Effects of Strength Training Using Unstable Surfaces on Strength, Power and Balance Performance Across the Lifespan: A Systematic Review and Meta-analysis

Devices with varying degrees of instability are frequently employed for athletic and everyday performance enhancement, balance promotion and musculoskeletal health to mimic the demands of the various tasks in applied settings [1, 2]. There are many devices that attempt to provide an unstable surface. These devices include air-pressurized balls (e.g. Swiss, physio or exercise balls), hemispherical balls with an inflated dome side and a hard rubber flat side (e.g. the BOSU^® ball), inflatable discs, wobble or balance boards, foam tubes, and high- and low-density foam platforms, as well as many other related devices. Unstable devices promote postural disequilibrium or imbalance, as postural sway may project the centre of mass beyond the device’s area of support. Unstable devices also promote postural disequilibrium as the surface distorts (e.g. a low-density foam cushion) readily in response to the reaction forces associated with changes in the centre of pressure.

According to the principle of training specificity [3, 4], training must simulate as closely as possible the demands of the task or activity. Tasks such as sports and fitness activities, occupational tasks and activities of daily living often occur on relatively unstable surfaces (e.g. skiing, snowboarding, skating, walking and working in icy or muddy conditions). Willardson [5] stated that “the optimal method to promote increases in balance, proprioception and spinal stability for any given sport is to practice the skill itself on the same surface on which the skill is performed in competition”. Similarly, Schmidtbleicher [6] stated that intermuscular coordination can only be developed by practising the movement for which coordination is sought. Unfortunately, with some seasonal sports, specific training is not possible year round (e.g. skiing in the summer or baseball in the snow). Therefore, alternative challenges using unstable surfaces could be included in training to provide a progressive overload and to stimulate strength and balance adaptations. Strength training on stable surfaces (STS), such as squats, deadlifts and Olympic lifts, is conducted with a moderate degree of instability [1, 7‐10]. Greater degrees of instability are provided when strength training is conducted on unstable surfaces (STU) or with unstable implements.

Proponents of unstable devices suggest that the greater instability may stress the neuromuscular system to a greater extent than STS [11, 12]. The rationale is that destabilizing training environments may enhance neuromuscular adaptations and training specificity, while providing a more varied and effective training stimulus. The Canadian Society for Exercise Physiology position stand [7] indicates that there are functional health benefits of STU (e.g. improved joint stability and reduced lower-extremity injury rates). In addition, improved strength, balance and functional performance have been reported following STU in primarily young healthy adults [13]. Further, STU appears to be a suitable training regimen to be implemented in the rehabilitative context and/or the geriatric context [8, 9]. In fact, the application of STU is not restricted to young healthy adults. For example, more ‘vulnerable’ cohorts due to biological aging (i.e. seniors) or maturational processes (i.e. children, adolescents) may particularly benefit from STU because surface instability allows and demands lower training loads but at the same time sufficiently and adequately stimulates the neuromuscular system of youth and seniors [1, 2, 7‐9]. Given that only a few studies have investigated the effects of STU as a promising training regimen in seniors, more research is needed to elucidate the effects of STU in seniors and to find out whether it is more effective than traditional STS. Further, the biological concept of ‘critical or sensitive maturational periods’, i.e. periods during which ontogenetic development reaches a qualitatively new level that provides opportunities for further improvement of an organ, tissue and/or physiological function [14], may imply that the adaptive potential following STU is also high in youth. In fact, Behm and Colado Sanchez [2] propagated the use of STU for performance enhancements in youth. However, it is not known whether increases in muscle strength, power and balance performances are comparable across the lifespan. A lifespan approach appears to be important because experts have reported in narrative reviews that STU is a meaningful and promising training regimen for youth, adults and seniors [2, 15]. However, these statements lack verification, which is why there is a need for this meta-analysis across the age continuum. Further, meta-analyses represent the highest evidence level on the evidence pyramid [16]. Given that this topic has been addressed by narrative reviews only [2, 15], a meta-analysis may further our knowledge in this area by providing in-depth information (representing a high level of evidence).

Therefore, a synthesis of the literature is needed to determine whether or not STU provides additional effects on measures of muscle strength, power and balance in comparison with STS. The purpose of the present systematic review and meta-analysis was to provide a study on the highest evidence level in evidence-based medicine regarding the effects of STU on measures of muscle strength, power and balance administered in the form of controlled trials in healthy individuals across the lifespan. It is hypothesized that STU produces similar or even larger performance enhancements than STS because the performance of STU is highly demanding for the neuromuscular system (i.e. additional joint and postural stability are needed during exercise). Further, on the basis of expert opinion [2, 15] and selected studies [17, 18], we expected that STU would be particularly suitable and effective in seniors and youth because it has previously been shown that strength training using low loads produced similar or even larger performance gains in these age groups.

We followed the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) statement guidelines provided by Moher et al. [19] when conducting our systematic review and meta-analysis.

This is the first systematic literature review and meta-analysis to examine the effects of STU on measures of muscle strength, power and balance, administered in the form of controlled trials in healthy individuals across the lifespan. Twenty-two controlled trials (four in adolescents, 15 in young adults and three in old adults) were included in this review. The major findings of this review were that STU as compared with CON is effective in improving variables of muscle strength, power and balance in healthy adolescents, young adults and old adults when tested on stable surfaces. Further, small overall effects were found for measures of strength and balance in adolescents and young adults in favour of STU compared with STS. However, our item-specific analyses (e.g. maximal strength) revealed no consistent advantage of STU when it was compared with STS particularly in adolescents and young adults.

Adherents of STU would propose that such exercises provide advantages over STS, due to training specificity principles (i.e. exercising on unstable devices for unstable task demands during sports, fitness and in the workplace) [3, 4], while the greater degree of instability provides greater stress and thus greater opportunities for training adaptations of the neuromuscular and balance systems [11, 12]. The question of training specificity could not be directly addressed in this review, as the testing measures were typically laboratory based. The studies often used tests such as a one-legged stance, as well as strength and power measures, such as bench press, squat, CMJ, SJ and others, that were performed under relatively stable and stationary conditions (Table 1). The proposed training specificity of STU should be more apparent in tests performed under more unstable conditions (e.g. jumping from or landing on a balance pad). However, a counter-argument would concede that exercises used for STU are also typically static (stationary) exercises (performed without translational movement) and thus STU training specificity would not apply to dynamic, mobile, unstable activities, such as ice hockey, beach volleyball or soccer on a muddy field. For example, static STU [13] and dynamic STU [44] did not provide additional benefit on a dynamic balance test nor on a 20-m hopping test for speed. Balance improvements with the hopping test would have been expected to improve limb power output and decrease contact time, resulting in faster speeds. While Sparkes and Behm [22] found no significant training-specific differences following 8 weeks of static STU and STS, they did report a trend (p = 0.06) for the STU group to improve the stable to unstable chest press force ratio to a greater degree (25 %) than the stable group (11 %). Although the trunk is supine upon a stable bench during the bench press, the load is suspended above the body, and the movement must be controlled with appropriate trunk and joint muscle stabilization. However, on the basis of the available data, it seems that STU does not provide systematic training-specific balance advantages over STS, irrespective of the age group considered (i.e. adolescent and young adults).

The lack of superiority of STU for balance measures may illustrate a dose–response or intensity–response relationship. While specific studies comparing the extent of centre of pressure excursions with STU versus STS are not available, it is generally accepted that STS involving free-weight lifts provides moderate levels of instability [1, 8, 9]. The placement and movement of bars, dumbbells and other implements on the shoulders (i.e. squats), above the body (i.e. shoulder presses, cleans, snatches) or in front of the body (i.e. bicep curls), for example, places disruptive torques outside the centre of gravity, challenging the system to maintain equilibrium. Although the challenges to postural stability may be greater during performance of a resistive exercise on an unstable surface, the present results demonstrate that this greater degree of stress does not lead to greater systematic balance improvements in adolescents and young adults.

The lower force and power outputs [1, 8, 9, 20, 49‐52]—as well as the decreased movement velocity and range of motion [50]—associated with STU could result in less rigorous strength and power training adaptations. However, not all studies that have investigated the effectiveness of STU reported force reductions under unstable conditions [43, 53, 54]. The Canadian Society for Exercise Physiology position stand [7] warns that “From a performance standpoint, unstable devices should not be utilized when hypertrophy, absolute strength, or power is the primary training goal, because force generation, power output, and movement velocity are impaired and may be insufficient to stimulate the desired adaptations, especially in trained athletes” (p. 110). However, the present results suggest that at least for non-elite athletes, there is a strength and power training stress/intensity plateau that is sufficient to induce positive training adaptations. In their review, Behm and Colado [8] reported that the mean force deficit with STU compared with similar STS exercises was 29 %. The present review indicates that in comparison with STU, exceeding that plateau by introducing greater strength or power challenges with STS does not provide significant advantages across the lifespan. As there were no age-specific differences in the training response, STU can be employed by the non-elite training population to improve strength and power and to achieve functional health benefits. In fact, the approximate STU-induced 30 % force deficit [8] could be viewed alternatively as a benefit, as the lower external forces or torques might decrease the chance or incidence of training-related injuries or might be more beneficial for rehabilitation of an injured muscle group [8].

Furthermore, the American College of Sports Medicine recommends that older adults should conduct strength training using light loads (40–50 % 1RM) at the beginning of training and moderate loads near the end of training (60–70 % 1RM) [55]. Similarly, low- to moderate-intensity strength training is recommended in youth [17]. Hence, the instability-related lower force outputs during performance of STU may not represent a compromising issue regarding neuromuscular adaptive processes in old adults and youth. Further, there is evidence in the literature that even in young healthy adults, strength training using low compared with high loads is equally effective in enhancing muscle strength. For example, isometric strength training at 100 versus 60 % of maximal voluntary contraction (MVC) [56] or dynamic strength training at 55–60 versus 80–90 % 1RM [57] resulted in similar improvements in measures of muscle strength. Thus, there is evidence that application of lower loads during STU provides a sufficient training stimulus to ensure similar strength or power training gains in comparison with STS using higher loads in different age groups. Of note, the study methodology of the included studies has to be taken into account when interpreting our findings. In other words, the included training studies ranged from 2 to 12 weeks and were conducted in primarily untrained or recreationally active individuals.

A potential reason for the observed similar training-induced adaptations following STU compared with STS could be related to similar or even higher levels of muscle activation during performance of STU [49, 58]. In fact, Anderson and Behm [49] found no significant differences in overall electromyographic (EMG) activity of trunk and shoulder muscles during performance of a chest press exercise on an unstable surface (i.e. a Swiss ball) as compared with a stable surface (i.e. a bench). Further, significantly higher trunk muscle activity was observed during performance of squat movements on an unstable surface (i.e. performed with a balance disc under each foot) versus a stable surface (i.e. performed with a Smith machine) [58].

Our findings did not support our initial hypothesis regarding the greater effectiveness of STU in youth compared with adults. When comparing STU with CON, our results revealed medium effects of STU on measures of maximal strength in young adults and, depending on the analysed parameter, no effects (i.e. maximal strength of the trunk rotators right) to medium effects [i.e. maximal strength of the trunk flexors, rotators, lateral flexors (left/right), lateral rotators (left)] in old adults. On the basis of these results, it can be concluded that STU provides an adequate stimulus to increase maximal strength in seniors, which is mostly equal to that observed in young adults. Further, comparison of STU and STS revealed similar training-induced performance enhancements for muscle endurance and dynamic balance in both adolescents and young adults. Methodological reasons may account for this somewhat unexpected finding. However, in terms of training, similar core strength, as well as plyometric exercises, on unstable surfaces/devices were included in adolescent [36‐38] and adult [23, 31] training protocols. Further, in terms of testing, differences in the sensitivity of the applied strength, power and balance tests may also have been responsible for the unexpected findings. Yet, similar test modalities (e.g. CMJ, one-legged stance), equipment (e.g. force plate) and parameters (e.g. jump height, postural sway) were used in studies that investigated adolescents [36, 38] and young adults [44, 47]. Therefore, methodological reasons appear not to be responsible for our findings. This is why we suggest that instability-related reductions in absolute training loads during STU as compared with STS may explain our results because the reduced loads are not challenging enough to induce extra adaptive processes in the adolescent neuromuscular system. In fact, adolescence is characterized by significant increases in levels of circulating androgens (e.g. testosterone), particularly in boys [59, 60], which is why high training loads appear to be more suitable to induce marked increases in muscle mass and strength in this age group. As a limitation of this study, it has to be noted that only four studies were found that investigated the impact of STU versus STS or CON in adolescents and only three studies were found that investigated the impact of STU versus STS or CON in old adults. On the basis of the rather small number of studies, we consider our findings as preliminary. Therefore, further research is needed to determine the general effectiveness of STU as compared with no training or regular training. Equally or even more important is the need to elucidate the specific effects of STU as compared with other strength training programs (e.g. STS using high/low loads).

This systematic review and meta-analysis shows that STU, when compared with no training or regular training only, is effective in improving strength performance in adolescents, young adults and old adults, as well as power and balance performance in young and old adults. However, heterogeneous effects were particularly found in adolescents and young adults when the effects of STU were compared with those of STS. Therefore, we conclude that the application of STU compared with STS has limited additional effects on measures of muscle strength, power and balance in healthy adolescents and young adults. Therefore, the use of unstable as compared with stable surfaces during strength training is only partially recommended. Because our systematic literature search did not identify studies that investigated the effects of STU versus STS in children, middle-aged adults and old adults, further research of high methodological quality (i.e. randomized controlled trials) is needed to determine whether there are extra effects of STU on muscle strength, power and balance performances in those age groups.