Why do muscles get sore after working them hard? I notice this especially when I just start working out after not working out for a while.
Soreness results from high force production when an exercise is new or a load is greater than normal. Furthermore, eccentric muscle contractions (in which the muscle lengthens, as when lowering a weight) cause more soreness in the days following the workout than either isometric contractions (in which the muscle does not change length, as when holding a weight) or concentric contractions (in which the muscle shortens, as when lifting a weight). This soreness in the days after exertion is called delayed onset muscle soreness (DOMS) (Armstrong 1984; Clarkson & Sayers 1999). Although many people think that lactic acid is the cause of muscle soreness, the fact is that lactic acid (lactate) is removed from the muscles within 30 to 60 minutes after exercise, so it is long gone by the time soreness develops. Muscle soreness results from an immediate mechanical injury and a biochemical injury occurring a few days after the workout (Faulkner et al. 1993). The mechanical injury is caused when the myosin heads pull away from the actin filament, causing microtears in the muscle fibers. The biochemical injury is characterized by increased plasma enzyme activity and a leaking of enzymes (e.g., creatine kinase) out of the muscle. Soreness typically increases in intensity during the first 24 hours postexercise, peaks in the next 48 hours, then subsides within five to seven days after the workout.
Following eccentric exercise, both ROM and muscular force production decrease (Balnave & Thompson 1993; Donnelly et al. 1992; MacIntyre et al. 1996; Mair et al. 1995; Newham et al. 1987; Weber et al. 1994). Structural damage, altered neural activation and a disruption in calcium ion homeostasis are possible reasons for the decrease in force production that occurs with DOMS (Armstrong 1984). DOMS is not associated with any long-term damage or reduced muscle function.
As your clients adapt to the training load, their muscles will be less sore following a workout. Eccentric training also reduces DOMS (Balnave & Thompson 1993; Mair et al. 1995).
In other words, beginning a new exercise causes more load than normal to be inflicted on your muscles. That causes microtears in the muscle, and in the days afterword you get increased plasma enzyme activity, and your muscles leak enzimes. It's normal, and goes away in a couple days. In addition, as you get used to training, this won't happen anymore.