title: Horses damp the spring in their step.
abstract: The muscular work of galloping in horses is halved by storing and returning elastic strain energy in spring-like muscle-tendon units.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 joints. Length change occurs by a stretching of the spring-like digital flexor tendons rather than through energetically expensive length changes in the muscle. Despite being apparently redundant for such a mechanism, 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.
authors.affiliation: Department of Veterinary Basic Sciences, The Royal Veterinary College, Hatfield, Herts AL9 7TA, UK. awilson@rvc.ac.uk
authors.forename: A M
authors.initials: AM
authors.lastname: Wilson
authors.forename: M P
authors.initials: MP
authors.lastname: McGuigan
authors.forename: A
authors.initials: A
authors.lastname: Su
authors.forename: A J
authors.initials: AJ
authors.lastname: van Den Bogert
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D004548:Elasticity; D005552:Forelimb; D005684:Gait; D006736:Horses / Q000033:anatomy & histology / Q000502:physiology*; D007867:Leg Bones / Q000502:physiology*; D008124:Locomotion; D008954:Models, Biological; D018485:Muscle Fibers, Skeletal / Q000502:physiology; D018482:Muscle, Skeletal / Q000033:anatomy & histology / Q000502:physiology*; D013710:Tendons / Q000033:anatomy & histology / Q000502:physiology*; D014732:Vibration