Muscle strength refers to the ability to produce force against an external resistance.^[8] It is often divided into specific types of strength, such as lower body strength or upper body strength.

Strength is commonly assessed using dynamic resistance exercise,^[9] which includes concentric (muscle shortening) and eccentric (muscle lengthening) muscle actions. The most popular method is a 1-repetition maximum (1RM) test,^[10] which involves lifting as much weight as possible for one repetition using either free weights or an exercise machine. A higher-repetition maximum test (i.e., a 2–6 RM) may also be used to assess strength and estimate 1RM strength.^[9]

Another option is an isometric strength test, which involves producing a maximal force against an immovable resistance.^[11] Unlike dynamic resistance exercise, the muscle length does not change during an isometric muscle action. Strong correlations have been reported between maximum dynamic and isometric strength.^[10]

In accordance with the principle of specificity — which states that training adaptations are specific to the demands imposed on the body — heavy loads (≥ 80% of 1RM) are superior to lighter loads for increasing 1RM strength.^[12][13] Training to muscular failure (i.e., the point at which another concentric repetition cannot be completed with proper form) is not necessary to increase muscle strength.^[14] In fact, ending each set a few reps shy of failure appears to be superior to training to failure for maximizing gains in 1RM strength.^[15][16] Also, rest intervals between sets should be at least 3 minutes.^[17][18]

Supplements marketed to enhance muscle strength typically claim to do so through one of the following mechanisms or a combination of them: increasing muscle contractile efficiency (e.g., by improving calcium handling in the sarcoplasmic reticulum), delaying muscular fatigue, increasing the availability of fuel sources (e.g., carbohydrate), and/or stimulating muscle protein synthesis.^[19]

The most effective supplements for increasing muscle strength appear to be creatine,^[20] protein,^[21] and caffeine.^[22] Other supplements that have been studied for muscle strength include nitrate, citrulline malate, HMB, alpha-GPC, taurine, ashwagandha, and omega-3 fatty acids.

Nutrition plays an important role in increasing muscle strength through fueling exercise and promoting recovery and exercise-induced adaptations. These processes are mainly influenced by protein and carbohydrate intake. Evidence suggests that a total daily protein intake of about 1.6 grams per kilogram of body weight is ideal for supporting increases in strength.^[23][21]

Muscle glycogen is a primary fuel source during resistance exercise,^[24] and glycogen depletion is associated with muscle fatigue and impaired muscle contraction efficiency,^[25] so consuming at least 3–5 grams of carbohydrate per kilogram of body weight per day is recommended to maximize strength gains.^[23]

With that said, many studies have not found differences in strength gains between higher- and lower-carbohydrate diets,^[26] particularly when the resistance exercise routine includes low volumes (< 10 sets per workout), high loads (≥ 80% of 1-repetition maximum), and long rest periods (≥ 3 minutes of rest between sets). However, there is a lack of evidence demonstrating benefits of lower-carbohydrate diets for muscle strength.

Differences in muscle strength between individuals seem to be mostly explained by differences in muscle mass,^{[27][28][29][30][31]} which is supported by the mechanistic rationale that a larger muscle has greater force-generating capacity.^[32] Other contributors to muscle strength include neural factors,^[8] such as the threshold at which motor units are recruited and the motor unit discharge rate, and genetics.^[33] Additionally, simply practicing the test used to examine strength (e.g., a back squat 1-repetition maximum) can promote increases in strength.^[34][35]