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Should You Change Exercise Design Based on Muscle Fiber Type?


leg muscles in a row machine

This study directly investigated whether a benefit exists to training muscles based on their fiber type. Participants were 30 healthy male volunteers (height: 175.7 cm; weight: 77.3 kg; body fat: 20.5%; age: 22.5 years) recruited from a university population.


The study employed an individually randomized within-group design where each participant performed both LIGHT (20–30 RM) and HEAVY (6–10 RM) RT for the calf muscles. One leg was randomly assigned to the LIGHT condition and the contralateral leg performed the HEAVY condition throughout the study period.


Prior to training, all participants underwent RM testing for their 8RM and 25RM to determine individual initial training loads (for HEAVY and LIGHT, respectively) in the respective randomized legs for each exercise.


A 1-week familiarization period was provided prior to the study whereby participants performed these exercises unilaterally over 3 non-consecutive days using their body weight for 3 sets of 5, 10, and 15 repetitions per set on Days 1, 2, and 3, respectively. This hypothetically promoted a repeated bout effect and thus helped to prevent unwanted muscle soreness from interfering with training.


After the strength testing, participants engaged in 8 weeks of intensive training of the plantar flexors, during which the two interventions were provided concurrently. The participants completed 2 sessions per week consisting of four sets per exercise per session with 90 s of rest afforded between sets and ~3 min of rest afforded between exercises. Sets were carried out to the point of momentary concentric muscular failure—the inability to perform another concentric repetition while maintaining proper form.


The load was adjusted for each exercise as needed on successive sets to ensure that participants achieved failure in the target repetition range. The Cadence of repetitions was carried out with a controlled concentric contraction and an approximately 2-s eccentric contraction as monitored by the research staff.


The study produced several novel and notable findings. First and foremost, muscular adaptations were similar for the soleus (a predominantly slow‐twitch muscle) and the gastrocnemius (a muscle with a mixed composition of both major fiber types) regardless of the magnitude of load used in training. Second, each of the calf muscles demonstrated robust hypertrophy, with the lateral gastroc showing greater gains compared to the medial gastroc and soleus. Third, both heavy and light loads elicited similar hypertrophic increases in the triceps surae. Finally, isometric strength increases were similar between loading conditions.


In short, don't worry about the fiber type when designing your training program. Focus on intensity, volume, and recovery.

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