Changes in muscle fascicles of tibialis anterior during anisometric contractions are not associated with motor-output variability of the ankle dorsiflexors in young and old adults.

This study examined the associations between the fluctuations of foot acceleration during shortening and lengthening contractions with the electromyographic (EMG) activity of lower leg muscles and ultrasound measures of tibialis anterior fascicle length and pennation angle. Young (24.9 ± 4.17 years) and old (74.8 ± 3.31 years) adults lifted and lowered a submaximal load with the foot at different speeds (3°/s-50°/s). The standard deviation (SD) of foot acceleration normalized to the load lifted was similar for young (12.2 ± 7.22 cm s(-2)/kg) and old adults (14.3 ± 8.03 cm s(-2)/kg; P = 0.093). The changes in tibialis anterior muscle fascicle length and pennation angle were similar for young and old adults (P ≥ 0.233), but greater for shortening (fascicle length: 0.937 ± 0.633 cm, pennation angle: 1.61 ± 0.918º) than for lengthening contractions (fascicle length: 0.806 ± 0.521 cm, pennation angle: 0.966 ± 0.632º; P ≤ 0.014). The changes in fascicle length and pennation angle were not associated with the SD of foot acceleration (r(2) ≤ 0.031; P ≥ 0.092). The surface EMG of tibialis anterior was greater for the shortening contractions than for the lengthening contractions (P < 0.001), but triceps surae EMG was similar for the two types of contractions (P = 0.304). The results suggested that the influence of movement speed on variability in performance was similar for shortening and lengthening contractions with the dorsiflexor muscles; furthermore, old adults were able to match the performance of young adults.