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First published online March 14, 2005
Journal of Experimental Biology 208, 1175-1190 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01486

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In vivo muscle function vs speed I. Muscle strain in relation to length change of the muscle-tendon unit

Donald F. Hoyt^1,*, Steven J. Wickler², Andrew A. Biewener³, Edward A. Cogger² and Kristin L. De La Paz²

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

View larger version (12K): [in a new window]   Fig. 1. Representative data for accelerometer and EMG of a forelimb (A) and hind limb (B) of a horse trotting on a level treadmill at 3.5 m s-1. The graphs show the accelerometer used to identify stance phase, starting with hoof contact (axis origin) and lift off (denoted by the broken line). Stance is indicated by the solid bar at the bottom. EMG activity patterns, below the accelerometer record, show that both muscles are active early in stance.

View larger version (19K): [in a new window]   Fig. 2. (A) Representative patterns of normalized triceps muscle length (solid line) and elbow kinematics (dashed line) from one horse at 3.25 m s-1. (Bi,ii) Representative patterns of normalized vastus muscle length and knee kinematics from two different horses at 3.25 m s-1 showing the two different strain patterns. Joint flexion is reflected in a decreasing angle. The dotted vertical lines, corresponding to inflections in the kinematics curve, show how the strain patterns were divided into phases. The numbers correspond to the phases described in the text. The vastus pattern (Bii), seen in two horses, lacked phase 2, seen in one horse at all speeds and in a second horse at low speeds. This phase is shown in Bi. Mean duration of EMG activity is indicated by horizontal, black bars.

View larger version (33K): [in a new window]   Fig. 3. Strain in triceps did not change as a function of speed during phase 3 but the strain rate was faster (more negative) at higher speeds. Values are means ± S.E.M. from the four horses used in the study. In the vastus, strain increased with speed during phases 3 and 5, the two concentric phases, but not during phase 4. Also during both concentric phases in the vastus the strain rate was faster at higher speeds.

View larger version (15K): [in a new window]   Fig. 4. EMG activity as a function of speed in the triceps (filled squares) vastus (filled triangles). (A) Activation phase advance did not change with speed in the triceps but it did in the vastus. (B) EMG burst duration, as a percentage of stance duration, did not change with speed in either muscle, averaging 40% in the triceps and 60% in the vastus. (C) Integrated EMG increased with speed in both muscles, indicating an increase in the volume of active muscle. Integrated EMG was normalized to the highest value measured in the same animal trotting up an incline (Wickler et al., 2005).

View larger version (24K): [in a new window]   Fig. 5. Stride parameters in the forelimbs (filled squares) and hindlimbs (filled triangles) of horses trotting on the level. (A) Stride period decreased with speed but was the same in both limbs. There were differences between the limbs for all of the other parameters (B-F). (B) Swing time is shorter for the forelimb and does not change with speed but it decreases with speed in the hindlimb. (C) Time of contact is longer for the forelimb and decreases with speed in both limbs. As a consequence of the longer time of contact in the forelimb (C), step length (D) is longer in the forelimb, the rate of force application (E) is lower, and duty factor (F) is larger. Notice that duty factor is greater than 0.5 in the forelimb at a speed of 2.5 m s-1.

View larger version (14K): [in a new window]   Fig. 6. Comparisons, in two horses trotting, of the patterns of vastus fascicle strain over the whole stride period at 3.0 m s-1 in two different years. Implants were placed in approximately the same area of the vastus muscle in each animal. Each strain pattern represents the mean of ten consecutive strides. The variance ratio is a quantitative measure of the similarity of two waveforms. It can range from a value of zero (identical) to 1.0 (no similarity). These two horses represent the range of constancy (variance ratios in two different years) obtained in this study: (A) variance ratio=0.203 and (B) variance ratio=0.028. Constancy is the term used to describe the similarity between two different experiments and reliability the similarity between successive strides from the same experiment. Constancy and reliability are reported in Tables 5 and 6 for three horses and four speeds. These two horses exhibit the two different patterns of vastus strain shown in Fig. 2. Focusing on the stance phase (up to 40% of stride) it can be seen that the two patterns are reasonably similar in the two different years.

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