Electromyographic and kinematic indicators of fatigue in horses: a pilot study.

This research aims to understand how the fatigue onset in galloping horses can be understood through electromyographic (EMG) signals combined with kinematic analysis. The primary hypothesis was that changes in muscle activity, as reflected in EMG median frequency, would link with changes in kinematic variables (the science of motion) during an exercise test performed by horses.

Study Design & Methodology

• The study was conducted using three Thoroughbred horses. These horses were fitted with retroreflective markers on their hooves and withers (the ridge between the shoulder bones), and surface electrodes were attached to the skin over the deltoid muscle in the forelimb. This was to aid in the tracking of kinematic data and the recording of EMG data.
• The horses were made to gallop at 110% of their maximum oxygen uptake (VO2max) on a treadmill set at an incline of 7.5%. This served as an exercise test.
• The onset of fatigue was determined as the point when the horses could not keep pace with the treadmill speed with minimal encouragement.
• The team recorded kinematic data at 200 Hz for 5 seconds at 30-second intervals, and raw EMG data were recorded at 1024 Hz for 3 seconds at 15-second intervals.

Findings & Analysis

• The researchers observed an increase in stride length (by 0.34 m), stride duration (by 0.03 seconds), and vertical excursion of the trunk marker (by 0.03 m) of the horses throughout the exercise test. These changes suggest the global indications of fatigue.
• Particularly for the horse that maintained the same lead (leading leg) during the exercise test, there was a decrease by 36% in the median frequency of the EMG signal (indicating muscle fatigue).
• For the other two horses that changed leads during their exercise test, smaller decreases in the median frequency were noted between the lead changes. Interestingly, the median frequency recovered to starting levels immediately following a lead change, implying quick recovery of muscle fatigue.
• The overall decrease in median frequency between lead changes ranged from 12% to 20%.

Conclusion

• The study reveals that kinematic changes such as stride length and duration could be considered as more overarching, global fatigue indicators.
• EMG variables, such as the median frequency, also serve as valid muscle fatigue indicators but are impacted by lead changes in the galloping horses.