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First published online March 14, 2005
Journal of Experimental Biology 208, 993-1009 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01473

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Hindlimb function in the alligator: integrating movements, motor patterns, ground reaction forces and bone strain of terrestrial locomotion

Stephen M. Reilly^1,*, Jeffrey S. Willey¹, Audrone R. Biknevicius³ and Richard W. Blob²

¹ Department of Biological Sciences, Ohio University, Athens, OH 45701, USA
² Department of Biological Sciences, Clemson University, Clemson, SC 29634, USA
³ Department of Biomedical Sciences, Ohio University College of Osteopathic Medicine, Athens, OH 45701, USA

View larger version (26K): [in a new window]   Fig. 1. Kinematic landmarks and angles used to describe hindlimb movements in alligators during locomotion. (A) Three-dimensional coordinates were digitized for one trunk landmark (T), and landmarks for the hip joint (H; on both sides: H left, H right); the knee (K), the ankle (A) and foot (F; on the skin on the lateral aspect of the metatarsal-tarsal articulation). (B) Three-dimensional angles were calculated for femoral retraction angle (the angle between landmarks T-H right-K indicating primarily femoral retraction/protraction movements); knee angle (angle H-K-A indicating knee flexion and extension), and ankle angle (angle K-A-F indicating foot flexion and extension). (C) Femoral adduction angle (angle H left-H right-K) was calculated to quantify the degree of hip adduction relative to a transverse line through the acetabula. (D) Ground reaction forces in body weight units (BWU) of a typical trial indicating key kinetic events: peak vertical force, peak braking force, braking-to-propulsive transition (zero fore-aft force), peak propulsive force and peak lateral force.

View larger version (30K): [in a new window]   Fig. 3. Integration of five levels of analysis of stance phase dynamics during hindlimb locomotion in alligators. (A) Dynamic phases of stance. (B) Torsional moment of the ground reaction force acting on the femur; illustrates torsional moments on the right femur as viewed from the acetabulum. (C) Three-dimensional ground reaction forces (top, vertical; middle, fore-aft; bottom, mediolateral) in body weight units (BWU). (D) Mean footfall patterns; R, right; L, left; H, hindlimb; F, forelimb. (E) Mean hindlimb kinematics. (F) Mean electromyographical patterns for 16 hindlimb muscles, constructed from the data in Table 1 and Gatesy (1997; indicated by *). All of the data (details in Materials and methods) are from alligators of similar size, moving at the same speed (0.146 m s-1), posture (femur adducted 51-60°), and gait (duty factors 0.70-0.73). Vertical green bars delineate the three dynamic phases of limb function (from A) during the stance phase: limb loading, support and propulsion and limb unloading, described in text.

View larger version (41K): [in a new window]   Fig. 2. Alligator hindlimb muscles involved in the high walking for which electromyographical data have been recorded. Lines of actions are indicated for muscles active in either stance phase (red), swing phase (blue) or with activity in both phases (black); gray shading in arrows indicates that the muscle lies medial to the femur. AMB1, ambiens, head 1; ADD1, adductor femoris 1; CFL, caudofemoralis longus; FMTI, femorotibialis internus; FTE, flexor tibialis externus; FTI2, flexor tibialis internus, head 2; G, gastrocnemius; ILFEM, iliofemoralis; ILFIB, iliofibularis; ILTIB1/2 iliotibialis, head 1/2; PIFE2/3, puboischiofemoralis externus, head 2/3; PIFI2, puboischiofemoralis internus, head 2; PIT, puboischiotibialis; TA, tibialis anterior. Data for muscles marked with asterisks are taken from Gatesy (1997). The continuation of the AMB1 tendon through the extensor sheet and into the Achilles tendon is not shown.

View larger version (26K): [in a new window]   Fig. 4. Typical two-dimensional hindlimb force vectors and femoral strains for alligators using a walking trot. Orientations of the (A) sagittal and (B) transverse hindlimb force vectors are shown in relation to the dynamic hindlimb stance phases shown in Fig. 3 (vertical bars). Note that these vectors are equal in magnitude but opposite in direction to the components of the ground reaction force; thus, when the hindlimb force vector is directed posteriorly (>90°), the ground reaction force is directed anteriorly. (C-E) In vivo principal, shear, and axial strains recorded from the alligator femur during the stance phase (at 0.37 m s-1; from Blob and Biewener, 1999). (C,D) Strain traces from the dorsal femur. (E) Strain trace from the anterior femur.