Effect of head and neck position on outcome of quantitative neuromuscular diagnostic techniques in Warmblood riding horses directly following moderate exercise



Reasons for performing study: There has been growing interest in training techniques with respect to the head and neck position (HNP) of the equine athlete. Little is known about the influence of HNP on neuromuscular transmission in neck muscles.

Objective: To test the hypothesis that different HNPs have effect on single fibre (SF), quantitative electromyographic (QEMG) examination and muscle enzyme activity directly after moderate exercise.

Methods: Seven Warmblood horses were studied using a standard exercise protocol in 5 HNPs: HNP1: unrestrained; HNP2: neck raised; bridge of nose around the vertical; HNP4: neck lowered and considerably flexed, bridge of nose pointing towards the chest; HNP5: neck raised and considerably extended; bridge of nose in front of the vertical; HNP7: neck lowered and flexed; bridge of nose pointing towards the carpus. Mean consecutive difference (MCD) of single muscle fibre potentials and motor unit action potential (MUP) variables (amplitude, duration, area, turns and phases) were recorded in each fixed position directly after exercise at rest using commercial EMG equipment. Muscle enzyme activity was measured before and 4, 6 and 24 h after exercise.

Results: Mean consecutive difference in all HNPs was higher than in HNP1 (22 µs, P<0.001) of which HNP4 was highest with 39 µs compared to 30 µs in HNP2 (P = 0.04); MCD in HNP 5,7 was with 25 µs lower than in HNP 2 and 4 (P<0.001). Odds ratio for MCD suggestive for conduction delay or block was 13.6 in HNP4 compared to HNP1 (P<0.001). Motion unit action potential variables followed the same pattern as MCD. Lactate dehydrogenase (LDH) activity increased in HNP4 at 4 h (P = 0.014), 6 h (P = 0.017) and 24 h (P = 0.038) post exercise and in HNP5 and HNP7 at 4 h (P = 0.037; 0.029).

Conclusions and clinical relevance: HNP4 in particular leads to a higher rise in LDH activity, MCD and MUP variables, indicating that HNPs have effect on variables characterising neuromuscular functionality.