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Horses damp the spring in their step


The muscular work of galloping in horses is halved by storing and returning elastic strain energy in spring-like muscle–tendon units1,2.These make the legs act like a child's pogo stick that is tuned to stretch and recoil at 2.5 strides per second. This mechanism is optimized by unique musculoskeletal adaptations: the digital flexor muscles have extremely short fibres and significant passive properties, whereas the tendons are very long and span several joints3,4. Length change occurs by a stretching of the spring-like digital flexor tendons rather than through energetically expensive length changes in the muscle5. Despite being apparently redundant for such a mechanism5, the muscle fibres in the digital flexors are well developed. Here we show that the mechanical arrangement of the elastic leg permits it to vibrate at a higher frequency of 30–40 Hz that could cause fatigue damage to tendon and bone. Furthermore, we show that the digital flexor muscles have minimal ability to contribute to or regulate significantly the 2.5-Hz cycle of movement, but are ideally arranged to damp these high-frequency oscillations in the limb.

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Figure 1: Equine distal forelimb (medial view) showing segments below the elbow, and the tendons and muscles that resist compression of the limb by the effect of gravitational and inertial forces during the stance phase of locomotion.
Figure 2: Plot of horizontal (in direction of locomotion) and vertical limb ground reaction force (GRF) against time for a 450-kg horse trotting at 3 m s-1 over a 6-mm-thick polyester/rubber-matting-covered conveyor-belt force plate.
Figure 3: Relationship between limb force and muscle length for an equine superficial digital flexor muscle loaded in situ in the unstimulated state (U), and two sequential cycles with maximal electrical stimulation (S1, S2).
Figure 4: Vertical and horizontal ground reaction force (GRF) data produced from the equine limb model described in the text in the reference condition (leg vertical, no muscle activation, loading on tarmac).
Figure 5: Plot of cycle work done on each cross-bridge in 10-21 J per cycle as a function of frequency (Hz) during a sinusoidal length change of 0.5%.


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We acknowledge the assistance of R. C. Woledge in carrying out the cross-bridge model calculations and thank him for comments on the manuscript. We thank the Horserace Betting Levy Board, London, for funding this work.

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Correspondence to Alan M. Wilson.

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Wilson, A., McGuigan, M., Su, A. et al. Horses damp the spring in their step. Nature 414, 895–899 (2001).

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