Figure 2.

Passive Forces Shape Active, Muscle-Driven Extensions

(A) The peak position reached by the tibia during active extensions elicited by a single FETi spike depended on the starting position. Each point is the mean peak amplitude of 5–9 extensions in one animal (N = 7, indicated by different symbols). Error bars were omitted for clarity, but the within-animal variability was very low (see Figure S2 for examples). The dashed gray line at y = x indicates “no movement.” The black line shows a linear fit applied to the pooled data of all animals, which is given by y = 10.72 + 0.57x, R = 0.97, p < 0.0001 (versus slope = 0). Extensions starting at more extended angles (to the left of the x axis) reached more extended maximal angles, but their amplitude (offset from dashed line) was smaller.

(B) The peak velocity of active tibial extensions driven by single FETi spikes increased systematically as the starting angle became more flexed. Values are mean peak velocity (±SD). Each animal is represented by a different symbol (n = 5–9 trials per data point). In three animals, active twitches were elicited from both extended (red) and flexed (black) angles. In a further four animals, active twitches were elicited only from extended angles (red).

See also Figure S2.