In mammals, play behaviour appears innate and, because of this, may provide insight into the frequency and intensity of load that is required to stimulate positive musculoskeletal development. The objective of this review was to explore the interaction between play and tissue (bone) development at a molecular through to whole-animal level, with specific focus on the horse as a model. The basis of our understanding of the response of bone to loading is the mechanostat theorem. This assumes that at a tissue level, bone attempts to keep localised strain within the physiological range of 1500-2500 microstrain. Loads above this range result in a modelling response to reduce strain, and strain below this threshold results in remodelling to maintain the localised physiological range. In foals, locomotor play is dramatic and vigorous, with cumulative increases in both intensity and complexity. Based on published literature describing locomotor play in foals and the microstrain at different gaits in the horse, it was proposed that locomotor play in foal aligns with the mechanostat theorem in both the magnitude and frequency of load cycles applied. The cumulative increases in the complexity and intensity of locomotor play as the foal develops, in turn, ensure the strain rates associated with play remain above the local physiological range and promote material and architectural changes in the distal limb bones. Thus, spontaneous locomotor play may be vital to ensure optimal bone development in the horse. Modern management systems need to provide appropriate opportunities for foals to perform spontaneous locomotor play to optimise bone development and reduce the risk of future musculoskeletal injury later in life.
Keywords: bone; foal; horse; mechanostat; play.