An Investigation Into the Short-Term Effects of Photobiomodulation on the Mechanical Nociceptive Thresholds of M. Longissimus and M. Gluteus Medius, in Relation to Muscle Firing Rate in Horses at Three Different Gaits

Back pain is a common condition in horses, yet despite this, quantitative assessments of the efficacy of treatment are scarce. Mechanical nociceptive thresholds (MNTs) and acoustic myography (AMG) recordings were obtained, both preinterventionand postintervention, from the left and right epaxial muscles in eight healthy general riding horses (mean age 17 ± 6 yrs). Using an algometer, MNT readings were taken at each of the 6 preselected points along the thoracolumbar M. longissimus and M. gluteus medius region. AMG recordings of the M. longissimus and M. gluteus medius were taken while walking, trotting, and cantering on a left or right hand 20m circle on a longe, on a waxed sand surface in an indoor arena. Horses were then treated using a class 1 laser. Therapy was applied for 1 minute at 1000 Hz to the same preselected points from which MNT measurements had previously been taken. Measurements were subsequently taken 1 hour and 24 hours post-treatment for MNT reading, and only 24 hours after for AMG measurements. No significant effect of treatment was noted for the MNTs. The AMG results were analyzed in terms of their temporal summation (T-score), where statistically significant improvements in the T-scores for M. longissimus and M. gluteus medius were noted for the different gaits. It is concluded that cold laser therapy has a positive effect on horse muscles that reveals a change in their firing frequency that is commensurate with changes seen with analgesia in subjects experiencing pain.