Fig. 4. (A) The effect of gape angle and tendon morphology on muscle strain in the anterior division of the adductor mandibula of the ratfish. In the untwisted condition (broken lines), strain increases much more rapidly in the anterior face (red line) than the posterior face (blue line) as the mouth is opened to maximum gape (40°). As a result, at maximum gape, there exists more than a 50% difference in strain between the faces of the muscle (relative to their respective resting lengths). This is an order of magnitude difference over what is seen in the twisted condition, where the strain differences in the two faces are never more than 5.5%. These results are represented schematically in (B) where strain differences (S) between the anterior and posterior faces of the muscle in each condition (twisted and untwisted) are colored in a gradient from yellow (equal strains) to green (an anterior face strain that is greater by 56% of resting length). AFS, anterior face strain; PFS, posterior face strain.