There is an abundance of training methodologies and exercises implementing various instability training devices; the Swiss ball being the most common1 closely followed by balance discs. Since the late 1980s, education programs have appeared through popular media and practitioner endorsements2 marketed to the athletic conditioning and rehabilitation sectors3 that favor unstable surface training (UST).
Studies are generally universal in their findings that UST provides the rehabilitation industry with valuable information regarding proprioceptive deficiencies related to joint stability post-injury,4,5 as well as improving the strength of the abdominal and back muscles, balance, co-ordination and range of motion of the joints of post-stroke patients.6 It is said that stroke patients experience reduced ability to maintain asymmetrical posture and balance, and using UST sensitizes the muscle spindles through the gamma motor neurons, thereby improving motor control output.7
Anecdotally, these studies suggest that UST may be a time-efficient and a cost-effective way of improving numerous performance elements simultaneously, which include improving neuromuscular pathways, proprioceptive awareness and balance, as well as the graduation of force production and establishing optimal agonist to antagonist co-contraction for joint stability.6 Because all movement requires a combination of stability and mobility, supporters of UST programs suggest that such training programs are ideal as they stimulate both of these modalities simultaneously.7
However, this information was obtained from studies performed on injured individuals; there are several limitations and potential conflicts when dealing with trained and healthy individuals using UST.8
A study by Anderson and Behm9 used UST and found a 59.6% decline in the maximum isometric force output when performing the horizontal bench press on a Swiss ball versus a stable surface. They concluded that the neuromuscular recruitment patterns of joint agonists and antagonists are changed when performing an exercise on an unstable surface, which may be due to the joint agonists playing a greater role in joint stabilization.10
This suggests that UST may not transfer to movements on a stable surface.11,12 This may be problematic for trainers, as these recruited neuromuscular patterns vary from the patterns of daily activities and sports, and attempting to increase training loads while using UST devices could increase the risk of injury.8
In contrast, the study by Cressey et al11 looked at UST and the vertical lunge exercise on a balance disc with trained and healthy individuals, and concluded that power outputs did increase when stability was reduced, due to the greater stiffness required of the lumbopelvic hip complex to stabilize the body in a vertical position.
The Cressey et al. study also provides a suggestion of how important it is to differentiate between instability at the foot (which is accustomed to stable surfaces in a closed chain motion) and the instability of the torso and arm (which accounts for instability while the base is stable) as most human activities position the body upon a stable surface with instability occurring further up the kinetic chain.
This correlates to their primary findings of the stability of the lumbopelvic hip complex providing a foundation for power. It also provides credibility to the principle of specificity regarding the client, environment and when endorsing training protocols. In fact, using free weights on a stable surface can provide an environment of moderate instability that stresses and forces training adaptations to an individual’s equilibrium13, and suggests how important the legs are within the functional synergy of movement in providing the stability and mobility required for optimizing performance.14
What Does This Information Mean?
A defined reason as to why and how to use UST or the effects it has on performance is well documented among injured individuals, but there is a strong suggestion that training adaptations will differ between injured individuals and those who are trained and healthy. There is also a suggestion that training parameters and their responses will be altered differently between the horizontal and vertical position. For example, the horizontal bench press movement reduces power and strength outputs and may compromise the neuromuscular recruitment patterns within vertical movements. In contrast, the vertical lunge movement produces a power increase due to an increased stability of the lumbo pelvic hip complex, which closely correlates the neuromuscular patterns in which individuals perform activities and sports.
Finally, using free weights on a stable surface may provide the appropriate] stimulus through the use of the functional synergy initiated by the legs. All of this information shows how important individual specificity is when guiding fitness professionals towards planning and delivering appropriate UST.
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