All Muscle Fibers Are Not Created Equal: Fast-Twich Verses Slow-Twitch
What are muscle fibers?
Muscle fibers are long cylindrical cells bundled together in groups inside the muscle.
Muscle fibers are innervated by nerve cells called motor neurons. A motor neuron and the muscle fibers it innervates are called a motor unit. When stimulated by a motor neuron, all the muscle fibers in a motor unit contract together.
Classification of muscle fibers
Muscle fibers can be classified in many different ways. Contraction or twitch time is the most common distinction between fast twitch muscle fibers and slow twitch muscle fibers.
Type I (slow twitch)
Type I muscle fibers are efficient at producing muscle contractions (twitch) for extended time periods. They carry more oxygen and as a result can sustain aerobic activity for hours. Unlike fast-twitch muscle fibers, slow-twitch fibers take a long time to develop force. Marathon runners and long distance cyclists have predominantly type I muscle fibers. An example of a slow twitch muscle is the soleus muscle of the calves, which can be trained using 15-20 reps or more.
Properties of Type I fibers:
Fatigue-resistant
High aerobic capacity
Low anaerobic power
Type II (fast twitch)
Type II muscle fibers produce short bursts of quick, powerful movements. They develop force quickly, but cannot sustain the force for long periods of time. Weight lifters and sprinters primarily have type II muscle fibers. An example of a fast twitch muscle is the hamstring muscle which is trained at 6 reps or below for best results.
Properties of Type II fibers:
Rapidly Fatigable
Low aerobic capacity
High anaerobic power
There is a third classification of muscle fiber type called type IIb intermediate, or oxidative muscle fibers. They have properties that are somewhere in between the fast twitch and the slow- twitch fibers, but tend to be more fatigue-resistant than pure fast twitch fibers.
