In the fitness class Jessie Syfko created for a nationwide gym chain, exercisers wear weighted vests that add a challenge to their workouts. “People start to realize how good it feels to work just a little bit harder and a little bit smarter” without actually changing what they’re doing, said Syfko, senior vice president for Life Time gyms. Weighted vests are increasingly showing up in fitness classes and on jogging tracks, touted by social media influencers as a way to keep bones healthy,