Understanding Muscle Fiber Types And Training For Them

The two broad categories

Type I (slow-twitch) fibers are more fatigue-resistant and suited to endurance activity. Type II (fast-twitch) fibers generate more force quickly but fatigue faster, and are more responsive to hypertrophy (growth) from resistance training.

Why rep range affects which fibers get emphasized

Heavier, lower-rep training tends to preferentially recruit and develop Type II fibers, while very high-rep, endurance-style training relies more heavily on Type I fibers — though there's meaningful overlap across most rep ranges.

Genetics play a real role here too

Individuals have naturally different ratios of fiber types (partly genetically determined), which is part of why some people respond better to strength-focused training and others to endurance-focused training, even with similar effort.

The practical takeaway

A training program that includes a range of rep schemes — some heavier, lower-rep work and some higher-rep work — trains a broader range of muscle fiber types than sticking rigidly to just one range.