An understanding of the predominant fiber composition of skeletal muscle can prove useful when designing an exercise program particularly with regards to chosing the appropriate load/intensity, rest period, set/rep parameters, & time under tension to create the optimal training effect.
In simplified terms, certain muscles are predominantly designed for low force production but excellent endurance and are composed largely of type I fibers; some muscles are best for quick, explosive movement and are composed largely of type IIx or IIb fibers; and finally some for best at a combination of both abilities so that are predominantly composed of type IIa fibers. The predominance of fiber type within a muscle help to determine the muscles’ function. Ideally, a muscle should be trained in accordance with the muscle’s designed purpose; if this rule isn’t followed, the desired training effect will not be maximized.
The knowledge of a muscle’s fiber composition can go a long way in training a particular muscle or muscles with the appropriate loading, set/reps, time under tension, and rest intervals to create the desired training effect. For instance, the rectus abdominus (“six pack”) is composed approximately of 53.9% type II fibers.(1) Type II fibers respond best to heavy loads, fewer repetitions, short time under tension, and require longer rest periods for full recovery. Training such a muscle with floor crunches for sets of 25+ reps like is commonly prescribed simply isn’t fruitful as most of the fibers simply will not be utilized (see Henneman’s Size Principle) because the load and rep range doesn’t support the muscle’s function. In such a case, the training program would be training the rectus abdominis against its function and physiology. Doing crunches on the floor is another issue in itself, but we won’t go there today.
Looking at research done at the Human Performance Laboratory at the University of Eastern Illinois shows us the effects that rest period and load choice can have on the fatigue of muscles designed for different functions. The study, published in the March 2010 edition of the Journal of Strength and Conditioning Research, looked at how performing consecutive sets of various loads with a constant rest period of 60 seconds affected performance of the back squat, leg extension, and leg curl exercises.
While the rest period remained constant, load was manipulated in the following way across four workouts: constant load for all sets, 5% load reduction after each set, 10% load reduction after each set, and 15% load reduction after each set. The researchers recorded the trainees’ repetition performance. The findings indicate that for the back squat and hamstring curl a 15% reduction in load was required to complete all repetitions successfully with the fixed rest period. However, load reductions were not necessary for the leg extension exercise.(2)
So what idea discussed here may contribute to these results? The quadriceps, which would be needed to perform the leg extension exercise, are predominantly type IIa fiber rich so they have a good combination of both force production and endurance. This combination allows the muscle to recover at a faster rate than fibers that possess greater force production and less muscular endurance (type IIx). Evidence for this fact can be seen in speed skaters who have massive quadriceps muscles as a result of the demands of their sport. The quadriceps are thus built for endurance.
There is no secret that a demanding exercise such as the back squat would need a longer rest period to maintain repetition performance; the exercise demands much neurological impulse and requires the participation of damn near the entire kinetic chain. As a rule, multi-joint exercise require longer rest periods when compared to single joint movements for this reason.
So why the performance degradation for the single-joint hamstring curl exercise? The answer lies at least in part to the fiber composition of the hamstrings. They are type IIx dominant and require longer rest periods as a result to recover adequately to meet the repetition demands. Remember that type IIx fibers are fantastic at producing force but can only do so for a short time; they also need more time in between sets to maintain muscle performance. This need for more rest in between sets for such musculature is exactly what the results of this study indicate. The fixed rest period of 60 seconds simply isn’t ample enough to allow the type IIx fibers to properly recover which leads to sub-optimal exercise performance. The hamstrings are a strength muscle and lack the endurance of the quadriceps. Take a look at the hamstring development of elite sprinters for confirmation who sprint for less than 20 seconds an sometimes less than 10 seconds.
The jist of this blog is that understanding the function and fiber composition of skeletal muscle can prove fruitful when designing science-based exercise programs for yourself or others. Exercises that require contribution from the hamstrings require longer rests in between sets to maintain performance and those that call for participation from the quadriceps require less. Thoughtful manipulation of set/reps, rest period, time under tension, & load choice are at the heart of the being an exercise professional or knowledgeable trainee. Theory is great but application is a must. Train smart!
Resources
(1) http://physiotherapy.curtin.edu.au/resources/educational-resources/exphys/97/fibretypedist.cfm#distrib
(2) http://www.ncbi.nlm.nih.gov/pubmed/19816212
