How do treadmill speed and terrain visibility influence neuromuscular control of guinea fowl locomotion?

Locomotor control mechanisms must flexibly adapt to both anticipated and unexpected terrain changes to maintain movement and avoid a fall. Recent studies revealed that ground birds alter movement in advance of overground obstacles, but not treadmill obstacles, suggesting context-dependent shifts in use of anticipatory control. We hypothesized that differences between overground and treadmill obstacle negotiation relate to differences in visual sensory information, which influence the ability to execute anticipatory manoeuvres. We explored two possible explanations: 1) previous treadmill obstacles may have been visually imperceptible, as they were low contrast to the tread, and 2) treadmill obstacles are visible for a shorter time compared to runway obstacles, limiting time available for visuomotor adjustments. To investigate these factors, we measured electromyographic activity in 8 hindlimb muscles of the guinea fowl (Numida meleagris, n=6) during treadmill locomotion at two speeds (0.7ms⁻¹ and 1.3ms⁻¹) and three terrain conditions at each speed: (i) level, (ii) repeated 5cm low contrast obstacles (<10% contrast, black/black), and (iii) repeated 5cm high contrast obstacles (>90% contrast, black/white). We hypothesized that anticipatory changes in muscle activity would be higher for 1) high contrast obstacles, and 2) the slower treadmill speed, when obstacle viewing time is longer. We found that treadmill speed significantly influenced obstacle negotiation strategy, but obstacle contrast did not. At the slower speed, we observed earlier and larger anticipatory increases in muscle activity and shifts in kinematic timing. We discuss possible visuomotor explanations for the observed context-dependent use of anticipatory strategies.