A method for detecting the temporal sequence of muscle activation during cycling using MRI

Surface electromyography (EMG) can assess muscle recruitment patterns during cycling, but has limited applicability to studies of deep muscle recruitment and electrically stimulated contractions. We determined whether muscle recruitment timing could be inferred from MRI-measured transverse relaxation time constant (T₂) changes and a cycle ergometer modified to vary power as a function of pedal angle. Six subjects performed 6 min of single-leg cycling under two conditions (E0°-230° and E90°-230°), which increased the power from 0°-230° and 90-230° of the pedal cycle, respectively. The difference condition produced a virtual power output from 0-180° (V0°-180°). Recruitment was assessed by integrating EMG over the pedal cycle (IEMG) and as the (post-pre) exercise T₂ change (ΔT₂). For E0°-230°, the mean IEMG for vastus medialis and lateralis (VM/VL; 49.3 ± 3.9 mV.s; mean ± SE) was greater (P < 0.05) than that for E90°-230° (17.9 ± 1.9 mV.s); the corresponding ΔT₂ values were 8.7 ± 1.0 and 1.4 ± 0.5 ms (P < 0.05). For E0°-230° and E90°-230°, the IEMG values for biceps femoris/long head (BFL) were 37.7 ± 5.4 and 27.1 ± 5.6 mV.s (P > 0.05); the corresponding ΔT₂ values were 0.9 ± 0.9 and 1.5 ± 0.9 ms (P > 0.05). MRI data indicated activation of the semitendinosus and BF/short head for E0°-230° and E90°-230°. For V0°-180°, ΔT₂ was 7.2 ± 0.9 ms for VM/VL and -0.6 ± 0.6 ms for BFL; IEMG was 31.5 ± 3.7 mV.s for VM/VL and 10.6 ± 7.0 mV.s for BFL. MRI and EMG data indicate VM/VL activity from 0 to 180° and selected hamstring activity from 90 to 230°. Combining ΔT₂ measurements with variable loading allows the spatial and temporal patterns of recruitment during cycling to be inferred from MRI data.