Which principle states that muscle fibers are recruited in order of size?

The principle that states muscle fibers are recruited in order of size is known as the Size Principle. This concept indicates that smaller motor units, which are typically composed of slow-twitch muscle fibers, are activated first during low-intensity activities. As the need for force increases, larger motor units are recruited, which are made up of fast-twitch fibers, allowing for greater force production.

Understanding the Size Principle is essential for both training and rehabilitation, as it highlights the natural progression the body follows in activating muscle fibers. This sequential recruitment optimizes performance and helps prevent fatigue, as smaller fibers can sustain activities for longer durations before larger, more powerful fibers are necessary.

The other concepts mentioned, while important in their own contexts, do not specifically reference the order of recruitment based on the size of the fibers. For instance, the Recruitment Principle is a broader term that can refer to the activation of muscle fibers without specifying the order. The Force Summation Principle relates to how muscle contractions can combine in force but does not address the recruitment hierarchy. The All or None Principle refers to the requirement that a motor unit will either fire completely or not at all, and does not pertain to the size order of recruitment.