Today, I'm excited to share insights from our recent pre-printed study on leg extension technique. While I’ve discussed the idea of training at longer-muscle lengths for muscle hypertrophy in a previous video, this latest study has some exciting results!
A recently pre-printed study by Larsen et al (2024) looked at the effects of different hip angles on quadriceps muscle growth. Using a within-participant design, 22 untrained participants performed leg extensions for 10 weeks with either 40 or 90° of hip flexion.
Why did the researchers manipulate hip angle? Well, the three vastii muscles are monoarticular, only crossing the knee joint. Therefore, hip angle shouldn’t impact the effectiveness of the leg extension. However, the fourth head of the quadriceps, the rectus femoris, is both a knee extensor and hip flexor. Therefore, when the hips are more flexed (90°), the rectus femoris is more shortened, and when the hips are more extended (40°), the rectus femoris is more stretched.
The researchers measured increases in muscle thickness of the vastus lateralis and rectus femoris muscles at both a proximal (closer to the hip) and distal (closer to the knee) site.
As the researchers hypothesized, vastus lateralis hypertrophy was similar. Rectus femoris hypertrophy was substantially larger (~50-170% more growth) with a 40° hip angle. The superiority of 40° of hip flexion was larger at the proximal site than the distal site. Interestingly, this is in contrast with most other research finding a benefit to longer-muscle length training for muscle hypertrophy. The mechanisms underlying the additional hypertrophy remain unclear.
Interestingly, this is in line with previous findings by Mitsuya and colleagues, who used magnetic resonance imaging to compare the impact of 40° vs. 80° hip flexion angle on distal, middle, and proximal transverse relaxation time (a proxy of muscle activation) of the rectus femoris during the leg extension exercise. They observed that a hip flexion angle of 40° led to significantly more muscle activation in the proximal and mid-belly rectus femoris compared to 80° of hip flexion. Some research now suggests that magnetic resonance imaging may predict hypertrophy, which is exciting.
Another interesting parallel can be drawn between this study by Larsen et al and a previous study by Maeo et al (2022). In this study, the researchers compared the effects of the seated vs lying leg curl on hamstring hypertrophy. Since three heads of the hamstrings are hip extensors, they are more lengthened during the seated leg curl, which flexes the hip to a greater extent than the lying leg curl. Conversely, the sartorius is a knee flexor, but also a hip flexor, meaning it is more lengthened during the lying leg curl than the seated leg curl. Just as you’d expect, the seated leg curl resulted in greater hypertrophy of the heads of the hamstrings that are hip extensors, and the lying leg curl led to more hypertrophy of the sartorius. Every muscle that was trained at longer-muscle lengths grew more, much like in our own study.
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 degree of stretch is one of the main things we should be looking for in a good exercise 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.
As far as your training goes, here’s how to apply these findings...
Takeaways
- Doing leg extensions with more extended hips substantially increases rectus femoris hypertrophy, without compromising vastus lateralis hypertrophy.
- A growing body of evidence suggests training at longer-muscle lengths may increase muscle hypertrophy.
- For powerlifters with weight class restrictions - and for whom vastus lateralis hypertrophy may be more desirable than rectus femoris hypertrophy - doing leg extensions with greater hip flexion may be preferable.
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That's it for today's newsletter. Remember to subscribe to the SBS YouTube for a new video this Friday. Then, we'll be back with a new podcast episode next week, and another edition of this newsletter in two weeks.
Have a great rest of your week.
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The SBS Team
Greg, Pak, Milo, and Lyndsey
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