Hypertrophy of Slow Twitch Muscle Fibers
A 2014 review paper by Ogborn and Schoenfeld does a great job of summarizing what is known at this point about training slow twitch muscles for hypertrophy. Here are some of the key issues:
• Type II (fast twitch) fibers display superior growth following high intensity (heavy) loading, approximately 50% greater than type I (slow twitch).
• If loading is greater than 50% of 1 RM, type II muscle growth exceeds type I growth
• Direct, head to head comparisons of 6-8 RM VS 20-30 RM demonstrated that 6-8 RM is superior for hypertrophy and that all fiber types showed some hypertrophy.
• Training at slow repetition speeds improved the hypertrophy response of light loads, but heavy loading was still superior.
• Aiming for the SINGLE protocol that maximizes hypertrophy may not be optimal. For example, in one study comparing 3 sets with 30% 1 RM, 80% 1 RM, and 1 set 80% 1 RM, hypertrophy overall was greater using 3 sets of 80% 1 RM, but type I muscle fiber hypertrophy was greater with the 30% 1RM protocol. Perhaps the optimal approach is a combination of heavy loading for low reps and lighter loading for high reps (and slower contraction speed) to maximize hypertrophy of both type I and type II fibers.
• Although recruitment of type II fibers increases with increasing load, type II fibers are also recruited in the latter repetitions of a set due to fatigue. This response is maximized by training to failure. There is some evidence that lower load, higher repetition training can produce hypertrophy responses similar to high loading protocols if the sets are taken to failure.
• If pure hypertrophy is the goal, it makes sense to train across a range of repetitions. High repetition, lower loading sets should be taken to failure.
• High load, low rep training maximizes type II muscle hypertrophy.
• Lower load, higher rep training (especially taken to failure) maximizes type I muscle hypertrophy.
• If the primary goal is strength, higher loads and lower repetitions are more optimal because higher loading leads to greater strength than lighter loading, even if the hypertrophy response is the same.
From a practical perspective this research gives credence to the old bodybuilder technique of including lighter “burn out” sets at the completion of training each body part, and methods like drop sets or high time under tension sets using slower performed higher reps. This would ensure that type I muscle fibers experience a maximum hypertrophy stimulus as well.
Ogborn D, and Shoenfeld BJ, the role of fiber types in muscle hypertrophy: Implications for loading strategies, Strength and Conditioning Journal, Volume 36, Number 2, April 2014
Mitchell CJ, Churchward-Venne TA, West DWD, Burd NA, Breen L, Baker SK, and Phillips SM. Resistance exercise load does not determine training-mediated hypertrophic gains in young men. J Appl Physiol 1134: 71-77, 2012.