Because muscle fiber length usually correlates closely with myofibril length, muscle fiber length is used as a marker for excursion potential or velocity. Therefore, muscle fiber length is used to determine excursion, while PSCA is used to determine force. The force–velocity relationship inherent in internal muscle design indicates that as the speed of a muscular action increases, force production drops and vice versa.

Based on these relationships, we can predict that there will be inherent trade-offs in muscle function based on the particular design of a muscle. If a muscle has both long fibers and a large cross-sectional area, it will be capable of both high excursion and high force, but will necessarily have a high mass and metabolic demand. On the other hand, muscles with long fibers, but small cross-sectional area, will have high excursion but low force potential. Contrary to the prior statement, muscles with a large cross-sectional area, but short fibers will have high force and low excursion potential. A fourth possibility would be observing short fibers with a small cross-sectional area. These muscles would have low mass and metabolic demand, but limited force and movement potential.

With the inherent trade-offs in function, we would expect muscles to be specialized to their functional role, so only the necessary features to accomplish the range of performance required would be present. That is, we would expect the structure of muscle to closely follow from the function of that muscle. Dr. Sam Ward has developed this concept of structural–functional relationships for muscles of the lower extremity by experientially determining the structure of individual muscles and proposing general categories for these specialized functions. Table 3.1 shows the experimentally determined architectural values for muscles of the leg.