It’s been suggested that free weights are better for increasing muscle strength because they require greater degrees of balance and muscle coordination than machine-based exercises.[1] These factors also lead to a greater recruitment of stabilizing muscles during free-weight exercises,[2] which contributes to the speculation that free weights are also better for muscle gain. Others have postulated that free weights are superior for muscle gain because they produce higher levels of muscle activation[3] and a greater release of anabolic hormones.[4]
Contrary to the above hypotheses, the available evidence does not indicate that free weights are universally better for increasing strength; rather, free weights are better than machines for increasing free-weight strength, and machines are better than free weights for increasing machine-based strength.[5] Ultimately, the efficacy of either tool is dependent upon how strength is tested.
With respect to muscle gain, specifically in terms of the direct muscle group being trained, limited evidence indicates that machines and free weights are equally effective.[5] Furthermore, in a 10-week study that assigned untrained men to perform a resistance exercise program consisting of free weights only, machines only, or free weights only for 5 weeks and machines only for 5 weeks, there were no differences between groups for changes in upper arm, chest, or thigh circumference.[6]
It stands to reason that stabilizing muscles would experience greater growth with free weights, although there is a scarcity of research directly examining these. Nevertheless, the best way to grow a specific muscle is to perform an exercise that directly, rather than indirectly, targets it. So, instead of prioritizing the performance of one exercise over another because it stimulates a greater number of muscles due to their role as synergists or stabilizers in the movement, it’s likely better to perform a variety of exercises, each of which directly targets different muscles, to maximize muscle gain. For further evidence in support of this hypothesis, see “Are a variety of exercises necessary to maximize muscle gain?”.