The effects of muscle starting length on work loop power output of isolated mouse soleus and extensor digitorum longus muscle.

Force-length (F-L) 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 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 soleus 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 association of passive work and active net work. Starting length affected maximal WL PO of soleus and EDL across evaluated cycle frequencies (P<0.030, ηp2 >0.494). For soleus, PO produced at -5% L0 was greater than most starting lengths (P<0.015, d>0.6), except vs. -10% L0 (P=0.135, d<0.4). However, PO produced at -10%L0 vs. L0 did not differ (P=0.138, d=0.35-0.49), indicating -5%L0 is optimal for maximal soleus WL PO. For EDL, WL PO produced at -10%L0 was lower than most starting lengths (P<0.032, d>1.08), except vs. -5%L0 (P=0.124, d<0.97). PO produced at other starting lengths did not differ (P>0.163, d<1.04). For soleus, 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 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 F-L curve during dynamic contractions is muscle specific.