Roger Enoka has been an interesting author and researcher for a number of years. He did an analysis of the pull in Olympic lifting that is a classic paper and also published an excellent textbook with Human Kinetics. He has a review article on fatigue in the February issue of the Journal of Biomechanics that is a summary of a lecture he gave in 2011. This article really challenges a lot of things that we thing we know about the nervous system and exercise.
This paper starts with the premise that we have not made advances on our understanding of fatigue during activities of daily living, this is because there are several outdated concepts that we cannot let go of:
1. Enoka argues against the existence of fast and slow-twitch muscle fibers. He points out that the studies originally establishing the existence of this were not looking at twitch contraction times but were looking at fatigability. According to Enoka, there don’t appear to be differences in contraction times. Another interesting point the author makes is that it appears that those fatigue-resistance motor units actually fatigue. It’s his view that we need to abandon the idea of motor unit types.
2. Enoka also argues that EMG is a poor measure of fatigue. This is because force production can change independent of any changes to EMG. The author points out that neither the amount of muscle activation nor the level of neural drive can be reliably estimated from EMG amplitude during fatiguing contractions.
3. According to the author, fatigue cannot be reduced to a handful of physiological or biomechanical measures and must include a more global perspective including psychological perceptions of fatigue as well as physiological/biomechanical ones.
For me, the first point in particular (slow vs. fast-twitch muscle fibers) is really interesting. This is the first time that I have encountered this idea. The original paper establishing slow, fast fatigable, and fast fatigue resistant motor units was based on looking at cat gastrocnemius muscle fibers in vitro. In a politically correct way, Enoka is essentially arguing that some of the results contradicted the conclusions and that successive researches and practitioners may have made some stretches on their application and interpretations of this paper.
Enoka, R.M. (2012). Muscle fatigue – from motor units to clinical symptoms. Journal of Biomechanics, 45: 427-433.
