I've asked a few questions on this site about, and have been doing some reading, on:

  • muscle fibre types (slow twich and fast twitch)
  • hypertrophy (myofibrillar and sarcoplasmic)
  • energy systems (ATP-PC, anaerobic and aerobic)

and I was hoping someone could tell me how the three interlink.

Are the following statements correct?

  1. As more muscle fibres are recruited (starting with slow-twitch and then progressing to fast-twitch), the energy systems used go from aerobic through to ATP-PC.

  2. Myofibrillar hypertrophy is an adaptation that allows for greater maximal strength output whereas sarcoplasmic hypertrophy is an adaptation for more prolonged exercise of the muscle.

Can the two different forms of hypertrophy occur in both muscle fibre types?

It would seem to me as though sarcoplasmic hypertrophy predominately occurs in slow-twitch and myofibrillar only occurs in fast twitch but I know that bodybuilders aim for sarcoplasmic in fast-twitch since slow-twitch cannot get very big.

  • I'm not well versed in this area, but statement #1 doesn't seem right. Aerobic exercise can involve fewer muscles (say, with cycling) or more muscles (say, with rowing), and I think the ATP-CP system can be used with fewer muscles (e.g. a one-arm push press) to many muscles (e.g. snatch). But I could definitely be wrong. Nov 7, 2013 at 20:18
  • @DaveLiepmann - You are correct. ATP-PC is basically used for high power, very short duration activities and relies on ATP already available in the muscle. This could be full body, like in a 100 yd sprint, or isolated as in a 3 rep maximum bench press. Once you pass the threshold, you move into the glycolitic (moderate duration/intensity) and down into oxidative (long duration, low intensity).
    – JohnP
    Nov 7, 2013 at 20:40
  • Am I correct in saying that when we use our fast-twitch muscle, we also recruit the slow-twitch - the fast-twitch acts as a "power-up" or bolster, if you will?
    – Dan
    Nov 7, 2013 at 20:45
  • 1
    MMmm....yes and no. You can really only have one type of fiber per motor unit. So, if you need a weak contraction, only type 1 fibers may be activated. The more intense, the more you recruit, but it's going to be a mixed bag, just like muscle composition. In more intense contractions, the faster fibers will get recruited first, but it's on the order of microseconds ahead of the other fibers.
    – JohnP
    Nov 8, 2013 at 16:31
  • so yes? sorry I don't quite understand your answer...
    – Dan
    Nov 8, 2013 at 17:14

1 Answer 1


They are both correct, however, the way they are stated is the source of confusion.

1) There is no correlation between the number of fibers and the energy system used. There is actually an exogenous explanation. The increased recruitment of muscle fibers occurs due to an increase in tension of the muscle. I.e. the force production is increased. When you are performing an explosive exercise, or moving a heavy weight, you need a high fiber recruitment. But what happens is that the fast twitch fibers are more selectively recruited, and they use the ATP-PC system.

Both fast twitch and slow twitch muscle fibers utilize all of the energy systems; the difference is that the fast twitch fibers get more of their energy from the ATP-PC system than from oxidation or glycolysis for example. And vice versa for the slow twitch.

2) Yes, both types of hypertrophy occur in both fiber types. But again there is a correlation between fiber type and type of hypertrophy. When exercising with high weights and low to medium reps, the ATP-PC system is primarily activated, and the muscle is stressed so as to induce myofibrillar hypertrophy. On the other hand, when doing high reps, the anaerobic, glycolytic, and oxidative pathways stimulate the muscle to undergo sarcoplasmic hypertrophy.

Even when performing low rep/high weights, the slow twitch that get activated are stimulated to undergo myofibrillar hypertrophy. But due to the fact that they contract weakly, and utilize a different energy system, the extent of their myofibrillar hypertrophy is lower than in fast twitch.

When discussing fast twitch fibers, it is interesting to note that they have a higher potential for sarcoplasmic hypertrophy than slow twitch have for myofibrillar hypertrophy. This is the explanation why performing cardio is detrimental to explosiveness; it induces a fiber type change, from fast twitch to slow twitch, which is in practice irreversible.

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