Are free weights or machines better for muscle growth?

Some consider free weights to be superior mainly due to the increased stabilization recruitment. It's theorized the instability with free weights results in greater muscle fiber recruitment.

On the other hand, a stable environment with machines could allow an individual to focus more on the intended muscles. Additionally, some machines are designed to match the strength curve of an exercise, meaning they challenge the working muscle or muscles to a high degree throughout the range of motion.

But what does the research say? luckily we have a recent study by Schwanbeck et al. to evaluate.

15 men and 21 women with an average of 2 years training experience were randomly assigned to either a free weight group or a machine group. For both groups, training consisted of cycling two days of training, with one day of rest. On day 1 both groups trained the chest, back, and triceps. On day 2 both groups trained the legs, shoulders, and biceps. This was cycle was repeated for 8 weeks.

Ultrasound was used to measure thickness of the quadriceps and biceps before and after the training period for both groups. 1 rep max on the barbell bench press and 1 rep max on the smith machine bench press was assessed before and after the training period. Additionally, a 6-10 rep max on the barbell back squat, and a 6-10 rep max on the smith machine squat were measured for both groups before and after the training period. These 6-10 rep maxes were used to predict one rep max values for both these exercises.

Starting off with the strength results, both groups actually experienced similar increases in barbell bench press 1RM. For the smith machine bench press 1RM, the machine group experienced greater gains compared to the free weight group. For predicted barbell back squat 1REP MAXES, both groups experienced statistically similar increases, however, it's very clear to see that percentage-wise, the free weight group experienced greater gains. For predicted smith machine squat 1REP MAXES, both groups experienced similar increases.

Moving on to the hypertrophy results, both groups experienced similar increases in biceps thickness and quadriceps thickness. In other words, both free weights and machines appear to produce similar hypertrophy. Now, of course, it would have been more informative if the researchers could have assessed hypertrophy of a few other muscle groups, as both groups did train the majority of their body. But Regardless, when looking at the mechanisms of muscle growth, reasoning does seem to suggest both free weights and machines can produce similar hypertrophy.

Mechanical tension is the best-categorized driver of muscle growth currently. Ultimately, both free weights and machines can induce high amounts of mechanical tension (active and passive), likely explaining a big part of why they can produce similar hypertrophy.
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References:
Schwanbeck et al. (2020) - [ Ссылка ]

DETAILS ON MECHANICAL TENSION:
Wakerhage et al. (2019) - [ Ссылка ] (mechanical tension is the best categorized driver of muscle growth currently + potential mechanosensors)
Schoenfeld (2010) - [ Ссылка ] (mechanical tension consists of active tension and passive tension)
Nishikawa (2020) - [ Ссылка ] (titin alters its function during eccentric contractions)
Cutts (1987) [ Ссылка ] (sarcomeres within the vastus muscles are stretched at long muscle lengths)

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