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First published online March 14, 2005
Journal of Experimental Biology 208, 1191-1200 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01485

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In vivo muscle function vs speed II. Muscle function trotting up an incline

Steven J. Wickler^1,*, Donald F. Hoyt², Andrew A. Biewener³, Edward A. Cogger¹ and Kristin L. De La Paz¹

¹ Equine Research Center, California State Polytechnic University, Pomona, CA 91768-4032, USA
² Biological Sciences Department, California State Polytechnic University, Pomona, CA 91768-4032, USA
³ Concord Field Station, Department of Organismic and Evolutionary Biology, Harvard University, Bedford, MA 01730, USA

View larger version (16K): [in a new window]   Fig. 1. Typical recordings for the entire stride for a forelimb and hindlimb during trotting up an incline. The top graphs include tracings of accelerometer records that were used to identify stance phase (gray shaded area with hoof lift-off denoted by vertical arrow). EMG activity patterns are shown below the accelerometer records to demonstrate when the muscle was active.

View larger version (18K): [in a new window]   Fig. 2. Normalized stance phase muscle lengths (means ± 1 S.E.M.) of the triceps and vastus for all horses at all speeds (N=4). The grey curve represents the mean of the same horses for locomotion over all speeds at 0% slope (from Hoyt et al., 2005). On the incline, both muscles shortened more than on the level.

View larger version (18K): [in a new window]   Fig. 3. Joint angles (broken lines) and normalized muscle lengths (solid lines, where 1.0 = length at rest), at 3.25 m s-1 for the triceps (A) and for the two patterns of muscle strain observed in the vastus (B; referred to in Hoyt et al., 2005). Phases (denoted by vertical broken lines and numbers), based on kinematics, were the same as used in the analysis of the data obtained during level locomotion. Mean duration of EMG activity is indicated by shaded gray bars.

View larger version (15K): [in a new window]   Fig. 4. Total strain (change in normalized muscle length) and strain rate (L s-1) of the triceps (during phase 3) as a function of trotting speed on a 10% incline (black symbols, mean ± 1 S.E.M.). Total strain on the level is denoted by gray symbols (Hoyt et al., 2005). Strain and strain rate during incline locomotion did not change with speed but were greater than during level locomotion.

View larger version (13K): [in a new window]   Fig. 5. The average strain (change in normalized muscle lengths; means ± 1 S.E.M.) of the vastus for the three phases of stance identified in Fig. 3 (phases 3-5). There was no overall effect of trotting speed. The muscle contracted concentrically during all three phases and the total concentric strain during the three phases was greater on an incline (black symbols) than on the level (gray symbols).

View larger version (12K): [in a new window]   Fig. 6. Shortening rates (L s-1; means ± 1 S.E.M.) of the vastus for phases 3-5. For phases 4 and 5 the shortening rates increased with speed and were greater on the incline (black symbols) than on the level (gray symbols).

View larger version (15K): [in a new window]   Fig. 7. EMG patterns (means ± 1 S.E.M.) for the triceps and vastus as a function of speed on the incline (black) versus the level (gray; Hoyt et al., 2005). (A) On an incline at low speeds, EMG activity of both muscles started near the time of foot contact, but as speed increased the muscle was activated earlier. (B) As speed increased on the incline, EMGs of both muscles were active for a smaller percentage of stance, but for a larger fraction than at the same speed on the level. (C) Integrated EMG activities of both muscles increased with speed and were greater on the incline.

View larger version (26K): [in a new window]   Fig. 8. Stride parameters (means ± 1 S.E.M.) for both limbs measured on the incline (black) versus on the level (gray; Hoyt et al., 2005). (A) Stride period decreased with speed and was longer on the incline than on the level, and was not different between the limbs. There were differences between the limbs for all the other parameters listed. (B) Swing time on the incline was not different from that on level for the forelimb, but was shorter for the hindlimb. (C) Time of contact on the incline was longer in the forelimb but not different in the hind limb. (D) Step length on the incline was longer for the forelimb but not different for the hindlimb. (E) The duty factor on the incline was not different from that on the level.