Force–length relationships derived from isometric activations may not directly apply to muscle force production during dynamic contractions. As such, different muscle starting lengths between isometric and dynamic conditions could be required to achieve maximal force and power. Therefore, this study examined the effects of starting length [±5–10% of length corresponding to maximal twitch force (L0)] on work loop (WL) power output (PO), across a range of cycle frequencies, of the soleus (SOL) and extensor digitorum longus muscle (EDL; N=8–10) isolated from ∼8 week old C57 mice. Furthermore, passive work was examined at a fixed cycle frequency to determine the association of passive work and active net work. Starting length affected maximal WL PO of the SOL and EDL across evaluated cycle frequencies (P<0.030, ηp2>0.494). For the SOL, PO produced at −5% L0 was greater than that at most starting lengths (P<0.015, Cohen's d>0.6), except −10% L0 (P=0.135, d<0.4). However, PO produced at −10% L0 versus L0 did not differ (P=0.138, d=0.35-0.49), indicating −5% L0 is optimal for maximal SOL WL PO. For the EDL, WL PO produced at −10% L0 was lower than that at most starting lengths (P<0.032, d>1.08), except versus −5% L0 (P=0.124, d<0.97). PO produced at other starting lengths did not differ (P>0.163, d<1.04). For the SOL, higher passive work was associated with reduced PO (Spearman's r=0.709, P<0.001), but no relationship was observed between passive work and PO of the EDL (Pearson's r=0.191, r2=0.04, P=0.184). This study suggests that starting length should be optimised for both static and dynamic contractions and confirms that the force–length curve during dynamic contractions is muscle specific.

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