Fig. 3. Modulation of the timing and intensity of hindlimb muscle activity between sprawling and high-walk locomotion in alligators. Significant changes in motor patterns (Table 2) as limb posture becomes more erect are illustrated by comparing mean electromyographic (EMG) patterns for more sprawling postures (about 30° of femoral adduction; gray bars) to those for more erect postures (about 50° of femoral adduction; black bars). Open bars indicate muscle patterns for which there was no significant change in motor patterns (timing or amplitude) as femoral adduction angle increased. Differences in bar thickness between sprawling and upright stance indicate significant differences in mean amplitude (in all cases where such differences occur, black bars for upright posture are thicker than gray bars for sprawling posture). The number of individuals from which EMG data were collected is listed in parentheses after each muscle name. In the top panel, mean three-dimensional limb segmental kinematics for the low (gray line) and high (black lines) walking postures are shown, illustrating differences between these postures (from Reilly and Elias, 1998). Note that more erect postures are produced by consistently greater femoral adduction, knee extension and ankle extension during the stance phase. Abbreviations: FTE, flexor tibialis externus; ADDFEM1, adductor femoris, head 1; PIFE3, puboischiofemoralis externus, head 3; PIT (a), puboischiotibialis (stance burst); PIT (b), puboischiotibialis (swing burst); FEMTIB, femorotibialis internus; ILTIB1, iliotibialis, head 1; ILTIB2, iliotibialis, head 2; GAST, gastrocnemius; PIFI2, puboischiofemoralis internus, head 2; ILFEM, iliofemoralis; AMB 1, ambiens, head 1; TA, tibialis anterior.