Work Capacity Could Be Holding Back Your Gains


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TRAINING TAKEAWAY: Poor capillary density, which influences the capacity to deliver blood and thus oxygen, nutrients, and growth factors to muscles, could be the bottleneck to progress in some lifters. Thus, aerobic conditioning and/or high repetition resistance training for the target muscles could be warranted to promote future hypertrophy, especially if aerobic fitness is poor.


Background: Periodization and Phase Potentiation

The study I’ll discuss specifically looked at the efficacy of aerobic training to promote future hypertrophy, but quickly discussing the state of the periodization literature will ensure we’re all on the same page.

Periodization can simply be defined as the long-term manipulation of training variables. We’re generally fans of periodization for long-term strength development, but we’ve also acknowledged that it may not be completely necessary. While meta-analytic findings indicate a benefit to periodization, our interpretation is that this is largely driven by higher peak loading (i.e., the highest % of 1RM within a training week) closer to the strength test in periodized training groups.

Also for long-term strength development, our viewpoint is that maximizing hypertrophy of the primary musculature used in the desired movement is vital. Since training studies are relatively short (rarely >16 weeks), it’s also worthwhile to dive into the potential effects of periodization on hypertrophy. The research up to this point has not indicated a clear benefit to periodization for hypertrophy. However, hypertrophy is typically a secondary outcome in study designs focused on strength and thus do not represent how practitioners may periodize training to maximize hypertrophy.

For this reason, I think there’s a decent possibility that there’s a benefit to periodization for hypertrophy - it’ll just have to be the “right” type of periodization for this specific training outcome. At the risk of being overly technical, I don’t think we care about periodization per se, but instead about phase potentiation. Periodization is a very broad term, and even the most anti-periodization coach will include long term manipulation in training variables, even if it’s just for the sake of reducing monotony or exploring other training approaches.

Phase potentiation proposes that there is inherent benefit to the sequence of training periods. For example, the results from block X are improved because it was preceded by block Y. That’s why I was excited to read this new study that investigated whether hypertrophy from 10 weeks of resistance training would be greater if it was preceded by a 6-week aerobic conditioning phase.

Before we discuss the study design and results, it’s important to mention the proposed mechanism of why aerobic training is a specific candidate for phase potentiation. The proposed mechanism lies at the intersection of satellite cells, capillary density, and recovery capacity. Satellite cells are muscle stem cells that become activated and then proliferate in response to a stimulus. Some of these satellite cells will donate myonuclei, which increases the capacity of the biochemical machinery necessary for generating additional muscle proteins, ultimately aiding hypertrophy. Interestingly, satellite cells are in close proximity to muscle capillaries, and some evidence suggests that capillary content may be a regulator of the activation and proliferation of satellite cells in response to a stimulus. If capillary density is higher, more satellite cells will be in close proximity to capillaries and thus will be exposed to a higher concentration of signaling factors in the blood. In addition to increasing the signal for hypertrophy, this could theoretically improve within- and between-session recovery time via improved oxygen delivery and waste removal. Since aerobic training increases capillary content, it may promote future hypertrophy. We just got a bit in the weeds, so here’s a graphical representation of the idea here:

Study Overview

To test this hypothesis, the researchers included 14 untrained men and women. We’ll come back to this, but it’s important to note that their aerobic fitness was rather poor (peak VO2 in the ~35th percentile). A within-subjects design was used, meaning each subject had one leg in one condition and the other leg in the other condition. This approach provides more precision and statistical power in a sample of only 14 individuals as factors including genetics, nutrition, and sleep are theoretically accounted for. Here is a graphical representation of the two conditions, focusing on the design elements most relevant to our discussion:

The cycling training was performed only on the AC+RT legs 3 times per week for 45 minutes. The specifics of the protocol are less important than the fact that it successfully increased capillary-to-fiber perimeter exchange index (a measure of capillary density) by ~33% in the AC+RT legs.

After the 6 weeks of cycling training in the AC+RT legs, there was not a meaningful difference between conditions in muscle fiber size. While I was surprised by this as aerobic training often leads to hypertrophy in untrained individuals, this does make for a clear comparison in hypertrophy between the AC+RT and RT Only conditions from the 10 weeks of resistance training.

The resistance training was 3 times per week, and the training relevant for quadriceps growth were 3 sets of each squats, leg press, and leg extension in each session. The repetition range was 10-12, and the authors reported that the last set was taken to failure on each exercise. Rest periods were not reported. If they were especially short, that would change my interpretation a bit. I’d imagine that would have been made clear if that was the case, so I’ll assume it wasn’t.

The AC+RT condition increased muscle fiber size by 29.8 ± 47.1% compared to 8.1 ± 19.24% in the RT Only condition. However, this difference was not quite statistically significant. The authors also performed an analysis using all 28 legs from both training conditions, and this indicated a significant correlation between capillary density and change in type II fiber size (r = 0.35; p < 0.05, note: we were not able to replicate this analysis - we got r = 0.31; p = 0.11). This relationship was not quite significant for type I fiber size (r = 0.37; p = 0.055 from our calculations). Lastly, the authors performed an analysis by “grouping” the 10 legs that had the greatest capillary density prior to resistance training and the 10 legs that had the lowest capillary density and comparing hypertrophy. The high capillary density “group” saw significantly greater hypertrophy (e.g., type II fibers: +54.6 ± 57.2% vs. +9.4 ± 27.4%):

Limitations & Considerations

While the high vs. low capillary density “group” analysis above is a bit jaw-dropping, there are a few methodological considerations when interpreting this analysis. Most importantly is that some of the legs in each “group” were from the same individual. This may have inflated the effect as a couple especially high or low responding individuals were essentially included twice in each “group.” For this reason, I would encourage the focus to be on the correlation analyses I mentioned. In these analyses, the correlation coefficients were just under the threshold for a moderate relationship. 

Beyond the statistics, I noted multiple benefits to the within-subjects study design used. However, there are drawbacks as well. Specifically, there are some mechanisms that may lead to a non-local effect in capillary density. In this study, the RT Only leg may have experienced some capillary density improvements while the other leg was undergoing AC. Unfortunately, we can’t know this for sure because we only have pre-AC data from the AC+RT leg. This has practical implications - if looking to increase capillary density of the biceps, there may be some benefit from cycling. However, the safest bet is likely to target the desired muscles, such as high repetition curls in this case.

Overall, a lot of questions remain unanswered. As noted, the aerobic fitness of these subjects was rather poor, and it may very well be that as long as aerobic fitness (of which capillary density is a part) is decent, there’s no benefit to improving capillary density. On the other hand, it could be that aerobic fitness is more consequential as training status increases as additional muscle mass requires additional oxygen demands during training.

Summary

This study demonstrates that greater capillary density at the onset of a resistance training program may improve hypertrophy. However, it appears that the change in capillary density in the time period prior to the onset of a training program is less consequential.

The r-value for the analysis I’m suggesting the focus to be on was ~0.35, so the effect here isn’t crazy; only ~14% of the variance in hypertrophy was explained by variance in capillary density. Nonetheless, training to improve capillary density may be a good troubleshooting tactic in the face of training plateaus.

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