Compared with shoulder-only training and with continued tennis training alone, will the addition of core strengthening to shoulder training produce significantly greater improvements in flat serve ball speed in healthy tennis players?
Secondary Hypothesis:
* Combined core + shoulder training will produce greater improvement in serve accuracy than shoulder-only training or tennis training alone.
* Combined core + shoulder training will produce greater improvement in TGAR than shoulder-only training or tennis training alone.
* Combined core + shoulder training will produce greater improvement in core strength/endurance and shoulder rotational strength than the comparator groups.