’re typically missing out on 40-50 degrees of stretch at the bottom, in terms of knee flexion ROM, which might reduce muscle hypertrophy.
