Stability balls, balance boards and similar devices are sold on the idea that an unstable surface makes each exercise harder for the core and stabilising muscles. Three published reviews have examined this idea, which the literature calls instability resistance training. This article summarises what they report about muscle activation, force output and trunk stability, and where the evidence remains unsettled. It covers general findings on healthy people and is not advice for anyone with an injury or health condition. None of the three abstracts reports a pooled meta-analytic effect size, so the findings are described as reviews of the literature.
What is being compared
Behm and Colado (Sports Health, 2013) define instability resistance training as the use of unstable surfaces and devices to strengthen the core or trunk muscles, and describe it as popular in fitness facilities. In practice the devices include stability balls and balance boards, and the comparison is with the same exercise on a stable surface. The three reviews come from different years: Behm and Anderson in 2006, Behm and Colado in 2013 and Zemkov in 2017.
Muscle activation is higher, force output is lower
Behm and Anderson (Journal of Strength and Conditioning Research, 2006) report that instability can decrease the externally measured force output of a muscle while maintaining high muscle activation, and attribute the high activation of limbs and trunk to increased stabilisation functions. Zemkov (Journal of Traditional and Complementary Medicine, 2017) reports that electromyography studies show significantly greater activity of trunk-stabilising muscles during exercises under unstable than stable conditions, and that it has been established that peak isometric force and rate of force development are significantly lower under unstable conditions. Power output was also compromised on unstable surfaces, though Zemkov notes this depends on the type of exercise, the device, the weight lifted and the person’s training background.
Effects on trunk stability and strength gains
Zemkov reports that intervention studies show a greater improvement in trunk stability after programmes using unstable devices than after floor exercises, and interprets this as instability training facilitating neural adaptation of trunk-stabilising muscles. He adds that both acute and long-term responses of the primarily activated muscles to exercises on unstable surfaces remain a matter of debate.
Behm and Anderson state that the lower force output may be detrimental to absolute strength gains. They report that some studies found increased co-contractions with unstable training, and that the positive effects of instability resistance training on sports performance had yet to be quantified. Behm and Colado report that studies document adaptations similar to stable resistance training in recreationally active individuals, and that similar progressions with lower resistance may improve balance and stability, increase core activation and improve motor control. They also note contradictory recommendations across the literature.
What the three reviews agree on
- Unstable conditions raise the activity of stabilising muscles.
- Unstable conditions lower the force or power a person can produce with the same exercise, so the loads used are typically lower.
- Recommendations differ on how useful the approach is, and the effect on sports performance was not established.
- Behm and Anderson conclude that a resistance programme aimed at musculoskeletal health should include both stable exercises, which emphasise higher force, and unstable ones, which emphasise balance stress.
Reading the evidence with care
Higher stabiliser activity does not by itself show a better training result. The reviews describe measurements of activation and intervention studies, and they contain no head-to-head comparison of specific products, such as one type of balance board against another. They also do not address the cost or space cost of a device. Findings from recreationally active adults should not be assumed for beginners or for athletes.
The lower force output has a practical consequence for loading. If the aim is to train the legs or chest with heavy weights, the reviews suggest unstable surfaces reduce the load that can be lifted, which is why the authors advise combining stable and unstable work. This site’s article on free weights against machines covers a related question about stabiliser demand.
Where equipment enters
Buying decisions can be framed by these findings without going beyond them. A device designed to add instability is relevant to a goal such as trunk stability or balance practice, and less relevant to a goal of lifting as heavy as possible. The article on mobility and balance equipment for home use compares this kind of kit as a product category.
Frequently asked questions
Do unstable surfaces build more core strength?
Zemkov reports greater activity of trunk-stabilising muscles and greater improvement in trunk stability compared with floor exercises, though he describes the responses of the main muscles as debated.
Can heavy lifting be done on an unstable surface?
The reviews report lower peak force and power on unstable surfaces, so lower loads are typically used.
The bottom line
Three reviews indicate that unstable devices increase stabiliser activity and trunk stability while lowering force and power output, which limits their usefulness for maximal strength. Recommendations in the literature differ and a benefit for sports performance was not established. The reviews support unstable work as an addition to, not a replacement for, stable exercises, and they do not compare individual products.
Sources
- Behm DG, Anderson KG, “The role of instability with resistance training”, Journal of Strength and Conditioning Research, 2006
- Behm DG, Colado JC, “Instability resistance training across the exercise continuum”, Sports Health, 2013
- Zemkov E, “Instability resistance training for health and performance”, Journal of Traditional and Complementary Medicine, 2017