Why are strength gains stability-specific? Part 2

NOTE: In Part 1 we discussed external load stability and machines vs. free weights.

Comparing training with stable or unstable machines

In some training studies, different types of machines have been compared with one another, where one type of machine makes use of fixed bar paths and the other type of machine uses cables that allow freedom of movement, to perform essentially the same multi-joint exercise (Spennewyn, 2008; Cacchio et al. 2008).

Strength gains are different between the two types of machine, and there is again definite evidence of stability-specific gains in strength going in both directions (Cacchio et al. 2008).

Interestingly, these studies also show that gains in dynamic strength when tested in the machine on which the training was performed are much higher when training with the cable machines than when training with fixed bar path machines (Spennewyn, 2008; Cacchio et al. 2008).

This suggests that there may well be a learning component inherent in using the cable machines that contributes substantially to the gains in strength.

Indeed, Cacchio et al. (2008) did note that training with the cable machines led to beneficial alterations in the EMG amplitudes of the stabilizers and of the antagonist muscles, while training with the fixed bar path machines did not.

We will come back to this point later on.

Comparing training on stable and unstable surfaces

The advantages and disadvantages of unstable surface training have been discussed ad nauseam (e.g. Hubbard, 2010; Behm & Sanchez, 2013). Here, I want to focus on if strength gains are stability-specific. To do this, we can look at studies exploring:

  1. Training with stable vs. unstable surfaces, then testing strength on stable surfaces
  2. Training with stable vs. unstable surfaces, then testing an athletic ability

#1. Comparisons of training on stable vs. unstable surfaces on strength on stable surfaces

Very few studies have compared the effects of training on stable vs. unstable surfaces on strength on stable surfaces. Those that have are summarized in a recent systematic review (Behm et al. 2015), although the measures used to test strength were not differentiated from one another, which makes the results difficult to interpret.

The most stable surface typically measured in studies is maximum isometric force, using a dynamometer. Training on unstable surfaces tends to produce similar gains in maximum isometric force as training on stable surfaces (Kibele & Behm, 2009; Sparkes & Behm, 2010; Prieske et al. 2016).

The second most stable surface typically measured in studies is maximum dynamic force, using the strength exercise used in the stable-surface training group, such as 1RM bench press (Cowley et al. 2007; Marinković et al. 2012; Premkumar et al. 2012; Maté-Muñoz et al. 2014), 3RM bench press (Sparkes & Behm, 2010), 6RM bench press (Saeterbakken et al. 2016), 1RM back squat (Marinković et al. 2012; Maté-Muñoz et al. 2014), and 3RM back squat (Sparkes & Behm, 2010). Training on unstable surfaces seems to produce similar gains in dynamic strength in the exercise used during training, compared to training with the same exercise on stable surfaces.

This suggests that there is no evidence of stability-specific strength when testing strength on stable surfaces after either stable or unstable surface training. However, although not as well-researched, there are some suggestions that gains in strength on unstable surfaces might be greater after training on unstable surfaces (Sparkes & Behm, 2010; Saeterbakken et al. 2016), which would mean that stability-specific strength gains still occur, albeit only in one direction.

Importantly, however, all of these studies were performed in untrained individuals. 

Since there are indications that unstable surface training does not lead to greater EMG amplitudes than stable surface training with the same absolute loads in resistance-trained individuals (Wahl & Behm, 2008; Li et al. 2013), training on unstable surfaces may not be as effective as training on stable surfaces in trained subjects.

#2. Comparisons of training on stable vs. unstable surfaces on athletic performance

Very few studies have compared the effects of training on stable vs. unstable surfaces on athletic performance measures. Those that have are summarized in a recent systematic review (Behm et al. 2015), although the measures used to assess athletic ability are not differentiated, which makes the results difficult to interpret.

Looking only at those studies exploring the effects of lower body strength training on countermovement jump height, a majority have found that performing the exercises on stable surfaces is better than performing the exercises on unstable surfaces (Cressey et al. 2007; Oberacker et al. 2012), although a minority have found no differences (Maté-Muñoz et al. 2014).

This suggests that lower body training on unstable surfaces may not transfer as well the same exercises performed on the ground to common tests of athletic ability, such as vertical jumping.

Summary of results

  • Unstable surface training doesimprovestrength on stable surfaces to a similar extent as stable surface training in untrained subjects. However, this may not apply to trained individuals.
  • Unstable surface training may notimprovecommon tests of athletic ability as well as the same exercises performed on the ground.

What mechanisms cause stability-specific strength gains?

Given that stability-specific strength gains do occur, what might be the underlying mechanisms?

One difference between more stable and less stable exercises is the amount of external load that is used. This could be a mechanism by which stability-specific strength gains occur, if the greater external load lifted in the more stable conditions then leads to different neuromuscular adaptations.

Another difference between more stable and less stable exercises is the amount of balance that is needed.

Differences in the need to balance could produce adaptations involving mechanisms by which stability-specific strength gains then occur. Balance requirements could produce improvements in strength simply by being a balance challenge (as balance training does have neural effects), or because they alter the way in which an exercise is performed, thereby changing the co-ordination and muscles involved in the movement.

Let’s take a look at both of these possibilities, and see which of them might be responsible for stability-specific strength gains.

In Part 3 we’ll discuss if the magnitude of the external load causes stability-specific strength gains.


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