Fig 4. Estimated muscle fascicle length.

(A) The Achilles tendon was assumed to be arranged in series with the triceps surae muscle fascicles. The measured musculotendon force was used to estimate tendon elongation. Muscle fascicle length was found by subtracting estimated tendon length from measured musculotendon length. A rigid tendon assumes changes in muscle fascicle length are equal to measured changes in musculotendon length. (B) Recorded IFR and measured musculotendon force in response to ramp-and-hold (left) and ramp-and-release stretches. Musculotendon force was used to compute estimated tendon elongation. (C) Example of estimated muscle fascicle length-related variables for ramp-and-hold stretch (left) and repeated ramp-and-release stretch (right). Top row is measured change in musculotendon length (red dashed trace) and two estimates of change in muscle fiber length (high tendon compliance = 2 mm^-1: yellow; low tendon compliance = 6 mm^-1: orange). The second and third rows are velocity and acceleration estimates, respectively, using the same coloring convention as for length.