Muscle Strength

Muscle strength refers to the ability to produce force against an external resistance.^[1] 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,^[2] which includes concentric (muscle shortening) and eccentric (muscle lengthening) muscle actions. The most popular method is a 1-repetition maximum (1RM) test,^[3] 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.^[2]

Another option is an isometric strength test, which involves producing a maximal force against an immovable resistance.^[4] 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.^[3]

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.^[5][6] 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.^[7] 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.^[8][9] Also, rest intervals between sets should be at least 3 minutes.^[10][11]

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.^[12]

The most effective supplements for increasing muscle strength appear to be creatine,^[13] protein,^[14] and caffeine.^[15] 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.^[16][14]

Muscle glycogen is a primary fuel source during resistance exercise,^[17] and glycogen depletion is associated with muscle fatigue and impaired muscle contraction efficiency,^[18] so consuming at least 3–5 grams of carbohydrate per kilogram of body weight per day is recommended to maximize strength gains.^[16]

With that said, many studies have not found differences in strength gains between higher- and lower-carbohydrate diets,^[19] 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,^{[20][21][22][23][24]} which is supported by the mechanistic rationale that a larger muscle has greater force-generating capacity.^[25] Other contributors to muscle strength include neural factors,^[1] such as the threshold at which motor units are recruited and the motor unit discharge rate, and genetics.^[26] Additionally, simply practicing the test used to examine strength (e.g., a back squat 1-repetition maximum) can promote increases in strength.^[27][28]