Final answer:
Increasing the length of the muscle fibers is not a method by which the force of muscle contraction can be increased. Increasing the number of motor units, the frequency of motor unit activation, and the size of muscle fibers are valid methods to enhance muscular force.
Step-by-step explanation:
The body can increase the force produced by a skeletal muscle in several ways. However, among the options listed, increasing the length of the muscle fibers is not a way by which the force of muscle contraction can be increased. Instead, the other methods such as increasing the number of motor units recruited, increasing the frequency of motor unit activation, and increasing the size of the muscle fibers (through hypertrophy) are all valid methods by which the force of muscle contraction can be increased.
Muscle fiber length is more associated with the optimal range of force generation rather than a means to increase force. When the length of the sarcomere falls within this optimal range, the muscle can generate the most force.