What is the effect of imagining doing an exercise on muscle growth? I recently read a study by Yue, G et al. (1992) which I found incredibly interesting. I have no background in physiology and I was hoping that someone who does can clear things up.

If I understood it right, the study had two groups of people who trained their Hypothenar eminence (pinky) muscles. One of the groups did so by doing the actual exercise and a second did so by imagining themselves doing the same exercise. The group that did the exercise improved by 30% and the group that imagined themselves doing it improved by 22%.

Did those who imagined themselves doing the exercise have a muscle increase in their muscle or did the improvement come from the brain having better control of that muscle? Also, what should I expect if I imagine myself lifting a dumbbell all day?


Yue, G., &amp; Cole, K. J. (1992). Strength increases from the motor program: comparison of training with maximal voluntary and imagined muscle contractions. Journal of Neurophysiology, 67(5), 1114-1123.
The article by Ranganathan et al (2004) provides some relevant information.
The authors discuss how it can be increased by two main factors, neural adaptation and muscle growth. They note how the motor skill acquisition literature supports the use of visualisation as an adjunct to actual practice as a means of improving performance:


  Research on motor skill acquisition has demonstrated clearly that
  mental practice leads to improved performance (Corbin, 1972; Feltz &amp;
  Landers, 1983). Thus, the neural events controlling the muscle
  parameters for performance (e.g., amplitude, timing) can be improved
  through mental practice.


They mention studies that have found no effect  of visualisation on strength (e.g., Herbert et al 1998).

However, in their study they did find an effect of visualisation on strength. They attribute this gain to neural adaptation rather than muscle growth.


  The key ﬁndings of this study were that mental training increases
  voluntary strength of both distal and proximal muscles of human upper
  extremities and the strength improvements accompanied elevations of
  time-locked (to MVC trials) cortical potential (MRCP). Based on the
  MRCP data (Figs. 3–5), we are conﬁdent that the primary mechanism
  underlying the strength increase is a mental training-induced
  enhancement in the central command to muscle. The data suggest that
  repetitive mental attempts of maximal muscle activation trained and
  enabled the brain to generate stronger signals to muscle.
  ...
  greater strength is a consequence of stronger brain activity. A
  stronger central command could recruit the motor units that were
  otherwise inactive in an untrained state and/or drive the active motor
  units to higher intensity (higher discharge rate), leading to greater
  muscle force


References


Corbin, C. B. (1972). Mental practice. In W. P. Morgan (Ed.), Ergogenic
aids and muscular performance (pp. 93–118). New York: Academic
Press.
Feltz, D. L., &amp; Landers, D. M. (1983). The effects of mental practice
on motor skill learning and performance: A meta-analysis. Journal of
Sports Psychology, 5, 25–27.
Herbert, R. D., Dean, C., &amp; Gandevia, S. C. (1998). Effects of real
and imagined training on voluntary muscle activation during maximal
isometric contractions. Acta Physiologica Scandinavica, 163, 361–368.
Ranganathan, V. K., Siemionow, V., Liu, J. Z., Sahgal, V., &amp; Yue, G. H. (2004). From mental power to muscle power—gaining strength by using the mind. Neuropsychologia, 42(7), 944-956. PDF