Theories of Acute vs. Chronic Responses

Acute refers to your body's responses to a single bout of exercise, such as a 40-yard dash, a marathon or a mountain hike. In contrast, chronic describes the effects of exercise performed over time, such as walking to work instead of driving, or engaging in a thrice weekly exercise program. Acute responses to aerobic exercise might include increased breathing rate and increased blood pressure. Over time, your body responds to regular cardiovascular activity by decreasing your resting heart rate and blood pressure, and enhancing your body's ability to use fat as an energy fuel.

Energy Conversion Theory

When talking about energy, you either say you have it or you don't. In its true, physiological sense, energy can neither be created or lost. It simply gets converted from one form to another, exercise physiologist Jason Karp told the "IDEA Fitness Journal." The transformation of high-energy phosphates -- called adenosine triphosphate, or ATP -- into a lower-energy adenosine diphosphate, or ADP, produces energy for physical activity. Since your muscles lack the capacity to store large quantities of ATP, you must constantly resynthesize it.

Metabolic Systems

Short duration power movements rely on your phosphagen or ATP-CP system, which provides the fastest ATP resynthesis. This anaerobic system does not rely on carbohydrates or fats for fuel. Intense activities lasting from 30 seconds to two minutes use the glycolysis system, which breaks down carbohydrate -- in the form of blood glucose or muscle glycogen -- and converts it to pyruvate. Pyruvate enters your cells and assists in ATP production. The oxygen-dependent aerobic system kicks in after two minutes, and uses blood glucose, glycogen and fat to fuel and resynthesize ATP. This system supports long duration continuous activities.

Delayed Onset Muscle Soreness

Your intense workout didn't cause muscle soreness until two days later. This phenomenon, called delayed onset muscle soreness, mystifies exercise physiologists. The book "Physiology of Sport and Exercise" details an event sequence that might trigger this reaction. First, tension in the muscle and its connective tissue causes structural damage, which in turn creates high calcium concentrations that inhibit cellular respiration and damage your muscles' cross striations. In response, your white blood cells, which protect you against conditions that threaten normal tissue function, accumulate and produce inflammation. Next, the inflammatory process increases tissue fluid pressure within your muscles, and activates their pain receptors.