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First published online January 17, 2007
Journal of Experimental Biology 210, 383-394 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.02668

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Running stability is enhanced by a proximo-distal gradient in joint neuromechanical control

M. A. Daley^*, G. Felix and A. A. Biewener

Concord Field Station, MCZ, Harvard University, 100 Old Causeway Road, Bedford, MA 01730, USA

View larger version (110K): [in this window] [in a new window]   Fig. 1. Still frames of a guinea fowl during an unexpected perturbation to illustrate experimental set-up. The ground force data reported in this paper were reported previously (Daley et al., 2006), where they were used to calculate changes in mechanical energy of the body center of mass (COM). Here, the experimental data were analyzed further by adding limb kinematics and inverse dynamics to investigate joint mechanics during the perturbation. A 0.6 m long force plate was placed at the midpoint of an 8 m long runway and rested 8.5 cm below the runway surface. White tissue paper pulled tightly across the gap created the appearance of a uniform substrate. Kinematics and ground reaction forces were measured through time (moving from frame A to frame B) for the perturbed step. These data were used to (1) evaluate whole limb mechanics and (2) calculate joint moments and work using inverse dynamics, as described in Materials and methods.

View larger version (9K): [in this window] [in a new window]   Fig. 2. Schematic illustration of variables used for calculation of external joint moments and work using inverse dynamics. Joint angles for the hip, knee, ankle and tarsometatarso-phalangeal (TMP) joints are shown in dotted blue. X marks the force-plate center of pressure (COP); red arrow, the ground reaction force vector (Fg). See Materials and methods for further details.

View larger version (26K): [in this window] [in a new window]   Fig. 3. COM height (sv), limb angle relative to horizontal (), limb length as the distance between hip and toe (L) and vertical (fv, solid line) and horizontal (fh, dotted line) components of ground reaction forces during the C, U and V treatments. The three U trials show typical examples corresponding to the three distinct COM energy response patterns (Daley et al., 2006). Silhouettes illustrate limb posture at the point of ground contact. Dotted line indicates the time of tissue paper contact, and the grey bars indicate duration of ground contact (tc). Ground reaction forces and COM position data were reported previously (Daley et al., 2006) and are shown here for reference. In the present paper we relate the limb loading and energy patterns to joint mechanics during the step following the perturbation.

View larger version (14K): [in this window] [in a new window]   Fig. 4. (A) Limb angle relative to horizontal () and (B) limb length as the distance between hip and toe (L) over the course of the perturbed step for all U trials from one individual (solid blue), with a typical C (broken green) and V (dotted red) trial from the same individual. Thin broken grey lines indicate the aerial phase. Thicker lines indicate the period of ground contact. The dotted vertical line indicates the time of tissue paper contact for U trials, and the time of ground contact for the C and V trials. Data are shown for the period from aerial phase peak in COM height to the end of the stance phase following the perturbation.

View larger version (14K): [in this window] [in a new window]   Fig. 5. Initial limb contact angle (i), initial relative limb length (Li/Lt), dimensionless limb stiffness (Kleg), and net work of the limb during stance (Elimb; calculated from inverse dynamics) during C (black), U (light grey) and V (dark grey) treatments with U trials subdivided by response mode. Values are mean ± s.e.m. (N=10, 7, 9, 3, 10 for the respective categories).

View larger version (34K): [in this window] [in a new window]   Fig. 6. Joint mechanics during stance. Joint angles (left), external moments (middle) and joint work loops (moment–angle plots, right) over the course of stance for the hip (A), knee (B), ankle (C) and TMP (D). A representative U trial for each of the 3 response modes is shown (broken colored lines) with a level running trial for comparison (C, solid black line). Increasing joint angles indicate extension, and positive moments indicate extensor moments. Arrows indicate the direction of work loops. Counter-clockwise indicates energy production by the joint, clockwise indicates energy absorption.

View larger version (10K): [in this window] [in a new window]   Fig. 7. Net external mechanical work in relation to limb contact angle (i) for (A) the hip and knee, (B) the ankle and TMP and (C) the entire limb. Black symbols are individual U trials, grey symbols show the mean ± s.e.m. for C trials (N=10).

View larger version (9K): [in this window] [in a new window]   Fig. 8. Net work at each joint during C (level running), U (unexpected drop) and V (visible drop) trials with U trials subdivided by response mode. Values are mean ± s.e.m. (N=10, 7, 9, 3, 10, for the respective categories).

View larger version (20K): [in this window] [in a new window]   Fig. 9. Initial stance phase knee angle determines limb posture and the work balance among the joints. The knee angle is the only joint angle that differs significantly at the onset of ground contact among the U response modes. If the knee is extended at contact (left silhouette) the limb has a lower initial angle and longer initial length. This extended posture is associated with larger decelerating forces, greater energy absorption by the ankle and TMP, and net energy absorption by the limb. If the knee is flexed at contact (right silhouette), the limb has a higher initial angle and shorter initial length. This flexed posture is associated with lower decelerating forces, spring-like function of the ankle and TMP, and net energy production by the limb. In cases with an extremely flexed knee, the distal limb simply collapses without supporting substantial weight (KEv mode, silhouette not shown). Values are mean ± s.e.m. (N=10, 7, 9, 3, for the respective categories).