Specificity and Transfer of Lower-Body Strength: Influence of Bilateral or Unilateral Lower-Body Resistance Training

Abstract

Appleby, BB, Cormack, SJ, and Newton, RU. Specificity and transfer of lower-body strength: Influence of bilateral or unilateral lower-body resistance training. J Strength Cond Res 33(2): 318–326, 2019—To examine the development of lower-body strength using either bilateral or unilateral resistance training. Developmental rugby players (n = 33; mean training age = 5.4 ± 2.9 years; 1 repetition maximum [1RM] 90° squat = 178 ± 27 kg) completed an 18-week randomized controlled training design (bilateral group [BIL], n = 13; unilateral group [UNI], n = 10; comparison, n = 10). The 8-week training phase involved 2 lower-body, volume-load matched resistance sessions per week (6–8 sets × 4–8 reps at 45–88% 1RM), differing only in the prescription of a bilateral (back squat) or unilateral (step-up) resistance exercise. Maximum strength was assessed by a randomized order of 1RM back squat and step-up testing and analyzed for within- and between-group differences using effect sizes (ES ± 90% confidence limits [CL]). Both training groups showed practically important improvements in their trained exercise (ES ± 90% CL: BIL = 0.67 ± 0.48; UNI = 0.74 ± 0.38) with transfer to their nontrained resistance exercise (BIL step-up = 0.27 ± 0.39: UNI squat = 0.42 ± 0.39). The difference between groups in adaptation of squat strength was unclear (BIL ES = −0.34 ± 0.55), while the UNI group showed an advantage in step-up training (ES = 0.41 ± 0.36). The results demonstrate that practically important increases in lower-body strength can be achieved using bilateral or unilateral resistance training and development of that strength may be expressed in the movement not trained, supporting the transfer of strength training between exercises of similar joint movements and muscles. Coaches may choose to incorporate unilateral strength training where the prescription of bilateral training may be inhibited.

Introduction

Specificity and transfer are important considerations for the design of resistance training programs to improve athletic performance (^37,41). Resistance exercises differ slightly in terms of contraction type (eccentric, concentric, or isometric), contraction velocity, and joint angles; each driving subtly different physiological adaptations (²⁹). Maximizing adaptation from resistance training to athletic performance is paramount in resistance programming. Many training studies demonstrating the transfer of strength to improved performance have incorporated bilateral resistance exercises (e.g., squat, deadlift, and power clean) (^11,16,18). An advantage of bilateral exercise is the magnitude of external load involved and the resulting development of maximal strength (^11,32,36). As a result, these exercises are frequently incorporated in resistance training for athletes.

However, given the unilateral nature of many sporting actions (e.g., sprinting and change of direction), unilateral exercises are deemed more sport specific (^25,31). Whilst the smaller base of support of unilateral compared to bilateral exercises requires altered neuromuscular coordination (stability and joint cocontraction) for successful performance, the cost is reduced external loading (^9,20,24). It is important for strength and conditioning coaches to maximize the benefits of resistance training within the busy training schedule of athletes. Given the importance of sport-specific resistance training in comprehensive athletic development, the comparison of the training benefit of unilateral with bilateral resistance training and performance requires further investigation.

Researchers have reported favorable transfer in relatively untrained individuals using the rear foot elevated split squat (RESS) as a unilateral training comparison to the bilateral back squat (³⁵). However, the external load used in the RESS is comparatively low to the back squat (approximately 50% of back squat load (³⁵)). Similar to an RESS, the barbell step-up may be a favorable alternative capable of combining instability and potentially higher external load (between 50 and 85% of 6RM squat loads (^13,40)). Therefore, the purpose of this study was to explore previously unexamined differences in lower-body maximum strength as a result of training using the back squat (or squat [bilateral]) only, or step-up (unilateral) only.

Methods

Experimental Approach to the Problem

This investigation involved an 18-week randomized controlled design training intervention. Subjects were assigned to 2 treatment groups (bilateral or unilateral resistance training) and a comparison group. The design comprised a 6-week familiarization phase (including training and testing practice and baseline testing), an 8-week training intervention (with mid-training and post-training testing), a recovery week, and a 3-week maintenance phase (concluding with final testing) (Figure 1). The maintenance phase was designed to replicate the minimum resistance training dose programed during an in-season period, common in competitive sporting environments (^4,38). Lower-body maximal strength testing was evaluated by a 1 repetition maximum (1RM) 90° squat and 90° step-up. Training was conducted during a development academy rugby preseason phase with both intervention groups participating in all training equally, with the only distinction being the volume-load matched prescription of squats (bilateral resistance training group [BIL]) or step-ups (unilateral resistance training group [UNI]) during 2 lower-body resistance training sessions per week.

Figure 1.: Schematic representation of study design.

Subjects

Twenty-three participants recruited from a state rugby union academy program and grade club competition completed required aspects of the testing and training (mean ± SD: age = 22.4 ± 4.1 years [age range: 18–29 years] , height = 185.3 ± 5.5 cm, and mass = 102.9 ± 12.0 kg). Training compliance was 96% attendance to training sessions for the intervention phase (weeks 1–8 of training), and 91% for the maintenance phase. At the completion of the baseline testing, balanced randomization procedures were used to allocate the participants into the experimental arms at a ratio of 1:1, stratified by resistance training experience (≤4 vs. >4 years) and maximal strength (≤1.5 vs. >1.5 squat 1RM to body mass ratio) (¹²). Given the training experience of the intervention cohort, accessing an appropriately matched control group (resistance training experience and relative strength), void of any training commitments was not possible. Therefore, a further cohort of 10 participants from the same rugby competition was included in a comparison (COM) group (Table 1). It was not possible to isolate this group of committed recreational athletes from their training commitments, as such, they were permitted to participate in similar club rugby playing and training requirements and individual self-regulated strength and conditioning. This group was required for testing only. All participants were notified of the potential risks involved and gave their written informed consent. This study was approved by the Research Ethics Committee at Edith Cowan University. All participants commenced free of injury or previous injury history that may have inhibited performance.

Table 1.: Participant characteristics at the commencement of the training intervention and testing.*†

Procedures

Squat Depth and Step-up Height Determination

To standardize the squat and step-up training stimulus between groups, a 90° knee angle was selected because it was observed in step-up piloting to facilitate a combination of loading and technical proficiency compared with preferential greater knee angles of squatting (¹⁰). Before the familiarization phase, participants attended an introductory session where individual squat depth and step-up box height were established. The 90° knee flexion squat depth was monitored by each participant squatting with a 20-kg Olympic barbell (Australian Barbell Company, Melbourne, Victoria, Australia) and Olympic weight plates (Eleiko, Halmstad, Sweden) to an elastic band placed on both sides of a power rack (York Fitness, Rocklea, Queensland, Australia) at their individually determined depth. For the step-up, participants were filmed performing 2 repetitions of barbell step-ups on a series of boxes of incremental step height of 20 mm from 300 to 420 mm. The 90° knee angle was defined as the minimum angle of the knee at contact of the lead foot on the step. All repetitions were analyzed, and the closest step-up box to that which resulted in a 90° knee angle was allocated to the participant.

One Repetition Maximum Testing

The 1RM protocol has been used for assessment of maximal strength (²²). The protocol involved participants completing a series of warm-up sets (4 repetitions at 50% of estimated 1RM, 3 repetitions at 70%, 2 repetitions at 80%, and 1 repetition at 90%) each separated by 3 minutes. After the warm-up, maximal attempts separated by a minimum of 5 minutes were performed until a 1RM was obtained (an average of 2.6 trials were required). Verbal encouragement was provided throughout the testing. An accredited S&C coach and at least 1 assistant observed each test for spotting, technique, and depth monitoring. The repetition was deemed a fail if the participant could not achieve the required depth or could not return to the upright position. The coefficient of variation of 1RM squat testing has been reported as 3.5% (³³). The coefficient of variation in the current cohort was 2.7% for the 1RM step-up test.

Training Programs

Training was conducted during a typical academy level rugby preseason preparation phase (Table 2) (³⁴), which involved 3 rugby skills sessions per week (60–90 minutes of duration, including rugby specific skills, tackling, passing, etc.), 2 upper-body resistance training sessions (individually prescribed for hypertrophy or strength; 4–7 exercises, 2–12 repetitions, 16–20 sets, 45–60 minutes of duration) 2 lower-body resistance training sessions (the training intervention, 60 minutes (Table 3)), 2 speed and agility sessions (30–45 minutes), and an additional cardiovascular session (30–45 minutes). The training intervention involved 2 lower-body resistance training sessions per week in which participants completed a periodized, volume-load matched (described below) program of squats (BIL group) or step-ups (UNI group). Each lower-body session was separated by 48 hours of recovery. The training venue, training equipment, and coach supervision were consistent. The only training aspect to differ between the 2 groups was the individually prescribed allocation load for squats or step-ups to the lower-body resistance training.

Table 2.: Weekly training schedule.*

Table 3.: Example of lower-body training program for each 4-week mesocycle.

Participants completed their intervention exercise, under the guidance of at least 1 coach to assist with load prescription, technical coaching, and performance monitoring. Barbell loads for the squat and step-up exercises were prescribed as a percentage of 1RM obtained at baseline, mid-testing, and post-testing (before the maintenance phase—Table 4). To determine the influence of either exercise to performance, it was critical to match the training stimuli as closely as possible using the following volume load equation: volume load = number of sets × total number of repetitions × %1RM (¹⁵) (Figure 2). In addition, a linear position transducer (LPT) (GymAware PowerTool Version 5; Kinetic, Canberra, Australia) was used to record barbell velocity and provide feedback for every repetition to each participant. The use of this device has been previously detailed (²). Performance feedback to each subject using an LPT has been demonstrated to produce superior performance during resistance training and ensured a maximal effort was achieved for all work repetitions during training (³).

Table 4.: The reps, sets, and percentage 1RM loading for squats and step-ups for each session.*†

Figure 2.: The prescribed volume load (VL) and training intensity (TI) as a percentage of 1RM of the training intervention (phases 2 and 3) based on repetitions × sets × %1RM (¹³). 1RM = 1 repetition maximum.

Testing Protocol

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.

Statistical Analyses

Descriptive statistics (mean ± SD) for strength were calculated for each testing occasion. The difference within the bilateral, unilateral, and comparison groups compared with baseline at weeks 9 and 12 was calculated using Excel (Version 2016; Microsoft, Redmond, WA, USA) (¹⁹). Data were log transformed to reduce bias due to nonuniformity of error and analyzed using the effect size (ES) statistic ± 90% confidence limits (CL) (¹⁹). In addition, the difference in the change between groups was also calculated. In all analyses, the outcome was adjusted to the mean of the stronger group in each performance task (¹⁹). The magnitude of the effect in both analyses was classified according to the following scale: 0.2–0.6 as small; 0.6–1.2 as moderate; and 1.2–2.0 as large (¹⁹). In addition, the likelihood of the effect exceeding the smallest practically important difference (0.2) was represented using the following scale: >75% as “likely”; >95% as “very likely”; and >99.5% as “almost certainly” (⁸). Effects less than 75% likely to exceed an ES of 0.2 were considered “trivial” and where the 90% CL crossed the negative and positive 0.2 values, the ES was classified “unclear.”

Results

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).

Figure 3.: Mean (±SD) and individual responses for 1RM squat (A) and 1RM step-up (B) for each treatment group. Training phase: Base = baseline testing; Mid = mid testing; End T. = end training; End M. = end maintenance. 1RM = 1 repetition maximum.

Table 5.: The magnitude of within-group changes in strength at weeks 9 and 12 compared with baseline for bilateral, unilateral, and comparison groups.*†

Table 6.: The magnitude of change in strength between the groups for each training cycle.*†

Discussion

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.

Practical Applications

Lower-body strength can be developed using unilateral (step-up) or bilateral (squat) resistance training and expressed in the nontrained variation. Coaches may be able to confidently substitute unilateral exercises for bilateral for lower-body strength development. Furthermore, the lower external loading used in unilateral exercises may beneficially unload anatomical structures, which may benefit athletes with acute or chronic injury who cannot tolerate large external loads. In addition, the integration of the step-up in a periodized plan may benefit further strength development and the improvement of advantageous secondary neuromuscular stabilizers.