White Muscle Vs. Red Muscle Fitness

Young women exercising with her personal trainer in the gym

Muscle fibers vary in composition, function and fitness requirements. Although genetic factors dictate distribution of muscle fibers, physical activity regulates fiber vitality. In fact, you may be unknowingly neglecting a significant percentage of your muscle mass. Training mode and intensity regulates muscle-fiber activation and resultant adaptation. Understanding the difference between white and red muscle fitness enables smart program design.

Muscle Physiology Basics

Muscle tissue contains an array of type I and type II fibers. Alternative names for type I fibers include slow-twitch fibers or red muscle, while type II fibers are also known as fast-twitch fibers or white muscle. Although both fiber types contribute to movement, exercise intensity initiates which fiber dominates force production, according to the American Council on Exercise’s Resources for the Personal Trainer. For example, red muscle fibers initiate all movement while white fibers activate only when intensity surpasses a given level, as stated by the American Council on Exercise. Both fiber types require weight-bearing exercise for fitness enhancement.

Red Muscle Fibers

According to the American Council on Exercise, red muscle fibers specialize in long-duration, low-intensity movement, such as walking, standing or lifting loads below 70 percent of your maximal ability. Red fibers fatigue slowly and dominate muscle composition in the human body. Because red fibers contribute to all muscular contractions, they are easier to target with exercise. For example, any repetitive, weight-bearing action above an accustomed-to intensity produces red muscle adaptations such as growth and increased endurance, according to the National Strength and Conditioning Association.

White Muscle Fibers

White muscle fibers create high-intensity actions lasting fewer than 30 seconds, such as jumping and lifting loads greater than 70 percent of your maximal ability. Because white fibers only activate during high-intensity action, sedentary individuals may go long periods of time without type II fiber activation, the American Council on Exercise reports. You can improve type II fiber fitness with heavy resistance training and ballistic exercise such as fast, yet controlled, weightlifting, as prescribed by the National Strength and Conditioning Association.


White muscle experiences detraining, or loss of fitness, due to reduced physical activity at a faster rate than red muscle mass. While white muscle fitness augments ability to sustain a given activity over time -- muscular endurance -- red muscle has a greater influence on muscle size and maximal strength. Endurance adaptations can be lost quickly without regular exercise. Generally, endurance athletes focus on red muscle fitness, while strength and power athletes build white muscle fitness, the National Strength and Conditioning Association says. Non-athletes can support muscular health with weekly weight training sessions. Furthermore, muscle requires 0.8 to 2.0 grams of protein per kilogram of body weight, regardless of fiber type, as prescribed by the National Strength and Conditioning Association. Consult a doctor before starting an exercise program.