It is unknown if higher antagonist muscle co-activation is a factor contributing to higher energy cost of cycling in older adults. We determined how age, power output, and cadence affect metabolic cost and lower extremity antagonist muscle co-activation during submaximal cycling. Thirteen young and 12 older male cyclists completed 6-minute trials at four power output-cadence conditions (75W-60rpm, 75W-90rpm, 125W-60rpm, and 125W-90rpm) while electromyography (EMG) and oxygen consumption were measured. Knee and ankle co-activation indices were calculated using vastus lateralis, biceps femoris, gastrocnemius, and tibialis anterior EMG data. Net rate of energy cost of cycling was higher in older compared to young cyclists at 125W (p=0.002) and at 90rpm (p=0.026). No age-related differences were observed in the magnitude or duration of co-activation about the knee or ankle (p>0.05). Our results indicated knee and ankle co-activation is not a substantive factor contributing to higher energy cost of cycling in older adults.