INTRODUCTION:
Eccentric training, which involves muscle lengthening under load, enhances athletic performance and prevents injuries. While most interventions focus on bilateral exercises, unilateral movements are crucial in real-life activities, aiding balance, coordination, and rehabilitation. However, differences in muscle activation between unilateral and bilateral exercises remain unclear. Therefore, the current study investigated these variations to guide training and rehabilitation programs.
METHODS:
A cross-sectional laboratory-controlled design was used. Twenty-five male participants performed four self-weighted eccentric tasks: single- and double-leg Nordic hamstring exercise (NHE-S/NHE-D) and eccentric leg extension (ELE-S/ELE-D). Electromyographic (EMG) data were collected using the Delsys Trigno Wireless system (Delsys Inc., Natick, MA, USA) from dominant side muscle groups: rectus abdominis (DRA), lumbar erector spinae (L3ES), multifidus (L5MF), biceps femoris long head (BFlh), semitendinosus (ST) at mid (50%) and distal (30%) regions, and gluteus maximus (Gmax). EMG parameters included RMS (normalized to MVIC), mean frequency (MF), waveform length (WAV), and tension index (TI). A Mann-Whitney U test was used to analyzed EMG features across four tasks. (P<0.05).
RESULTS:
The BFlh and Gmax exhibited significantly higher RMS values during unilateral tasks compared to bilateral tasks (P<0.001). While the ST showed no significant difference in the NHE, it demonstrated higher RMS values during ELE-S (P<0.01). In terms of trunk muscle (DRA, L3ES, L5MF) responses, most of them showed no significant differences in activation, except for the DRA during NHE-S exhibited increased RMS (P=0.01). The lumbar muscles (L3ES, L5MF) had significantly higher activation levels compared to the DRA (P<0.01, all comparisons), with the L5MF displaying higher levels of MF and TI than the L3ES during ELE (P<0.01, all comparisons). Regarding regional patterns, MF values consistently followed the hierarchy: BFlh50 > ST50 > ST30 > BFlh30 (P<0.05). The RMS of ST50 exceeded that of BFlh50 only during NHE-D (P=0.02), and ST50 demonstrated greater WAV and TI compared to ST30 during NHE (P<0.05, all comparisons). These differences in inter-muscle comparisons were consistent across both unilateral and bilateral patterns.
CONCLUSION:
Unilateral eccentric training elicited greater activation in BFlh and Gmax than bilateral training, while ST showed increased activation only in ELE. Trunk muscle activation remained largely unchanged, except for higher DRA activation in NHE-S. L5MF exhibited greater fatigue and tension than L3ES in ELE, and muscle fatigue patterns followed a consistent ranking (BFlh50 > ST50 > ST30 > BFlh30). These findings suggest that unilateral training can enhance hamstring activation and may be valuable for athletic performance, rehabilitation, and injury prevention.