Tendon strain in the forelimbs as a function of gait and ground characteristics and in vitro limb loading in ponies

Strains in the tendons of the m. flexor digitalis superficialis (superficial digital flexor, SDFT) and m. flexor digitalis profundus (deep digital flexor, DDFT) tendons, the accessory ligament of the deep digital flexor muscle (inferior check ligament, ICL) and the m. interosseus medius (suspensory ligament, SL) of 5 ponies were recorded at the walk and trot using mercury-in-silastic strain gauges (MISS), on a hard surface (brick pavement) and on sand. The horses were shod with normal, flat shoes. On pavement, strain in the SDFT, DDFT and SL increased significantly from the walk (2.19%, 1.15% and 3.36%, respectively) to the trot (4.15%, 1.70% and 5.78%, respectively), but that in the ICL did not change significantly (5.36% at the walk, 4.88% at the trot). Strains in the ICL and SL were higher on pavement than on sand (P < 0.1) and strains in the SDFT and DDFT were not significantly different. Tendon strain in the SDFT and SL, but not in the ICL and DDFT, increased (P < 0.1) in a pony at the walk on pavement with a rider. Post mortem loading of the same instrumented limbs revealed that the metacarpophalangeal joint could be further extended when the elbow joint was extended. The in vitro tendon strain was different from that in vivo, implying that results from in vitro limb loading tests have only limited value for assessing tendon functioning in vivo.