Participants had a minimum of 3 days of recovery between their last lower-body strength session and strength testing. Participants followed a standardized warm-up that included stationary bike riding and lower-body mobility exercises. One repetition maximum strength testing began with a series of warm-up sets (4 repetitions at 50% of 1RM, 3 repetitions at 70%, 2 repetitions at 80%, and 1 repetition at 90%) each separated by 3 minutes rest, then a series of maximal attempts until a 1RM was achieved. The order of squat or step-up was randomized between all participants. Testing occurred inside a power cage, with safety bars. A squat was deemed a fail if the participant did not descend to the required depth or failed to achieve full extension without assistance. A step-up was judged as a fail if the participant could not fully extend the leg without assistance from the uninvolved limb. All repetitions were observed by an accredited strength and conditioning coach.
Strength performance for the BIL, UNI, and COM groups and individual responses are presented in Figure 3. The magnitude of change within each group at the end of the 8-week training intervention and 3-week maintenance phase is presented in Table 5. Both the BIL and UNI groups showed meaningful improvements in 1RM strength (BIL 1RM squat ES 0.79 ± 0.40; UNI 1RM average step-up ES 0.63 ± 0.17) during the training period (Table 5). The between-group changes at the end of the 8-week training intervention and 3-week maintenance phase are presented in Table 6. The results of 1RM squat strength between the BIL and UNI groups were unclear at all time points, while small differences in average 1RM step-up strength were observed when comparing the BIL and UNI groups during the 8-week training intervention (ES = 0.41 ± 0.36, favoring UNI group) (Table 6).
This investigation sought to explore the specificity and transfer of isoinertial strength training between bilateral and unilateral movements. In accordance with the principle of specificity, both the bilateral and unilateral training groups demonstrated moderate improvements in their trained movement. In addition, both groups also demonstrated small improvements in the nontrained movement. The primary finding being that the underlying physiological and biomechanical stimuli of neuromuscular adaptation can be developed bilaterally or unilaterally and may be exhibited to a lesser extent in performance of the nontrained variant.
It has been suggested that the closer the mechanical specificity of a training exercise to a performance, the greater the transfer of performance gain (37,39,41). For example, lower-body maximal strength is often assessed by a 1RM squat, and strength training usually involves squatting (7,11). The results of this study support this concept because both groups showed the greatest improvement in their trained exercise (Figure 3 and Table 5) and these improvements are in line with those previously reported in bilateral and unilateral training (11,35).
The phenomena of transfer is dependent on mechanical specificity (contraction type, contraction velocity, and joint angle) between the training stimulus and the performance; the closer the two, the greater the transfer (37,39). In the current study, both groups showed small strength increases in their nontrained movement indicating a level of transfer between the exercises (Table 5). These findings are similar to research in bilateral and unilateral training investigations (26,35). Notably, the improvements in strength of both groups in both exercises highlight the importance of the underlying physiological and biomechanical demands of an exercise driving adaptation, and not the outward appearance. This has practical implications where strength and conditioning coaches may experience constraints with equipment (i.e., in the case of travel or large athlete numbers) or the athlete (through acute or chronic injury) where the substitution or incorporation of a similar exercise can yield transfer benefits.
Neuromuscular differences have been reported between bilateral and unilateral movements (1,24). This is attributed to the greater stability requirements of the unilateral exercise and the neuromuscular control required for efficient performance (24). The results of this study suggest that strength improvements from a unilateral exercise can improve strength in a bilateral movement. An advantage of unilateral exercises may be in the development of coordination and stabilizer musculature that may not be sufficiently stimulated in stable, bilateral movements (24). For example, decreasing the stability of an exercise can result in increased balance requirements, antagonist recruitment and cocontraction, and trunk/hip activation levels (1,5,30). In addition, unilateral exercises require a lower total external load that would be valuable in unloading anatomical structures such as the spine (17,27). However, the increased requirement for stability has been shown to decrease the force output of agonists and, when combined with the lower external resistance possible, suggests that unilateral exercises are perhaps less effective for the development of maximal strength (23,24). However, the results from this investigation support previous work (35) and suggest that unilateral exercises can effectively develop strength and also transfer strength to bilateral performance (Table 6). However, a small difference (0.41 ± 0.36) existed between the improvement in step-up strength, in favor of the unilateral group. This suggests that training the unilateral exercise facilitated an adaptation necessary for step-up performance that the bilateral group did not experience. Whether the strength development benefits of the step-up exercise transfers to sprint and change of direction performance requires further investigation.
A unique feature of this investigation was the presence of a short maintenance phase, representative of short-term in-season phases in elite team sports often necessitated by competition, recovery, and travel. As a result, the opportunities for physical development are limited, shifting to a focus of maintaining capacity developed during the precompetition phase. Previous research has reported that 1 resistance training session per week is sufficient to maintain strength (4,6,14,28). In the current investigation, although much shorter in duration than the previously mentioned studies, both intervention groups remained relatively unchanged in their trained exercise (trivial ES changes) during the 3-week period of only one resistance session per week. This suggests that in phases of competition or travel where strength training may be limited to 1 session per week, unilateral or bilateral resistance training is sufficient to maintain strength for short periods. It should be noted that a small ES decrease in 1RM step-up strength occurred for the comparison group during this maintenance phase. This change may have been due to less variation in individual responses of this group and the maintenance periodization cycle of training prescription of intervention groups nearing competition.
Although rigorous planning was implemented, in a training study involving “real-world” athletes, it is not possible to control every aspect. The following limitations should be considered when interpreting the results. First, complexity exists in balancing workloads between groups that has been identified in previous research attempting to fairly observe the influence of bilateral and unilateral training, which may result in unequal training stimulus between the intervention groups (21,35). In addition, a 90° knee flexion angle was used to compare bilateral and unilateral exercises, and future research may investigate angles deeper than 90° (10). Finally, because of the squad nature of the group training, it was not possible to blind participants and coaches from the training interventions.
The results of this study demonstrate that lower-body strength can be developed using bilateral or unilateral means and that strength can be transferred between movements as indicated by the degree of change in the nontrained exercise in the current study. The findings of this study support the use of unilateral or bilateral exercises for improved strength development where muscular intensity is matched. Further studies should ascertain the transfer to measures of sport performance such as speed and change of direction.
The authors have no conflicts of interest to disclose.
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