If a stimulus exceeds an activation threshold, all muscle fibers innervated by the same motor unit will contract.
The all-or-none law means that the strength of a muscle fiber’s response is not dependent on the strength of the stimulus. The more muscle fibers are recruited, the more force is ultimately produced. On the other hand, fast-twitch muscle fibers are only activated when slow fibers are unable to produce enough force. Because slow-twitch muscle fibers are naturally smaller, they are also recruited first. The size principle refers to the way motor units are activated from smallest to largest. These motor units are activated according to two principles the size principle and the all-or none-law. A single motor neuron and all the muscle fibers it innervates is known as a motor unit. Muscle fibers are activated by a motor neuron, which controls muscle contraction by delivering a signal from the brain to the muscle. If you’re looking to know more about slow-twitch muscle fibers, we’ve written a dedicated article about it here. This post explains the basics of fast-twitch muscle fibers and what makes them so integral for athletic performance.
This is because you must maintain good posture at all times. For example, postural muscles (tonic muscles) often contain more slow muscle fibers than muscles responsible for movement (phasic muscles). However, this also varies between individuals and where the muscle is located. Fast-twitch muscle fibers generate more force but fatigue faster, whereas slow muscle fibers produce less force while excelling in sustained physical performance.Īn important thing to note is that all skeletal muscles consist of both muscle fiber types. The latter can also be further divided into type IIa (fast oxidative) and type IIb/IIx ( fast glycolytic) muscle fibers.Įach of these muscle fibers types have their own specific properties and responses to physical activity. Skeletal muscles are comprised of two different muscle fiber types slow-twitch muscle fibers (type I) and fast-twitch muscle fibers (type II).