Which is better for strength growth: stable or unstable supports

In this text we have left out the safety of exercising on unstable platforms – if you have a self-preservation system you are unlikely to squat with a barbell standing on a fitball. However, why many lifters seek to get an unstable platform under their feet (back, etc.).

For example, the bench press can be performed in highly stable mode (lying on a bench with a barbell), in moderately stable mode (on a bench, but with dumbbells) or in very unstable mode (on a fitball with dumbbells).

Many strength coaches are strongly against performing exercises on unstable surfaces because smaller weights are used in less stable conditions, and therefore in their opinion the use of smaller weights is less effective for strength development. What does the science say about this?

Specificity of Strength Development
There is a specificity of strength development that has been proved by numerous studies: if you train on machines, you show better results on machines than those who train with free weights. And vice versa: those who train with free weights perform better with free weights.


Does the same specificity apply with unstable platforms?


This study has again proved the principle of specificity: if you train on cable-powered machines, you perform better on cable-powered machines than on machines with a fixed trajectory and vice versa:

Those who used cable-propelled trainers (in green) show better gains than those who used fixed track trainers.

Training on unstable surfaces generally produces the same strength effect as on stable surfaces, as measured by maximum isometric (as well as dynamic) strength (according to a number of studies: Kibele & Behm, 2009; Sparkes & Behm, 2010; Prieske et al. 2016, Cowley et al. 2007; Marinković et al. 2012; Premkumar et al. 2012; Maté-Muñoz et al. 2014, Sparkes & Behm, 2010, Marinković et al. 2012; Maté-Muñoz et al. 2014, Sparkes & Behm, 2010).

Thus, there is no evidence for specificity of strength gains (as a function of platform stability) when tested on stable surfaces after training on both stable and unstable surfaces.

However, it has been suggested that strength gains on unstable surfaces should be higher after training under similar conditions (Sparkes & Behm, 2010). This would imply that specificity in strength development still occurs, at least in one direction.

For example, in studies examining the effect of lower-body strength training on counter-movement in the high jump, it appears that in most cases results are better when performed on stable surfaces than when performed on unstable surfaces (3, 4), although in some studies again – no difference was found (5).

Unstable platforms: less weight but same result – why?
When using unstable surfaces to perform exercises with the same relative load, the applied force is usually (but not always) less than when performing similar exercises on a stable surface (6, 7). Many studies prove the relationship: more stability = more applied force; less stability = less applied force. However, the effect on force growth is almost the same. How can this be?

In a study of exercises performed on stable and unstable platforms (with different weights but the same relative load, respectively), some scientists have found that the EMG amplitude (electromyography, which shows the degree of muscle activity) of the muscles is the same for exercises performed under both unstable and stable conditions. The weights are different and the muscle activation is the same. This was found for both isometric (8) and dynamic muscle contractions (8, 10-13).

The magnitude of the load and the degree of muscle involvement may be different
Some studies even reported higher EMG amplitude of agonist muscles under unstable conditions compared to stable conditions (9, 14-17).

However, there are also quite the opposite data: many researchers note that the EMG amplitude of the agonist muscles is lower in unstable conditions than in stable ones, both for isometric (18, 19) and dynamic (9, 13, 19, 20) muscle work.

Although the results are conflicting, it seems clear enough that it is not always the case that a significant external force in more stable exercises leads to an increase in internal muscular force.

At lower loads but in unstable conditions, the intrinsic muscular force may be higher than expected because there is an increase in coactivation of the antagonist muscles and accretion of the synergist muscles.

Here is the EMG amplitude shown by different muscles in unsteady (cable machines) and steady (fixed trajectory) bench presses:

graph_unstable_ supports_2

For example, the middle deltoid muscle is more actively engaged in the bench press with free weights compared to the same exercise in the Smith (11). Similarly, the EMG amplitude of the broadest back, posterior delts, biceps brachii, upper and lower trapezius muscles is greater on cable-powered bench presses compared to fixed bar trajectories (1).

This suggests that the significant external force seen in stable exercises appears to produce only a small increase in internal muscle force compared with training in less stable conditions, because the muscles have to work harder against antagonists and stabilisers in an unstable situation. This leads to a strengthening of the agonist muscles even when the external load is less.


Training under more stable conditions (i.e. on machines rather than with free weights, or with barbells rather than dumbbells) results in a greater external force application (more weight than in exercises under unstable conditions). However, the increased external force only partially increases the internal muscle force (and will probably be even lower in trained people), as the antagonist and stabiliser muscles are more activated in unstable conditions.

So less weight (in unstable conditions) is compensated by more muscle engagement, so many studies show no difference in strength gains in unstable and unstable training.

The mechanism that leads to strength gains in balance training and strength training is at least partially the same. This also helps to explain some cases of greater than expected strength gains from training on unstable surfaces for untrained people.

Training on unstable surfaces limits the activity of the antagonist muscles and increases the activity of the synergists. These changes lead to more efficient coordination of muscle groups under specific conditions, which results in improved stability-specific strength performance.

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