MACHINES & SYSTEMIC FATIGUE

While the literature tends towards the modalities being comparable, with preference, access, and adherence being the ultimate deciding factors (1), there are several reasons why machines may be the preferable option for most trainees.

Systemic fatigue

Training with machines generally presents less systemic fatigue, as they are designed to isolate particular muscles or muscle groups, compared to exercises normally completed using free weights.

Let's take an example for quad hypertrophy, comparing the use of the barbell back squat and the leg press.

Both intensity and volume are essential for long term muscle growth. As you go through your training career, it will be necessary to add training volume in some respect (load/ sets/ reps/ time etc.) in order for the desired stress to match your fitness level in such a way as to disrupt homeostasis and provide a stimulus for further adaptation and growth.

In scenario A, using the barbell back squat, an increase in training volume will tax more than just the quadriceps. Being a free weight compound movement, we can expect the glutes, hamstrings, spinal erectors, among other muscles to be heavily taxed in conjunction with the quads. This is what we refer to as systemic fatigue. While this stress may be tolerable and even sufficient in the beginning of your training career, as you progress you may find systemic fatigue a limiting factor to ramping up your sessions, inhibiting further progress.

In scenario B, using the leg press, the total systemic fatigue is greatly reduced as the quadriceps are the only muscles being used. This generally allows for a greater stress to be applied, which can scale more easily with your abilities over time.

Imagine completing 5 sets to near failure with the leg press vs. the back squat, then having to hit a hamstring movement to complete your lower body session. Are you choosing the barbell RDL or the seated hamstring curl…?

Consider this through the eyes of a strength athlete, if most of your training is dedicated towards high intensity squatting, benching, and deadlifting, are you going to voluntarily add sets of high relative intensity free weight movements that only add to your systemic fatigue, producing marginal gains and adding to an already significant workload? Probably not.

Skill

Exercises involving free weights have a higher skill threshold compared to those involving machines.

Improvements in rate coding (the rate at which action potentials are discharged) on a neuromuscular level facilitate early skill adaptations are responsible for the vast majority of strength gains when a novel stimulus is applied (2). While this is true for both machines and free weights, the skill threshold required to tax specific areas during a free weight exercise is higher compared to those using machines.

As intra and inter set fatigue sets in, standardising repetitions on exercises with a larger skill demand becomes more difficult compared to those with a lower skill demand. This is likely to inhibit consistent, repeatable sets of high relative intensity (3> reps in reserve) which precipitate hypertrophy.

Quality of movement is essential for hypertrophy, as the goal is to stimulate the target muscle as much as possible, not to load the exercise as much as possible.

Individual anatomy

Some free weight movements may be well suited to stimulate the target muscle for some trainees, others may not be so lucky. As we alluded to, the goal for hypertrophy is to stimulate the target muscle as much as possible, not to load the exercise as much as possible.

Taking the back squat vs leg press for quad hypertrophy, a shorter limbed lifter may find the barbell back squat to be an excellent movement that does not incur sufficient systemic fatigue due to their ability to maintain an upright posture whilst maximising hip and knee flexion. On the other end of the spectrum, a longer-limbed lifter such as myself may struggle to accrue sufficient training volume at high relative intensities due to the fatigue accumulating in the glutes and spinal erectors, necessitated by a more bent-over posture. In the second scenario, a leg press or hack squat may be more appropriate due to the anatomy of the lifter.

Many modern machines have multiple points where customisation/ optimisation is available. These can be taken advantage of to either reduce systemic fatigue or increase the stress on desired tissue, remembering that anatomy determines function, which determines movement.

References: Machines and free weight exercises: a systematic review and meta-analysis comparing changes in muscle size, strength, and power PMID: 34609100, DOI: 10.23736/S0022-4707.21.12929-9 Enhanced motor unit rate coding with improvements in a force-matching task PMID: 15491836, DOI: 10.1016/j.jelekin.2004.04.005