Why are strength gains stability-specific? Part 4

In Part 3 of this series, we touched on magnitude of external load affecting stability-specific strength gains.

Here in Part 4, we’ll discuss if the need to balance causes stability-specific strength gains.

Does the need to balance affect stability-specific strength gains?

When using machines to perform an exercise, the balance challenge involved is smaller than when using free weights to perform a very similar exercise. Similarly, when using unstable surfaces, the balance challenge is greater than when using the same exercise on a stable surface.

More stability = less need to balance; less stability = more need to balance.

Bench presses with different stability requirements

Surprisingly, balance training on its own can increase strength.

This could mean that the balance aspect of unstable surface training could lead to strength gains irrespective of the loading used.

Studies show that balance training even without concomitant strength training leads to strength gains (Heitkamp et al. 2001; 2002; Bruhn et al. 2006; Myer et al. 2006; Beurskens et al. 2015; Cug et al. 2016). Such gains seem to be connected with increases in rate of force development (Gruber & Gollhofer, 2004; Bruhn et al. 2006; Gruber et al. 2007; Behrens et al. 2015), probably caused by increases in early phase neural drive, through faster motor unit firing rates (Gruber & Gollhofer, 2004).

What is behind these changes is unclear.

Increases in neural drive after strength training appear to be partly caused by increases in corticospinal excitability (Beck et al. 2007; Griffin & Cafarelli, 2007; Kidgell et al. 2010), and partly because of reductions in corticospinal inhibition (Latella et al. 2012; Weier et al. 2012; Christie & Kamen, 2014; Rio et al. 2015).

At first glance, it might seem that balance training produces completely different neural adaptations, as it causes reductions in corticospinal excitability in balance tests (Taube et al. 2007; Beck et al. 2007; Schubert et al. 2008). However, these reductions in corticospinal excitability are very task-specific, just like improvements in balance (Kümmel et al. 2016). In fact, corticospinal excitability is elevated after balance training in tests that have not been practiced, including strength tests.

This shared mechanism could explain why additional gains in strength do not arise either when balance training is preceded by a period of strength training (Bruhn et al. 2006), nor when a program of balance training is performed together with a program of strength training (Manolopoulos et al. 2016). It may also explain how strength training can improve balance in a range of populations (Heitkamp et al. 2001; Anderson & Behm, 2005; Orr et al. 2008; Manolopoulos et al. 2016), and also increases co-ordination (Carroll et al. 2001).

This shared mechanism of strength gains by changes in neural drive may partly account for the larger-than-expected gains in strength after training on unstable surfaces, but given the similarity between the changes after balance and strength training, probably cannot explain stability-specific gains in strength.

The need to balance seems to affect the co-ordination patterns of muscles during multi-joint exercises. This affects the extent to which force can be produced during specific, dynamic movements.

Performing an exercise in an unstable environment produces greater activation of the synergist and antagonist muscles compared to the exact same exercise performed under more stable conditions, even where agonist activation is similar (Cacchio et al. 2008; Schick et al. 2010; Ostrowski et al. 2016; Signorile et al. 2016).

More importantly, training in the unstable environment reduces the antagonist activation, and increases the activation of the stabilizers.

These changes lead to a more efficient pattern of muscular contractions in that specific, dynamic movement under unstable conditions, which improves strength very substantially, in a stability-specific way.

For example, when comparing training with cable machines and with fixed bar path machines, Cacchio et al. (2008) found that training with the cable machines led to increases in the EMG amplitudes of the stabilizers, and reductions in the EMG amplitudes of the antagonist muscles during a cable machine strength test, while training with the fixed bar path machines did not.

Training in an unstable environment improves muscular co-ordination.

Given that performance in balance tasks is very task-specific (Kümmel et al. 2016), and also that changes in neural drive after balance training are very task-specific (Beck et al. 2007; Schubert et al. 2008), it therefore seems very likely that such changes in inter-muscular co-ordination during specific dynamic movements are the underlying mechanism that causes stability-specific strength gains.

Since free weight exercises performed on the ground (like barbell squats) are most similar in terms of stability requirements to athletic ability tests (like vertical jumps), this also explains why free weights could indeed be described as “just right” in terms of external load stability, and therefore transfer most effectively to sport.

Conclusions

Training in more stable environments (i.e. machines rather than free weights, or barbells rather than dumbbells) involves greater externally-applied forces. These greater externally-applied forces are only partly reflected in greater internal muscle forces (and possibly even less in trained individuals), because of the greater antagonist and stabilizer activation in unstable environments.

This suggests that more stability is better for enhancing force production, when stability is not a factor. Even so, levels of force production are probably not an important mechanism by which stability-specific strength gains occur.

Balance training and strength training produce strength gains at least partly through a common mechanism. This may account for the some of the larger-than-expected gains in strength after training on unstable surfaces in untrained individuals, although again it does not explain stability-specific gains in strength.

The need to balance in an unstable environment affects the co-ordination patterns of muscles in multi-joint exercises, increasing synergist and antagonist activation. Synergist and antagonist activation affect the extent to which force can be produced. Training in an unstable environment reduces antagonist, and increases synergist activation in a stability-specific way. These changes lead to a more efficient pattern of muscular contractions under the specific stability conditions, which in turn improves strength in a stability-specific way.

Since free weight exercises performed on the ground are most similar in terms of stability requirements to athletic ability tests, this is probably why conventional free weight training transfers most effectively to sport.


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