As workouts intensify, a harmful side effect grows more common

Before starting a new program, do a less intense version of it first

Three years ago, Christina D’Ambrosio went to her first spin class, pedaling fast on a stationary bike to the rhythms of popular music as an instructor shouted motivation.

But Ms. D’Ambrosio, who exercises regularly, found the hourlong class was harder than she anticipated. By the end her legs were sore and wobbly.

“I thought my body just wasn’t used to that kind of muscle ache because it was my first class,” said Ms. D’Ambrosio, a kindergarten teacher from Pleasantville, N.Y.

Over the next two days, her legs throbbed with excruciating pain, her urine turned a dark shade of brown, and she felt nauseated.