Why Daily Weight Lifting Can Be Dangerous

Q: I have always read that in weight training, one should rest a day between workouts targeting the same muscle groups. Is there proof that targeting them on consecutive days is harmful? What about people who have some heavy lifting in their day jobs?

A: Weight training, especially if your body is not used to lifting weights, harms muscle tissue in the short term while also prompting the tissue to repair itself and become stronger, said Stuart Phillips, a professor at McMaster University in Canada who has long studied weight training. In 1997, he and colleagues published a seminal study showing that within three hours of a bout of strenuous weight training, men and women develop significant increases in markers of tissue breakdown that remain elevated for 24 hours. At the same time, their muscles begin to show rising levels of other markers related to tissue repair and growth that linger for a full 48 hours after the workout.
These findings, which have been replicated many times since, strongly indicate that muscles benefit from a day or two of rest between training, Dr. Phillips said, in order for them to complete the Nietzschean rip-and-repair cycle.
“Plus it’s not just muscle” affected by weight training, he said. “Connective tissue in joints also needs a recovery period.” In fact, joints can be more prone to injuries from daily weight training than muscles, he said.
As for people whose jobs involve frequent heavy lifting, they generally develop a tolerance to the loads that they regularly lift, he said. But even with that familiarity, they often develop overuse injuries in their backs, wrists and other joints, Dr. Phillips said.
So his advice is to wait 48 hours between sessions of strenuous weight training that target a particular muscle group. You could focus on upper-body muscles on one day and leg and lower-back muscles the next, if you enjoy visiting the gym daily. But don’t exercise all of the muscles every day. “Muscles and joints need rest to recover and regenerate,” Dr. Phillips said.