“Muscle size” refers to the goal of muscle mass gain (or hypertrophy), which occurs primarily as a result of an increase in the size and/or number of myofibrils (i.e., bundles of protein filaments within muscle fibers that produce muscle contraction).^[1] “Muscle strength” refers to the ability to produce force against an external resistance.^[2]

Muscle size is measured both at the whole-body and muscle-specific level. In the former, generally fat-free mass or lean mass is distinguished from fat mass and is commonly measured using either dual-energy x-ray absorptiometry (DEXA-DXA|DXA) or bioelectrical impedance analysis (BIA). Other assessment methods include ultrasound, computed tomography, and magnetic resonance imaging.^[3]

Muscle strength is most commonly assessed using dynamic resistance exercise in the form of a 1-repetition maximum (1RM) test.^[4][5] A higher-repetition maximum test (i.e., a 2–6 RM test) may also be used to assess strength and estimate 1RM. Another option is an isometric strength test, which involves producing a maximal force against an immovable resistance.^[6] Unlike dynamic resistance exercise, the muscle length does not change during an isometric muscle action.

Resistance exercise is the most effective means for increasing muscle size and strength. For muscle gain, a wide spectrum of loading ranges (approximately 40%–85% of 1-repetition maximum) are similarly effective,^[7] whereas for strength, heavy loads (≥ 80% of 1-repetition maximum) are superior to lighter loads.^[8][9]

About 10–20 sets should be performed per week for muscle gain,^[10] whereas muscle strength is effectively built with lower training volumes of about 5–9 sets per week.^[11][12]

For both muscle size and strength, a rest interval of at least 3 minutes between sets is best for most exercises.^[13] A shorter rest interval of about 60–90 seconds may be employed for single-joint and certain machine-based exercises during muscle gain-oriented workouts.^[14]

Supplements marketed to improve muscle size and strength are purported to do so through by enhancing resistance exercise performance (i.e., by enhancing force production or muscular endurance), by stimulating muscle protein synthesis, by increasing the availability of fuel sources (e.g., carbohydrate), and/or by supporting recovery.^[15]

Some of the most effective options are creatine,^[16] caffeine,^[17] and protein.^[18] Other popular options include citrulline, nitrate, beta-alanine, ashwagandha, betaine, alpha-GPC, and taurine.

Nutrition plays an important role in increasing muscle size and strength by fueling exercise, promoting recovery, and providing the materials to build up muscle. These processes are mainly influenced by protein and carbohydrate intake. Evidence suggests that a total daily protein intake of 1.6–2.2 grams per kilogram of body weight is ideal for supporting increases in muscle size and strength.^[18][18]

Muscle glycogen is the primary fuel source during resistance exercise,^[19] and glycogen depletion is associated with muscular fatigue and impaired muscle contraction efficiency,^[20] so consuming at least 3–5 grams of carbohydrate per kilogram of body weight per day is recommended to maximize increases in muscle size and strength.^[21][22]

Nonetheless, many studies have not found differences in strength gains between higher- and lower-carbohydrate diets.^[23] This is likely due to the fact that compared to muscle-gain-oriented workouts, those focused on strength adaptations tend to feature lower volumes and longer rest periods, relying less on carbohydrates for energy.

How much muscle and strength an individual gains in response to a resistance exercise program is influenced by a myriad of factors, including their age, genetics,^[24][25][26] lifestyle (i.e., sleep, nutrition, and stress management), and training history. For instance, the magnitude of muscle gain tends to decrease with age^[27][28][29] and training experience.^[22][9]

Generally speaking, the majority of a person’s strength can be attributed to their muscle mass, ^{[30][31][32][33][34]} which is supported by the mechanistic rationale that a larger muscle has greater force-generating capacity.^[35] Put simply, in most cases, the best way to improve one’s strength is to increase one’s muscle mass. Neural factors (e.g., the threshold at which motor units are recruited and the motor unit discharge rate^[2]) also contribute to muscle strength, and the muscle strength one can deploy in a given exercise can be increased simply by practicing that exercise.^[36][37]