Background The modality of resistance training (e.g., plyometric training [PLYO], traditional resistance training [TRT]) can specifically affect gains in muscle strength and power measures in soccer players. Further, effects of resistance training modality on neuromuscular performance measures (e.g., electromechanical delay [EMD]) are inconsistent in non-athletes. Objective To examine the effects of PLYO, TRT, and combined PLYO and TRT (i.e., complex training [CT]) on muscle strength, power, and EMD in highly trained male soccer players (age: 21.7 ± 1.6 years). Methods Participants were randomly assigned to eight weeks of CT (n = 30), PLYO (n = 28), and TRT (n = 26) in addition to their regular soccer training, or soccer training control condition (n = 29). Pre- and post-measurements comprised tests for maximal eccentric knee flexor and concentric knee extensor peak torque (PT), their functional ratio (FUNC-H/Q), and the corresponding EMD of the knee flexors at three angular velocities (60, 180, and 300°/s). Results Results showed a significant main effect of group for concentric and eccentric PT (p<.001, 1.67 = d = 3.17). Concentric and eccentric PT were larger for CT, PLYO, and TRT groups compared to control group (CG) with the greatest effects for CT across all angular velocities (p<.001, 0.95 = d = 3.57). Additionally, a significant group effect was identified for FUNC-H/Q at 300°/s (p=.002, d = 0.83) with lower ratios in PLYO and TRT groups but not CT compared with CG (p<.001, -0.94 = d=-0.83). Further, significant and large-sized group effects for EMD were found at 60, 180, and 300°/s (p<.001, 0.96 = d = 2.89). Post-hoc test showed lower EMD values in CT compared with PLYO, TRT, and CG (p<.001, -2.88 = d=-0.87). Moreover, EMD was lower in PLYO and TRT compared with CG at 60 and 300°/s (p<.001, -1.84 = d=-1.10). Conclusions Complex training, PT, and TRT were effective training modalities in improving neuromuscular performance (i.e., maximal eccentric knee flexor/concentric knee extensor PT, knee flexor EMD) in highly trained soccer players. More specifically, CT appears to be superior in maintaining higher FUNC-H/Q values compared with PLYO and TRT. Trial Registration: This study does not report results related to healthcare interventions using human participants and therefore it was not prospectively registered on a clinical trial registry. Key Points - Complex training, plyometric training, and traditional resistance training are effective resistance training modalities in improving lower limb muscle strength and knee flexors` electromechanical delay in highly trained male soccer players. - Complex training appears to be more effective to improve maximal isokinetic muscle strength when compared with plyometric and traditional resistance training potentially due to larger decrements in electromechanical delay. - The functional hamstrings-to-quadriceps strength ratio could be maintained with complex training, whereas strength ratios were lower following plyometric and traditional resistance training particularly at high angular velocities (i.e., 300°/s).
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