Why are strength gains stability-specific? Part 1

Many strength coaches recommend against training on unstable surfaces when using free weights for improving sports performance. Leaving aside safety issues, the main reason given is that lighter weights are used in less stable conditions. And this makes sense, because lighter weights lead to smaller strength gains.

Many coaches also recommend against using multi-joint machines like the leg press, when preparing athletes for sport. The reason given for this is that the strength gains will not transfer to a less stable environment (they are less “functional”), where some degree of balance is required.

So do free weights involve a level of external load stability that is just right when preparing athletes for sport?

Or is all of this just wishful thinking?

What is external load stability?

The stability of the external load determines either how much balance is needed when doing an exercise. External load stability exists on a continuum from very stable, to neutrally stable, to very unstable.

Less stability means more balance is needed, while more stability means that less balance is required.

Changes in stability can involve altering either:

  • The type of resistance used
  • The type of surface on which the lifter is standing, sitting, or lying

For example, a bench press can be performed in a highly stable set-up (on a bench with a barbell), in a moderately stable set-up (on a bench with dumbbells), or in a very unstable set-up (on a Swiss ball with dumbbells).

Bench presses with different stability requirements

Changing from a barbell to dumbbells involves altering the stability of the external resistance directly, while changing from a bench to a Swiss ball involves altering the stability of the surface upon which the lifter is resting.

Both of these changes alter the stability requirements, and therefore the need to balance, but in different ways.

Why is external load stability important?

If strength gains are stability-specific, then gains in strength in a very stable exercise set-up (such as sitting on a machine) might not transfer to strength displayed in less stable environments (such as standing on the ground), while gains in very unstable environments (such as balancing on a wobble board) might not transfer to those in more stable environments (Willardson, 2004).

This is important for athletes, because they tend to spend much of their time exerting force into the ground.

There are clues that strength is indeed stability-specific.

For example, the association between a machine 1RM and a similar exercise free weights 1RM can range between moderate and strong (Willardson & Bressel, 2004; Cotterman et al. 2005; Langford et al. 2007; Lyons et al. 2010; Mayhew et al. 2010; Ferraresi et al. 2013).

This suggests that some people are better able than others to transfer strength from one degree of stability to the next. Otherwise, the relationship between machine and free weight 1RMs would be very strong. Everyone would simply scale their strength levels between stable and unstable exercises in the same way.

But can strength training on stable or unstable surfaces increase our ability to exert force under different stability conditions?

Comparing training with machines and free weights

The advantages and disadvantages of training with either machines or free weights for transferring strength gains to sporting movement have been debated at great length (e.g. Stone et al. 2000; Haff, 2000).

Ultimately, the degree of transfer between a strength training exercise and a sporting movement will depend partly on the extent to which the strength gains after training are stability-specific, and partly on the extent to which the stability requirements used in training match the stability requirements of the sporting movement.

So are strength gains after training stability-specific? We can investigate this by looking at studies comparing the following:

  1. Training with machines vs. free weights, then testing free weights strength
  2. Training with machines vs. free weights, then testing an athletic ability

#1. Comparisons of machine and free weights training on free weights strength

More studies than you might expect have assessed the effects of long-term training programs with machines on strength tested with free weights in young adults (Boyer, 1990; Augustsson et al. 1998; Langford et al. 2007; Lennon et al. 2010; Mayhew et al. 2010; Ratamess et al. 2016; Rossi et al. 2016; Saeterbakken et al. 2016).

Without exception, every single study has shown that training using machines can improve strength tested using free weights.

Several of these studies have also compared the effects of training with machines with the effects of training with free weights on strength tested with free weights (Boyer, 1990; Augustsson et al. 1998; Langford et al. 2007; Mayhew et al. 2010; Lennon et al. 2010; Rossi et al. 2016; Saeterbakken et al. 2016).

Only Langford et al. (2007) found no evidence of stability-specificity when training with machines or free weights, possibly because the difference in stability between conditions may not have been that substantial, as indicated by the small difference in force between exercise variations of just 3 – 8%.

However, most researchers have found evidence of stability-specificity in at least one direction, both when the exercises performed are very similar (Boyer, 1990; Lennon et al. 2010; Mayhew et al. 2010; Rossi et al. 2016; Saeterbakken et al. 2016), and when they are different (Augustsson et al. 1998).

#2. Comparisons of machine and free weights training on athletic performance

Many studies have assessed the effects of training the lower body with machines on sports performance in young, healthy adults.

Improving leg press strength seems to transfer reasonably well to greater vertical jump height (Silvester & Bryce, 1981; Papadopoulos et al. 2014; Wirth et al. 2015; 2016; Manolopoulos et al. 2016; Rossi et al. 2016) as well as single-leg hop for distance (Wawrzyniak et al. 1996). Smith machine squat training improves vertical jump height and sprint running ability (De Hoyo et al. 2015b). Even the knee extension can improve vertical jump height (Augustsson et al. 1998; Friedmann-Bette et al. 2010).

Flywheel training is an increasingly popular form of machine training, although arguably since it uses a cable to direct the force, this requires more control than a more traditional machine with a fixed bar path. Flywheel squat training transfers well to vertical jump height (Sheppard et al. 2008; Gual et al. 2015; De Hoyo et al. 2015a), and flywheel leg curl training transfers well to sprint running (Askling et al. 2003; De Hoyo et al. 2015a).

So training using machines does transfer to sporting performance, despite what some alarmists might tell you.

Far fewer studies have actually compared the effects of machine and free weights training in young adults on athletic performance (Augustsson et al. 1998; Wirth et al. 2015; 2016; Rossi et al. 2016).

Even so, these studies confirm that machine training does not transfer as well to vertical jumping ability as free weights, both when the exercises used are similar (Wirth et al. 2015; 2016; Rossi et al. 2016), and when they are different (Augustsson et al. 1998), which is evidence that stability-specific strength gains do occur.

Summary of results

  • Machine training improves free weights strength, but not by as much as free weight training (and the other way around). So stability-specific strength gainsdo happen.
  • Machine weight training improves athletic ability, but not by as much as free weight training.

In Part 2 we’ll discuss Training comparisons with stable or unstable machines

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