Delayed onset muscle soreness (DOMS) is a common post-workout sensation characterized by sore muscles and joint stiffness caused by intense exercise activities.
According to the American College of Sports Medicine, “Any type of activity that places unaccustomed loads on a muscle may lead to delayed onset muscle soreness.”
DOMS will usually develop anywhere from 12 to 24 hours after an intense exercise session and commonly elicits the most pain between 24 to 72 hours after completion of a workout. This is why many people might feel even more sore the second day after an intense workout
Most research supports that DOMS is caused by tiny tears in your muscle fibers which, among other types of exercise, can result from workouts involving weight lifting, running, walking downhill, step aerobics and jumping. It is believed that these tears are formed during the eccentric phase of an exercise, like when you lower your arm to the starting position of a bicep curl or the flexing of your thigh muscle while running or walking downhill.
While DOMS can be caused by all different types of exercise, the severity of soreness will depend on the intensity. For example, running downhill would cause greater soreness than walking downhill and performing a high number of repetitions for a certain strength exercise would result in more distress than performing less repetitions of the same exercise.
DOMS is a natural reaction to intense exercise but its effects can potentially be reduced by implementing a few helpful techniques, such as progressing your workout program slowly in order to allow your muscles adequate time to adapt and leaving enough time for your muscles to recover in between workouts. For example, avoid exercising the same muscle groups two days in a row.
Soucre: ACSM's Resources for the Personal Trainer; Third Edition
Related: How to Relieve Muscle Soreness