Effect of increased kick frequency on propelling efficiency and muscular co-activation during underwater dolphin kick

•Swimmers performed dolphin kick swimming trials at different kick frequencies.•Surface electromyography of trunk, thigh, and leg muscles was performed.•Muscular co-activation phase increased with increasing kick frequency.•Increasing kick frequency affects propelling efficiency and muscular co-activation.•Reduced propelling efficiency correlated with co-activation phase increment change.