The effect of activation level on muscle function during locomotion: are optimal lengths and velocities always used?

Abstract

Skeletal muscle exhibits broad functional diversity, despite its inherent length and velocity constraints. The observed variation in morphology and physiology is assumed to have evolved to allow muscle to operate at its optimal length and velocity during locomotion. Here, we used the variation in optimum lengths and velocities that occurs with muscle activation level to experimentally test this assumption. Muscle ergometry and sonomicrometry were used to characterize force-length and power-velocity relationships, and in vivo operating lengths and velocities, at a range of activation levels. Operating lengths and velocities were mapped onto activation level specific force-length and power-velocity relationships to determine whether they tracked changing optima. Operating velocities decreased in line with decreased optimal velocities, suggesting that optimal velocities are always used. However, operating lengths did not change with changing optima. At high activation levels, fibres used an optimal range of lengths. However, at lower activation levels, fibres appeared to operate on the ascending limb of sub-maximally activated force-length relationships. This suggests that optimal lengths are only used when demand is greatest. This study provides the first mapping of operating lengths to activation level-specific optima, and as such, provides insight into our assumptions about the factors that determine muscle performance during locomotion.

Keywords: force–length; force–velocity; locomotion; recruitment.

Figure 1.

The effect of activation level, on the force–length relationship. Sample force–length curves at all relative force levels are shown (a). From these relationships, optimum length (L₀/L_0max) and activation level (F₀/F_0max) were determined for each stimulus condition (b). There was a significant effect of activation level (F₀/F_0max) on optimum length (p < 0.01) (b). n = 11, 6, 7 and 4.

Figure 2.

The effect of activation level on the power–velocity relationship. Power–velocity curves at all relative force levels are shown (a). From these relationships, optimal velocity (V_opt) was determined. There is a significant effect of relative force on the power–velocity relationship (p < 0.001) (b); optimal velocity decreased with decreasing activation level (c). n = 6, 5, 4 and 3.

Figure 3.

Representative traces of muscle fibre lengths and muscle activation during a short (a) and long (b) hop. Data are shown from the same individual and highlight the increase in fibre length changes and EMG intensity with increasing hop distance.

Figure 4.

The effect of hop distance on take-off time (a), EMG intensity (b), fibre lengths (c) and fibre velocities (d). There was no effect of hop distance on take-off time, the time from EMG onset to take-off (p = 0.25) (a). EMG intensity increased with increasing hop distance (p < 0.001) (b). There was no effect of hop distance on initial fibre length (p = 0.14) (c; filled circles), however, final fibre length decreased with increasing hop distance (p < 0.001) (c; open circles). Shortening velocity increased with increasing hop distance (p < 0.005) (d).

Figure 5.

The relationship between muscle operating lengths and force–length relationships. The effect of activation level on the force–length relationship is shown (a). Initial operating lengths (circle), final operating lengths (arrow) and the resultant muscle shortening, are shown as a function of hop distance and aligned with the force–length relationship (b). In the longest hops (b) muscles shortened over the plateau of the maximally activated force–length relationship (a). In shorter hops (b), muscles operated on the ascending limb of sub-maximally activated force–length relationships (a).

Figure 6.

The relationship between muscle operating velocities and the power–velocity relationship. The effect of activation level on the power–velocity relationship is shown (a). Operating velocities are shown as a function of hop distance and aligned with the power–velocity relationship. Operating velocities decrease with decreasing hop distance (b), tracking the decrease in optimum velocity with decreasing activation level (a).