Summary Strains in the superficial digital flexor tendon (SDFT), deep digital flexor tendon (DDFT), accessory ligament of the deep digital flexor muscle (inferior check ligament [ICL]) and the interosseus medius muscle (suspensory ligament [SL]) in the right forelimb of 5 ponies were measured using mercury‐in‐silastic strain gauges a few hours after implantation. Tendon strains were recorded at the walk with normal flat shoes, egg‐bar shoes, a 7° increased hoof angle accomplished by application of a heel‐wedge and a 7° decreased hoof angle using a toe‐wedge, consecutively. Ground reaction forces were recorded with all 4 shoe types preoperatively and with flat shoes post operatively. The strain patterns of the SDFT, DDFT and SL showed a rapid increase at the beginning of the stance phase, followed by a plateau with a small incline or decline and a rapid decrease at the end of the stance phase. The SDFT had its maximal strain in the first half of the stance phase in all ponies. The DDFT and SL reached their maximal strain in the first half of the stance phase in 2 ponies and in the second half of the stance phase in the other 3 ponies. The ICL was strained maximally in the second half of the stance phase in all ponies. Averaged over all 5 ponies, the maximal strains in the SDFT, DDFT, ICL and SL with normal flat shoes were 2.4, 1.3, 5.4 and 3.7%, respectively. If an egg‐bar was applied the mean peak strain in the DDFT was 0.13% lower and strain in the SL was 0.22% higher. With a heel‐wedge, strain decreased in the DDFT and ICL (0.19% and 0.4%, respectively) and increased by 0.24% in the SL. A toe‐wedge increased strain in the ICL by 0.8%. All changes mentioned were statistically significant (P<0.1). The changes in tendon strain as a result of different types of shoeing correlated with changes in calculated torque's of the ground reaction force acting on the coffin joint.

Summary 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.

The load distribution over tendinous structures in the equine forelimb was studied by computing forces from in vivo signals of implanted liquid-metal strain gauges in 5 ponies. For validation, these tendon forces were converted to joint moments, which were summed and compared to the calculated joint moments caused by the ground reaction force. Mean peak forces per kilogram body weight (n = 5) amounted to 5.2 N/kg for the superfical digital flexor tendon, 3.8 N/kg for the deep digital flexor tendon, 7.3 N/kg for the distal accessory (check) ligament and 8.4 N/kg for the third interosseous muscle (suspensory ligament). The maximal moment exerted by the tendons about the fetlock joint differed 11 ± 7% (average ± SD, n = 5) from the maximal ground reaction force moment, which difference amounted to 17 ± 15% for the coffin joint moments. These differences appeared to result to a substantial extent from errors in the moment arms. Therefore, the computed tendon forces were considered to be sufficiently reliable.

The in vivo strains of the musculus interosseus medius (suspensory ligament) and its rami extensorii (extensor branches) in the forelimb of the horse were determined from angular changes of the metacarpophalangeal and the distal interphalangeal joints. For this purpose, regression models were fitted to strains and joint angle combinations measured in in vitro limb loading experiments. The in vivo strains were computed from the kinematics of 8 horses at the walk, the trot and the canter. It was found that the extensor branches were strained about 1.0% at hoof impact, which indicates that they passively extend the interphalangeal joints just prior to impact and prevent flexion of the pastern joint just thereafter. The maximal strain of the suspensory ligament amounted to 3.4% at the walk, 5.6% at the trot and 6.3% at a slow canter.

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