Figure 3.17 Passive insufficiency. Stretching the hamstrings to the end of their available length over a flexed hip and extended knee limits further hip flexion either from the contraction of hip flexors or from additional passive stretching of the hamstrings.
Figure 3.18 Tenodesis can be used to assist with grasp and release if there is no voluntary ability to open and close the fingers. (A) As the wrist flexes, finger extensors reach a maximal length, become passively insufficient, and extend the fingers. (B) Passive insufficiency of the finger flexors occurs as the wrist extends, causing the fingers to passively flex.
TABLE 3.4 Factors Related to Muscle Contraction Force Factors
Contraction Force Fiber architecture
Pennate fi ber arrangements have a greater number of fi bers for a given area with the capability of a greater cross-sectional area. This formation can produce greater forces compared with a fusiform parallel muscle fi ber arrangement.
Muscle contraction type Contraction speed Length Motor units
Isometric contractions produce greater tension than concentric contractions, and eccentric contractions produce greater forces than isometric contractions.
As speed increases, force decreases during a concentric contraction. Eccentric contractions produce greater tension as the speed of lengthening increases.
Greater tension is produced the closer the sarcomeres are to their optimal length.
Greater muscle tension is associated with: • Larger motor unit size • Increase in the number of units fi ring • Increase in the frequency of fi ring of motor units
