Quantitative differences in activities of back and pelvic limb muscles during walking and trotting between chronically lame and nonlame horses American Journal of Veterinary Research September 2009 70 9 1129-1134 Objective—To assess differences in activities of back and pelvic limb muscles by use of surface electromyography (SEMG) in chronically lame and nonlame horses during walking and trotting. Animals—12 nonlame horses and 12 horses with unilateral chronic mild to moderate pelvic limb lameness. Procedures—On each horse, bipolar electrodes were attached to the skin over the midpoints of the right and left longissimus thoracis (Lot), semitendinosus (Set), biceps femoris (Bif), gluteus medius (Glm), and extensor digitorum longus (Edl) muscles. For each muscle, synchronous kinematic and SEMG recordings were made during walking and trotting on a treadmill; mean, maximum, and minimum muscle activities and maximum-to-mean and minimum-to-mean activity ratios were determined. For each lame or nonlame horse, data from both pelvic limbs were averaged; in lame horses, data from the nonlame and lame pelvic limbs were also examined separately (NL-L and L-L values, respectively). Comparisons were made among the 4 data sets and between gaits. Results—During walking, the NL-L maximum-to-mean ratios for Bif and Glm muscles were significantly greater and in lame horses, L-L and NL-L minimum-to-mean ratios for Set, Bif, Glm, and Lot muscles were significantly less than those for nonlame horses. During trotting, minimum-to-mean ratios for Set, Glm, and Lot muscles in lame horses were significantly lower than those for nonlame horses. Activity of the Edl muscle was not affected by lameness. Conclusions and Clinical Relevance—In lame horses, variation in muscle use was detectable via SEMG. In chronically lame horses, back and pelvic limb muscle activities were affected differently during walking and trotting.