Doing squats with your legs spread apart will give you a better training effect on your glutes and hamstrings. Sports scientists at Illinois State University discovered this when they did an experiment with 9 experienced strength athletes.
In gyms and popular sports magazines the experts claimed – and still claim – that keeping your feet apart when doing squats will train your inside leg muscles. This position gives a stronger stimulus to your adductors and the vastus medialis, the tear-shaped groups of muscles above your knee on the inside of your legs, so the story goes.
The researchers attached electrodes to their subjects’ leg muscles, and got them to do squats with a low load [60 percent of their 1RM] and a high load [75 percent of their 1RM]. In one session the subjects kept their feet close together [narrow stance], in another they positioned their feet a shoulder’s width apart [shoulder stance] and in yet another session they placed their feet two shoulders’ width apart [wide stance].
When they measured the electrical activity in the muscles, the researchers noticed that for the quadriceps muscle it made no difference how the subjects positioned their feet. And it made no difference for the vastus medialis either. See the figure immediately below. In the next figure down you see the effect on the adductors. Again, no difference.
Descent phase = while lowering the weight; ascent phase = while lifting the weight.
But the positioning of the feet did have an effect on the stimulus for the gluteus muscle [immediately below] and the hamstrings [bottom figure].
So the best way to do squats is with your feet spread. It doesn’t make any difference for your quads, but for your gluteus and hamstrings the wide stance is the best.
A more important factor than the position of your feet is the amount of weight you train with. Training with a high load will always provide a bigger stimulus than training with a low load. Which means that heavy squats in wide stance will have a tiny bit better effect than heavy squats with your feet together.
Med Sci Sports Exerc. 1999 Mar; 31(3): 428-36.