Many exercise enthusiasts are interested in the nuances of fiber types and their responses to exercise. It is commonly thought that the large, powerful, and rapidly fatiguing fast twitch fibers work to build size and strength, while the smaller, fatigue-resistant slow twitch fibers contribute most during cardio and muscular endurance training. A recent study in the Journal of Strength and Conditioning Research answered some questions about the role of fiber type, and also raised a few more.


Because we all have various goals and programs, it’s important to know how these muscle fibers influence the composition of our muscles. To find out more about this interaction, the participants in this study performed six sets of fifteen reps of maximal eccentric exercise for the quads every day for six days. Yes, it was a brutal workload. However, for those who participate in sports or exercise daily, the frequency may not be so foreign as it seems. The results of the study, however, might be a little surprising.


Despite the brutal six-day schedule, most recovery markers showed the athletes adjusted in a normal amount of time. DOMS, for example, had a fairly standard curve. It got steadily worse over the first couple days, hitting a peak on day three (which was the second day after the first exercise, when many people normally experience the strongest DOMS), and then steadily declined each day. This is what typically happens when you subject the body to one day of ninety reps of intense eccentric exercise. However, in this study the athletes did this protocol every day, and yet recovery seemed unhindered.

Breaking Muscle Shop


It wasn’t just DOMS that recovered normally. Peak torque recovered quickly as well. It was at its lowest on day three and had nearly returned to normal by day six. Clearly the participants adjusted quickly to this extreme method of exercise.


Something that did not recover, however, were the blood markers for muscle damage. Namely, the researchers measured creatine kinase (CK) and lactate dehydrogenise (LDH), which both remained elevated beyond day six. Since the other features of exercise recovered, however, I’m curious what would have happened if a second group had done the same protocol but only on the first day, while taking the same tests.


But the meat and potatoes of the study was the analysis of how muscle fiber types responded to exercise. There were two major take home points. First, fast twitch fibers experienced measurable muscle damage before slow twitch fibers. The fast twitch fibers were damaged on day three when biopsied, but the slow twitch weren’t until the seventh day, just after the end of the testing.


Second, muscles with slow fiber dominance were more susceptible to negative results from the long bouts of eccentric exercise. They lost strength faster and recovered slower. You may be wondering how this could be, since it is often thought that slow twitch fibers are more resistant to damage. The researchers didn’t ruminate long on this topic, but I suspect the primary reason was because the fast twitch fibers shouldered most of the burden with this type of exercise. A slow twitch dominant muscle simply doesn’t have as many of these fibers, so the burden is relatively higher.


The way the researchers applied this new knowledge seemed a bit odd to me. They recommended doing a lot of eccentric work multiple days in a row, but for only three days if you’re slow twitch dominant. I think more research is needed for exercise prescription, so take these results for what they are and make your own conclusions.



1. Dóra Ureczky, et. al., “The Effects of Short-Term Exercise Training on Peak-Torque Are Time- and Fiber-Type Dependent,” Journal of Strength and Conditioning Research, DOI: 10.1519/JSC.0000000000000414


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