Reported in our previous study on passive cycling support, the energy cost of knee extension can be reduced using the energy stored from knee flexion by torsion spring. In the current study, the planar spiral spring is applied to attain the compact design of the cycling augmented knee exoskeleton (CAKE-2). The surface electromyography (EMG) results over the rectus femoris muscles of three healthy male participants performing constant power cycling on a trainer at 200 W and 225 W are analyzed in time-frequency via the continuous wavelet transform. In all cycling tests with and without the exoskeletons worn on both legs, no sign of peripheral muscle fatigue or significant change in the EMG median power spectral frequency (MDF) appears throughout the two-minute cycling trials. At the same cycling speed and leg cadence, the average of EMG-MDF increases with the exercise intensity. At the same cycling power, less quadriceps activity can be observed from all the participants when the spring support was used during cycling. The capability to modify the unbalanced effort required from the quadriceps and the hamstring during cycling without requiring an external energy source is applicable for cycling enhancement and rehabilitation applications.