As recently as 2010 (Kohler, Flanagan, & Whiting, 2010) found no clear benefit in training with either unstable loads or unstable surfaces. The theory is clear: heavier weights can be lifted in stable conditions than in unstable conditions and most sport is conducted on stable surfaces. Exceptions to this are surfing and snowboarding or similar orientated sports where the participant needs to be able to move a solid mass (board) in highly unstable and unpredictable conditions.
However, in the last few years there continues to be increasing interest in instability resistance training (IRT) using either unstable surfaces (Bosu or Reebok core board) or using instable weights (Tendo-destabilizing bar (TBB)(Fletcher & Bagley, 2014), barbells, sandbags or water filled bags (Calatayud, 2015)) to challenge the body to achieve greater truncal muscle activation particularly in anterior deltoid (AD), lumbar erector spinae (LUMB), external oblique (OBLIQ) and gluteus medius (GM). Difficulties have arisen analysing relative 10RM loads across different training equipment (Fletcher & Bagley, 2014). Looking at training with an element of instability in the weight itself would seem a viable alternative (Fletcher & Bagley, 2014).
The same amount of weight cannot be lifted for example in the chest press between unstable weights and stable weights due to the delay in the turn-around time (amortization phase) between eccentric phases and concentric phase and therefore a loss of stretch shortening cycle (SSC) and elastic energy (Zemkova & Hamar, 2013). This effect is noticeably less for the squat on the Bosu than for the chest press on the Swiss ball (Zemkova & Hamar, 2013). However the increased training effect on the trunk muscles using unstable surfaces (BOSU) or unstable weight (Aquahit bag) can be of benefit in a periodisation programme both on static balance (Aquahit > Bosu) and dynamic balance (Bosu>Aquahit ) (Zemková, 2016). The biggest challenge would seem to be to not only train these core muscles but make it meaningful to sport which is mainly carried out in upright stance. Of interest is research (Calatayud, 2015) which looked at the clean and jerk manoeuvre with 20kgs barbell, sandbag and water filled bag. The water bag showed higher muscles activation across OBLIQ, GM and LUMB with AD being similar in all three testing groups.
Further research needs to be conducted with other water-filled equipment (CorMax AQA Ball) and heavier water equipment (CorMax Beast) for elite sports persons conditioned to training with heavier weights to further extend their training capacity.
Encouraging is the position statement made by the Canadian Society for Exercise Physiology which see the role of IRT and greater core activation as part of the periodisation of athletes, whitst providing a safer low load higher core muscle activation alternative for general population and those undergoing rehabilitation. The road is clear for IRT in rehabilitation (D. Behm & Colado, 2012; D. G. Behm & Colado Sanchez, 2013) and physiotherapists and exercise science graduates can benefit from recent research and this gap in the market between conventional gym training and softer rehabilitation options.
Behm, D., & Colado, J. C. (2012). The effectiveness of resistance training using unstable surfaces and devices for rehabilitation. International journal of sports physical therapy, 7(2), 226.
Behm, D. G., & Colado Sanchez, J. C. (2013). Instability Resistance Training Across the Exercise Continuum. Sports Health: A Multidisciplinary Approach, 5(6), 500-503. doi:10.1177/1941738113477815
Calatayud, J. (2015). CORE MUSCLE ACTIVITY DURING THE CLEAN AND JERK LIFT WITH BARBELL VERSUS SANDBAGS AND WATER BAGS. International journal of sports physical therapy, 10(6), 803-811.
Fletcher, I. M., & Bagley, A. (2014). Changing the stability conditions in a back squat: the effect on maximum load lifted and erector spinae muscle activity. Sports Biomechanics, 13(4), 380-390. doi:10.1080/14763141.2014.982697
Kohler, M. J., Flanagan, P. S., & Whiting, C. W. (2010). Muscle Activation Patterns While Lifting Stable and Unstable Loads on Stable and Unstable Surfaces. Journal of Strength and Conditioning Research, 24(2), 313-321. doi:10.1519/JSC.0b013e3181c8655a
Zemková, E. (2016). Instability resistance training for health and performance. Journal of Traditional and Complementary Medicine. doi:10.1016/j.jtcme.2016.05.007
Zemkova, E., & Hamar, D. (2013). Utilization of elastic energy during weight exercises differs under stable and unstable conditions. The Journal of sports medicine and physical fitness, 53(2), 119.