Does muscle length affect regional hypertrophy?

October 23, 2024

At TGL, we post articles that we find interesting or helpful. Our sharing of an article does not constitute an endorsement of its content. This article by Stronger By Science originally appeared ​here.

Training at longer-muscle lengths has gained significant attention in recent years, largely due to studies suggesting that exercises emphasizing more extended joint angles may enhance muscle growth.

While the mechanism underlying the additional muscle growth remains unclear, nearly all comparisons of shorter- and longer-muscle length training have shown greater muscle growth from longer-muscle length training. However, one surprising aspect of these studies was that, usually, distal measurement sites would benefit disproportionately from longer-muscle length training.

Let’s back up, though. What is a distal measurement site? A distal measurement site refers to a point located farther from the center of the body or the attachment point of a muscle. In muscle growth studies, distal sites typically refer to the muscle regions closer to the ends of the limbs (e.g., near the elbow or knee) rather than the mid-belly or proximal regions (closer to the torso or point of muscle origin). For the triceps, a proximal measurement point would be closer to its origin - the shoulder - whereas a distal measurement point would be closer to the insertion site - the elbow.

Physique athletes and recreational lifters alike usually desire uniform, balanced muscle hypertrophy. So, if longer-muscle length training were to cause “uneven” muscle growth, this could be an additional consideration when incorporating longer-muscle length training. This study sought to determine whether muscle length actually induced regional muscle hypertrophy.

In our recently pre-printed meta-analysis, we searched for all studies that manipulated muscle length and measured at least a couple of measurement sites (i.e. proximal, mid-belly, distal).

We found 12 studies examining the effects of training at different muscle lengths on hypertrophy in various regions of muscles.

To briefly touch on methods: since we employed a Bayesian meta-analysis, we opted to employ a few different priors to reflect the prior beliefs of different authors. Likewise, we discussed and agreed on how large an effect needed to be to be deemed meaningful or worth caring about.

When data were analyzed this way, there was some evidence for a regional effect of muscle length, but the effect was very modest at best (i.e. trivial). Directionally, longer-muscle length training did slightly increase muscle growth at all sites, and the effect increased slightly when looking at more distal measurement sites. However, at all measurement sites, there was only a ~3-40% probability that the effect was large enough to be meaningful.

Importantly, though, the contrast in muscle length within existing studies was usually quite modest, making it difficult to gain a clear understanding of the relationship between muscle length and regional hypertrophy. On average, there was only a 21.8% difference in average muscle length trained between the shorter- and longer-muscle length groups in a given study. So, if the shorter-muscle length condition trained at an average muscle length of 40%, the longer-muscle length condition trained at an average muscle length of 61.8%. Very few studies used a larger contrast than this.

Let’s assume that the effect of muscle length on regional hypertrophy grows as contrast grows - assuming a linear relationship, for example. Since the average difference in muscle length between groups was relatively small in existing studies, the resulting effects were slated to be modest, by design.

However, it’s also possible that this modest difference reflects the typical real-world effects of muscle length on hypertrophy. For example, if someone did half squats (which typically correspond to about 90 degrees of knee flexion), and someone else squatted below parallel (which typically requires about 120 degrees of knee flexion), the average degree of knee flexion in those two exercises would be approximately 45 degrees of the half squats, versus 60 degrees for the below-parallel squats. Since muscle lengths scale with joint ranges of motion, the differences in average quad muscle length for that comparison would likely be around 15%. Larger contrasts than that would require even larger differences in ROM (for example, comparing full squats to quarter squats).

So, while longer-muscle length training likely causes at least slightly more muscle growth than shorter-muscle length training, to ascertain whether there could be a meaningful regional hypertrophy effect, future studies would ideally look at larger contrasts (i.e. bigger differences in muscle length trained).