Hind limb extensor muscle activation timing during trotting in Beagles shows limb-specific variability and can be referenced to kinematic events swing vertical moment and stance vertical moment

Objective: To characterize hind limb extensor muscle activation timing during trotting in clinically normal Beagles using surface electromyography (EMG) within the context of canine gait analysis and to evaluate whether event-based kinematic reference points provide a physiologically meaningful framework for interpreting activation timing. Methods: An observational exploratory study was conducted in 4 clinically normal Beagles. Synchronized surface EMG (1,000 Hz) and 3-D motion capture (240 Hz) were recorded from 8 hind limbs during unrestrained overground trotting. Threshold-based muscle onset and offset timing were determined for the middle gluteal, vastus lateralis, biceps femoris, and gastrocnemius muscles. Two kinematic events, the swing vertical moment (SwV) and the stance vertical moment (StV), were defined to anchor activation timing relative to gait mechanics. Results: Substantial inter- and intraindividual variability in muscle activation patterns was observed, including differences between contralateral hind limbs. Despite this variability, consistent intermuscle features emerged: the vastus lateralis most frequently exhibited delayed activation, whereas the middle gluteal muscle tended to terminate activity earlier during stance. Across most hind limbs, muscle onset clustered within the SwV-touchdown interval, and activity termination occurred after the StV. Conclusions: Hind limb muscle activation timing during trotting is characterized by pronounced limb-specific variability. Event-based kinematic references, such as the SwV and StV, provide a refined temporal reference for EMG interpretation without assuming fixed gait-cycle proportions. Clinical Relevance: These findings support limb-specific neuromuscular assessment and event-based temporal analysis in canine gait evaluation, with potential relevance for veterinary biomechanics research and rehabilitation planning.