Final answer:
As force increases, there is an increase in the proportion of motor units recruited in skeletal muscles, ranging from smaller, more excitable units to larger units for stronger contractions. This progressive recruitment allows for both fine control and powerful movements, while preventing muscle fatigue through staggered unit activation.
Step-by-step explanation:
The recruitment of motor units in skeletal muscles is a process by which the number of active motor units increases, resulting in a stronger muscle contraction. When force increases, there is an increase in the proportion of motor units (MUs) recruited. This recruitment starts with the smaller, more excitable motor units and, as more strength is required, progresses to larger, higher-threshold motor neurons that activate larger muscle fibers.
In this way, finer control over muscle movement is possible at lower forces, and maximum contraction occurs when the most and largest units are recruited. However, to prevent fatigue, not all motor units may be active simultaneously, allowing for longer muscle contractions. The concept of length-tension relationship is also important, as it relates to the optimal sarcomere length for generating maximal force during a muscle contraction.