Energy Systems

Muscle contraction depends on a continuous supply of ATP (adenosine triphosphate). The body produces this energy through three distinct systems, and the dominant system at any given moment depends on the intensity and duration of the activity.

Phosphagen System — 0 to ~10 seconds. Draws on stored ATP and creatine phosphate to produce energy immediately, with no metabolic byproducts. This system powers short, maximal efforts such as a heavy set of three repetitions, a sprint start, or a vertical jump.

Glycolytic System — ~10 seconds to 2 minutes. Breaks down glucose to produce ATP rapidly. This process generates hydrogen ions as a byproduct, which is the actual source of the "burning" sensation during intense effort (commonly, though inaccurately, attributed to lactic acid). This system predominates in efforts such as a hard 400-meter run or a demanding set of squats.

Oxidative System (Aerobic) — beyond 2 minutes Produces ATP more slowly but far more sustainably, metabolizing carbohydrates and fats in the presence of oxygen. This is the dominant system for endurance activity.

These systems do not switch on and off discretely; they operate concurrently, with the balance between them shifting according to effort and duration.

Neuromuscular Adaption

In the early weeks of a new training program, strength gains are driven primarily by the nervous system becoming more efficient at recruiting existing muscle fibers, rather than by an increase in muscle size. This process, known as neuromuscular adaptation, typically precedes measurable muscular growth and accounts for the rapid strength gains often observed when beginning a new program.

Disclaimer

The information on this page is provided for general educational purposes only and is not intended as, and should not be substituted for, professional medical, nutritional, or fitness advice. It is not a substitute for individualized guidance from a qualified physician, registered dietitian, or certified fitness professional.

Individual physiology, health status, and response to training and nutrition vary. Before beginning any new exercise program or making significant changes to diet, consult a physician, particularly if you are pregnant, have an existing medical condition, are taking medication, or have a history of injury.

Muscular Adaption and Recovery

Resistance training does not build muscle directly — it initiates a process that is completed during recovery.

Training induces microscopic damage to muscle fibers. In the hours following exercise, the body repairs this damage and adds new protein to the fiber through a process called muscle protein synthesis, which is the mechanism underlying muscular growth. Adequate recovery time is therefore essential to this process, not simply beneficial to it.

Research on protein timing supports this directly: studies indicate that approximately 30 grams of protein consumed after exercise is sufficient to maximize the muscle protein synthesis response, and whole-body protein balance improves with increasing post-exercise protein intake up to that threshold.

Micronutrients

Iron - Essential for oxygen transport to working muscle. Training itself can deplete iron stores, through sweat, foot-strike hemolysis, and elevated levels of a regulatory hormone called hepcidin that limits iron absorption following intense exercise. This is a particular consideration in endurance training and for individuals training at high frequency.

Calcium and Vitamin D — Support bone density, which is critical given the repeated mechanical stress of training.

Magnesium — Involved in muscle contraction, relaxation, and energy production.

B Vitamins — Support the conversion of food into usable energy.

Electrolytes (sodium, potassium, chloride) — Lost through sweat and required for proper muscle and nerve function; the demand for replacement increases with exercise duration and heat exposure.

Macronutrients

Carbohydrates - The body's primary fuel source, stored as glycogen in muscle and the liver. Depleted glycogen stores are a principal cause of fatigue during training. Consuming carbohydrate together with protein after exercise has also been shown to accelerate glycogen resynthesis relative to carbohydrate alone.

Protein - Supplies the amino acids required for the muscle protein synthesis described in Section 02. Distributing protein intake across meals throughout the day, rather than concentrating it in a single serving, supports a more consistent synthesis response, as the body can only utilize a finite amount at one time.

Fats - The primary fuel source for lower-intensity, longer-duration activity, and a necessary component in hormone production, including the hormones involved in tissue repair and muscular growth.

Cardiovascular Adaption

Consistent aerobic training produces structural changes to the heart and vascular system beyond the immediate caloric expenditure of a given session. Stroke volume (the amount of blood pumped per heartbeat) increases, capillary density within muscle tissue improves, and over the course of sustained training, resting heart rate decreases while VO2 max — a measure of the body's efficiency in using oxygen — increases.

Sources

Areta et al., "Timing and distribution of protein ingestion during prolonged recovery from resistance exercise alters myofibrillar protein synthesis," Journal of Physiology, 2013

Trommelen et al., "Dose-response effects of dietary protein on muscle protein synthesis during recovery from endurance exercise in young men," American Journal of Clinical Nutrition, 2020

Wilkinson et al., "Effect of exercise and recovery on muscle protein synthesis in human subjects," PubMed/American Journal of Physiology, 1990

Craven et al., "The Effect of Consuming Carbohydrate With and Without Protein on the Rate of Muscle Glycogen Re-synthesis During Short-Term Post-exercise Recovery: A Systematic Review and Meta-analysis," Sports Medicine Open, 2021

Pedlar, Brugnara, Bruinvels & Burden, "Iron balance and iron supplementation for the female athlete: A practical approach," European Journal of Sport Science; and "Iron and the endurance athlete," PubMed, 2014

Sim, Dawson, Landers, Trinder & Peeling, "Iron regulation in athletes: exploring the menstrual cycle and effects of different exercise modalities on hepcidin production," PMC systematic review, 2021