|Rachel training while waiting for her patients|
Previous studies showed mixed results about using mirrors for exercise. Some studies show workout benefits, some no effects while others show negative effects depending on the specific task and experience level of subjects.
Currently, there is a lot of evidence showing that external focus leads to better performance than internal focus while performing physical tasks. Let's say you're shooting a basketball on the free throw line. Focusing on the rim rather than the movement of your wrist will get you better results. One reason is external focus (focusing on the rim) allows well-practiced movements to take place on auto pilot. This is more efficient than trying to directly control wrist action (internal focus).
A group of researchers studied the role of mirrors in attentional focus by getting subjects to do two series of tests. One involved flexing the elbow as hard as possible (single joint movement). The other test involved jumping as high as possible (multi joint movement).
In both cases, tests were done four times under the following conditions. Internal focus, external focus, neutral and finally with a mirror.
To sum up, external focus was best for both series of tests, while internal focus was worse. Using mirrors were no different (statistically) from the neutral condition.
So for both tasks, the mirror didn't really matter. Perhaps while doing resistance type training with heavier weights, mirrors may be helpful for maintaining symmetry of movement or correct form. What is important is that external focus trumps internal focus.
Runners take note that a previous study found that focusing on your form or your breathing (internal focus) results in worse running economy than if you focused on the surroundings (external focus).
Halperin I, Highes S et al (2016). The Effects Of Either A Mirror, Internal Or External Focus Instructions On Single And Multi-joint Tasks. PLOS One.
Schucker L, Hagemann H et al (2009). The Effect Of Attentional Focus On Running Economy. J of Sports Sciences. 7(12) : 1241-1248. http://dx.doi.org/10.1080/02640410903150467.