An important study by Purvis, Landgren, Cekan, and Diczfalusy indicated that some hormone levels, including testosterone, were actually slightly elevated in the blood immediately after masturbation. While they did detect some elevated hormones, these increases were not significant enough to help with muscle building or athletic performance, nor were these increases in baseline testosterone levels permanent.
This is supported by Isenmann, E., Schumann, M., Notbohm, H.L. et al's research which found that there could be a correlation between testosterone levels and masturbation, but the results were not significant enough or with a large enough sample size to be entirely conclusive.