An in vivo equine forelimb model for short-term recording of peak isometric force in the superficial and deep digital flexor muscles

Objective— To develop and test an experimental model for in vivo short-term recording of peak isometric forces of the digital flexor muscles in the forelimb of adult horses. Study Design— In vivo experimental study. Sample Population— Four healthy, anesthetized, adult Thoroughbred horses (3 to 7 years old; 527 ± 87 kg) Methods— In dorsal recumbency, ulnar and median nerves were exposed and instrumented with insulated bipolar cuff stimulation electrodes for later connection to an electrical stimulator. In left lateral recumbency, a biplanar fixator was applied to the right humerus and a custom-made, rigid, aluminum frame connected to it, to allow loading of muscles distal to the fixator. Threaded transfixation pins through the radial and metacarpal condyles were clamped to the rigid frame so that the humerus, radius, ulna, and metacarpus were fixed in position. Each digital flexor muscle insertion tendon was transected just above the metacarpophalangeal joint, extracted from the carpal canal, and secured in a metal clamp positioned at the distal myotendinous (MT) junction. Distally, the clamp was connected in series to a load cell and a pneumatic actuator to record force and to maintain muscle length during nerve stimulation. A linear potentiometer was connected in parallel to the actuator to record MT junction position. Initial trials were conducted to identify median and ulnar nerve stimulation variables to achieve maximal muscle contraction. Isometric contractions were performed at different muscle lengths and peak forces registered during 3 seconds of supramaximal dual (ulnar and median) nerve stimulation. Results— A stimulation voltage of 2.5 to 5.0 V at 50 Hz usually produced maximal force for both the superficial digital flexor (SDF) and deep digital flexor (DDF) muscles. Single ulnar and median nerve stimulation elicited force development not only in the DDF muscle but also in the SDF muscle. At voltages higher than 1 V, normalized force was greatest with combined median and ulnar nerve stimulation for both the DDF and SDF muscles; however, normalized force was greater for median nerve stimulation than ulnar nerve stimulation in the DDF muscle, and the opposite relationship was observed for the SDF muscle. Final recording of dual supramaximal nerve stimulation of SDF and DDF muscles resulted in peak isometric forces of 716 ± 192 N and 1,577 ± 203 N, respectively. Conclusions— The instrumentation technique and experimental protocol enabled recording of peak isometric forces in the SDF and DDF muscles of anesthetized adult horses. Clinical Relevance— Studies using this model will improve knowledge of SDF and DDF muscle mechanics with insight to functional implications of the complex architecture of these muscles. Knowledge of the dynamic performance of the SDF and DDF muscles would also be useful for the development of new treatment strategies for flexor deformities and tendon injuries in horses.