Muscles left unchallenged turn soft, leaving owners unable to win pillow fights, bench-press thimbles, or stand up to the bullying advances of a light breeze. Toughen up with today's Groupon: for $25, you get a 10-visit punch card to Flex Fitness in Holland (a $50 value).

Flex Fitness Center challenges both novice and seasoned exercisers to rebuild their bodies and redefine their fitness goals with cutting-edge cardio equipment and results-oriented weight training tools. Trim extra inches with sprints on one of the gym’s Precor treadmills, or go the extra 26.2 miles with a marathon jog soundtracked by overhead TVs playing news, sports, or congressional karaoke sessions. With six squat cages and 40 yards of dumbbells, Flex Fitness lays out a smorgasbord of muscle-bulking and power-lifting opportunities. Equipment ranging from free weights to machines helps bench pressers add bulk and subtract chances of being unwillingly buried by crabs during trips to the beach.