Why are strength gains stability-specific? Part 3

In Part 2 of this series we discussed training comparisons with stable or unstable machines.

Here in Part 3 we’ll touch on magnitude of external load affecting stability-specific strength gains.

Does the magnitude of the external load cause stability-specific strength gains?

Although light load training to muscular failure can produce similar gains in muscular size to heavy load training, the external load used during strength training is usually still a key determinant of the resulting adaptations, for several reasons.

According to the strength-endurance continuum, heavy load training causes greater increases in maximum strength, while light load training (to muscular failure) causes greater increases in repetition strength, or muscular endurance.

Heavy load training tends to cause greater gains in maximum strength because of a range of peripheral factors (increased lateral force transmission and tendon stiffness) and central factors (increased neural drive and coordination).

Therefore, it is feasible that the heavier loads that can be used during training under stable conditions might lead to greater gains in strength through these factors, although such strength might only be demonstrated under the very stable conditions used in training, because of the inability to control the application of force under less stable conditions.

Does the magnitude of the external load cause stability-specific strength gains?

When using machines to perform an exercise with the same relative load, the external load involved is usually greater than when using free weights to perform a similar exercise (Cotterman et al. 2005; Cowley et al. 2007; Lyons et al. 2010). Similarly, when using unstable surfaces to perform an exercise with the same relative load, the external load involved is usually less than when using the same exercise on a stable surface (Goodman et al. 2008; Behm & Colado, 2012).

However, the size of the difference in external load caused by the stability changes depends on the exercise (Cotterman et al. 2005), on the muscle group (Lehman et al. 2006), and of course on how much instability is involved.

More importantly, for the greater external load during more stable exercises to produce greater adaptations, it may need to translate to greater internal muscle force.

Muscle activation, as measured by EMG amplitude, is a good proxy for internal muscle force production, particularly when measurements are taken under non-fatiguing conditions, and when the muscle action is isometric.

When investigating exercises performed with the same relative load (which means a lower absolute load in the unstable condition), some researchers have found that EMG amplitude of the prime movers is similar in exercises performed in unstable and stable environments. This has been found both during isometric (Anderson & Behm, 2004; Saeterbakken & Fimland, 2013a) and dynamic muscle actions (Anderson & Behm, 2004; Welsch et al. 2005; Goodman et al. 2008; Schick et al. 2010; Saeterbakken et al. 2011; Andersen et al. 2014).

Some have even reported that the EMG amplitude of the prime movers is higher when using an unstable environment, compared to a stable environment (McCaw & Friday, 1994; Schwanbeck et al. 2009; Saeterbakken & Fimland, 2013b; Fletcher & Bagley, 2014; Campbell et al. 2014).

On the other hand, many other researchers have reported that the EMG amplitudes of the prime movers are lower under unstable conditions than under stable conditions, both during isometric (McBride et al. 2006; Chulvi-Medrano et al. 2010) and dynamic (Kohler et al. 2010; Chulvi-Medrano et al. 2010; Saeterbakken & Fimland, 2013c; Andersen et al. 2014) muscle actions.

Although the findings of these studies are conflicting, it seems fairly clear that the greater external force in more stable exercises does not always translate to greater internal muscle forces.

When lighter loads are used in unstable environments, internal muscle force production might be higher-than-expected because of increased antagonist co-activation, and synergist activation (Anderson & Behm, 2005; Sparkes & Behm, 2010).

Force produced by the prime mover muscles is greater, because they are working to co-contract with the antagonist and synergist or stabilizer muscles in order to hold the body and/or the weights in place, as well as move them through space.

Indeed, less stable environments often produce greater activation of the synergists or stabilizers and antagonists (Schick et al. 2010; Ostrowski et al. 2016; Signorile et al. 2016).

For example, middle (but not anterior) deltoid activation tends to be greater during free weight bench presses compared to Smith machine bench presses (Schick et al. 2010). Similarly, latissimus dorsi, posterior deltoid, biceps brachii, upper trapezius, and lower trapezius EMG amplitudes are greater in a pressing exercise performed with a cable machine compared to with a fixed bar path machine (Cacchio et al. 2008).

Antagonist and synergist activation differs when stability is altered!

This suggests that the greater externally-applied forces observed when performing an exercise under stable conditions probably only lead to slightly greater internal muscle forces compared to training under less stable conditions, because the muscles have to work harder against the antagonists and synergists or stabilizers in less stable environments.

As explained above, agonist muscle forces (as indicated by the proxy of EMG) are probably largely similar when lifting the same relative load (but a different absolute load) under stable and unstable conditions.

Therefore, it seems unlikely that the magnitude of the external load will contribute substantially to stability-specificity, such that strength gains when tested under stable conditions are greater after strength training under stable conditions than after training under unstable conditions.

The factors that are affected by the magnitude of the external load are:

  • Tendon stiffness
  • Lateral force transmission
  • Neural drive
  • Coordination

It is feasible that despite the similar internal muscle forces under both stable and unstable conditions (because of greater synergist and antagonist activation), some of these factors could still be influenced by the greater external load used when training under stable conditions. However, whether this happens, and which factors might be affected, is still unclear.

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


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