Walk into any fitness club this time of year and witness the phenomenon: What started on Jan. 1 as an unwavering commitment to get in shape has become a struggle to keep the momentum, if not an outright surrender. Gyms that were packed with people three weeks ago are starting to look deserted. For many people, fitness has lost ground to time and inertia.

Of course, a few display an exceptional level of perseverance. If you are a member of a gym, you likely know who they are; you notice “the new guy or girl” and remark at the ...