Muscle soreness is most often associated with eccentric exercise— muscle contractions that occur as the fibers lengthen. Examples include walking or running downhill and lowering the weight to the chest during a bench press. Muscles can exert more force eccentrically (lengthening contractions) than concentrically (shortening contractions), so muscle soreness and injury is more common in exercises involving eccentric contractions. Muscles increase in strength and size largely by repairing small injuries to the muscle fibers, and adapting by adding muscle protein so that similar loads are less stressful in the future.
Researchers from Wayne State University in Detroit found that creatine kinase (a marker of muscle injury and inflammation) and resting energy expenditure were higher in untrained people than in trained people in the days following an intense weight-training workout. The program, designed to induce muscle soreness, involved 8 sets of 6 repetitions for eight exercises. The training cadence was 1 second for each concentric contraction and 3 seconds for each eccentric contraction. The study showed the weight-trained people adapted to intense muscle contractions and developed protection against potentially damaging exercise. Conversely, muscle damage and the resulting repair process were greater in untrained people.