Fig. 4. Mean kinematic profiles of humerus protraction and retraction (i.e. angle from the transverse plane) during turtle swimming. (A) Trachemys scripta, (B) Apalone spinifera. Each kinematic trial was normalized to the same duration, and values of kinematic angles were interpolated for 100 equally spaced increments through the stroke cycle (Walker, 1998), allowing mean angles and standard errors to be calculated for each 1 % increment through the stroke for each species. Angle values ±1 S.E.M. are plotted for every second increment (every 2 % through the cycle); N=15 trials for each species. Angles of 0° indicate that the humerus is perpendicular to the anteroposterior midline of the turtle, with negative angles indicating that the distal end of the humerus is directed posteriorly and positive angles indicating that the distal end of the humerus is directed anteriorly. An angle of +90°, for example, indicates that the distal end of the humerus is directed straight ahead of the turtle. The bold vertical line in each plot demarcates recovery phase (R) from thrust phase (T).
