Variation of Lower-Limb Muscle Activation Asymmetry in Step Incremental and Constant-Power Pedaling Exercise

Asymmetry, defined as unequal neuromuscular activation or mechanical performance between contralateral limbs, plays a critical role in cycling efficiency and injury risk. While kinematic and kinetic measures are commonly used to assess asymmetry, surface electromyography (EMG) signals offer an additional perspective on neuromuscular asymmetry. This study evaluated muscle activation asymmetry during cycling using the Normalized Symmetry Index (NSI), a metric that quantifies differences in kinematics and kinetics between limbs, where higher values indicate greater asymmetry. NSI was calculated from EMG recordings of seven lower-limb muscles under two test conditions: step incremental and constant-power cycling to task failure. Twenty recreationally active participants performed both tests on a stationary ergometer while EMG data were collected bilaterally. Step incremental cycling resulted in a significant reduction in NSI for key muscles in the quadriceps group: vastus medialis (from 44% to 21%, <i>p</i> < 0.001), vastus lateralis (from 45% to 22%, <i>p</i> = 0.002), rectus femoris (from 54% to 24%, <i>p</i> < 0.001), and biceps femoris (from 52% to 29%, <i>p</i> = 0.003). No significant changes were observed for the tibialis anterior, soleus, or gastrocnemius medialis. In contrast, under constant-power conditions, NSI values remained unchanged over time for all muscles (all <i>p</i> > 0.05), with average NSI values ranging from 12% to 30%, indicating consistent bilateral activation. These findings highlight the sensitivity of surface EMG in detecting workload-dependent changes in muscle activation asymmetry and suggest that higher cycling intensities, compared to lower ones, may promote more balanced engagement of primary cycling muscles.