Satellite cell responses

Satellite cell responses to exercise are sometimes referenced as proxy markers for the amount of hypertrophy that is stimulated by a strength training workout. Even so, there are good reasons to argue that the satellite cell response to exercise is totally unrelated to muscle growth, as I explain in this article.

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#1. Satellite cell responses to exercise are stimulated by calcium ions

First of all, it is important to note that the stimulus that activates satellite cells is the release of calcium ions inside the cytoplasm of the muscle fiber. This is most likely because the primary role of satellite cells is to facilitate muscle damage repair processes. Hence, whenever the calcium ion influx into the cytoplasm is suppressed, this prevents the activation of the satellite cells.

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This is important, because muscle fibers can be strongly activated (leading to a very high degree of calcium ion influx into the cytoplasm) without any meaningful hypertrophy occurring. Therefore, we might expect that satellite cell activation can also occur without this then leading to much muscle growth happening.

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#2. Satellite cell responses occur similarly after exercise that causes hypertrophy and after exercise that does not cause hypertrophy

After a bout of strength training exercise that involves multiple sets to failure, we might reasonably assume that there is a signaling cascade that is responsible for stimulating muscle growth. In contrast, after a bout of exercise that involves lifting a light load and stopping a long way from muscular failure, we might reasonably assume that there is no such cascade, and that hypertrophy is not stimulated. Interestingly, the satellite cell responses after both of these types of exercise is basically identical, suggesting that the satellite cells play no role in limiting muscle growth after individual workouts.

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Indeed, satellite cell responses after strength training programs and after aerobic exercise programs are very similar, despite the fact that only the strength training programs produce hypertrophy.

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It seems most likely that the role of satellite cells after both strength training workouts and aerobic exercise workouts is to repair muscle damage, as indicated by the activation of the most satellite cells around those (hybrid) muscle fibers that display the most signs of muscle damage (centrally-located nuclei). This makes sense if the initial signal for satellite cell activation is the influx of calcium ions into the muscle fiber cytoplasm, because it is calcium ion accumulation inside the muscle fiber that then produces the majority of the muscle damage.

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#3. Satellite cells are NOT required for myofibrillar addition

Importantly, animal models have shown that satellite cells themselves are not actually required for myofibrillar addition to occur, but they are required for muscle regeneration after damage.

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They might also be required for sarcomerogenesis, although not all studies have reported this finding.

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Conclusions

Satellite cell responses to exercise are sometimes argued to indicate the potential for the type of exercise in question to stimulate hypertrophy. Indeed, this assumption is a key argument put forward by researchers who argue that more muscle-damaging types of exercise (like eccentric contractions) are more stimulating for hypertrophy than less muscle-damaging types of exercise (like concentric contractions) because more muscle-damaging exercise typically produces greater satellite cell responses than less muscle-damaging exercise. Yet, the satellite cell responses to a workout do not indicate the hypertrophic potential of that workout for three key reasons. Firstly, satellite cells are activated by the influx of calcium ions into the muscle fiber (we known this because suppressing calcium ion influx prevents satellite cell responses) and we can achieve high levels of calcium ions inside muscle fibers without stimulating hypertrophy. Secondly, satellite cell responses to exercise can be similar irrespective of hypertrophy stimulus that is produced. Workouts involving training with light loads to failure (which causes hypertrophy) and with light loads very far from failure (which does not cause hypertrophy) nevertheless cause similar satellite cell responses. Moreover, both aerobic exercise workouts and strength training workouts cause similar satellite cell responses despite one being famous for producing hypertrophy and the other being much less effective. Thirdly, satellite cells are not actually necessary for myofibrillar addition to happen, whereas they seem to be much more important for muscle damage repair and regeneration (and perhaps also sarcomerogenesis).