Certain muscles which are commonly considered to have similar functions were found to operate on very different portions of the force-length curve. Splenius capitis and semispinalis capitis both generate an extension moment. For all muscles studied, fascicle length variations were greatest for those directions with the largest moment arms. Fascicle operating ranges of sternocleidomastoid during head movements in pitch, yaw and roll. The vertical line indicates fascicle length at neutral head position.
Vertical lines indicate fascicle lengths at neutral head position.
The range of joint angles over which a muscle can generate active force is proportional to the ratio between optimal fascicle length and moment arm. A larger ratio indicates less variation in force-generating capacity with changes in head position. The ratio of optimal fascicle length to moment arm varies among neck muscles, more often because of differences in moment arm rather than fascicle length. The optimal fascicle length of splenius capitis is 9.5 cm, but the extension moment arm is close to 5 cm. This explains why sternocleidomastoid remains on the plateau region during flexion-extension movements while splenius capitis undergoes large changes in fascicle length.