biostudies-literatureSun SYNICHD NIH HHSNational Institute of Child Health and Human Development357-366https://www.ebi.ac.uk/biostudies/studies/S-EPMC9969848biostudies-literatureUnknown59(3)Prior to hatching, chick embryos spontaneously produce repetitive limb movements (RLMs), a developmental precursor to walking. During RLMs, flexor and extensor muscles are alternately active as during stance and swing phases of gait. However, previous studies of RLMs observed that flexor muscles were rhythmically active for many cycles, whereas extensors often failed to be recruited. Thus, we asked if flexor muscles are preferentially recruited during RLMs in chick embryos 1 day before hatching and onset of walking. Using a within-subject design, we compared EMG burst parameters for flexor and extensor muscles acting at the hip or ankle. Findings indicated that flexor burst count exceeded extensor count. Also, flexor muscles were consistently recruited at the lowest levels of neural drive. We conclude that there is a bias favoring flexor muscle recruitment and drive during spontaneously produced RLMs. Potential neural mechanisms and developmental implications of our findings are discussed.Developmental psychobiologyDifferences in flexor and extensor activity during locomotor-related leg movements in chick embryos.PMC9969848R01 HD053367ROI HD 053367Bradley NSSun SYfalseDifferences in flexor and extensor activity during locomotor-related leg movements in chick embryos.Prior to hatching, chick embryos spontaneously produce repetitive limb movements (RLMs), a developmental precursor to walking. During RLMs, flexor and extensor muscles are alternately active as during stance and swing phases of gait. However, previous studies of RLMs observed that flexor muscles were rhythmically active for many cycles, whereas extensors often failed to be recruited. Thus, we asked if flexor muscles are preferentially recruited during RLMs in chick embryos 1 day before hatching and onset of walking. Using a within-subject design, we compared EMG burst parameters for flexor and extensor muscles acting at the hip or ankle. Findings indicated that flexor burst count exceeded extensor count. Also, flexor muscles were consistently recruited at the lowest levels of neural drive. We conclude that there is a bias favoring flexor muscle recruitment and drive during spontaneously produced RLMs. Potential neural mechanisms and developmental implications of our findings are discussed.2017-01-01T00:00:00Z2017 Apr2025-04-04T00:35:05.405Z2025-04-04T00:35:05.405ZS-EPMC99698482832334810.1002/dev.21500