Strength refers to the ability to overcome resistance. It not the same as power, which has to do with the speed and explosiveness with which resistance can be overcome.
Muscle strength refers to the ability to produce force against an external resistance.[reference|url=https://pubmed.ncbi.nlm.nih.gov/29372481|title=The Importance of Muscular Strength: Training Considerations.|published=2018-Apr|authors=Suchomel TJ, Nimphius S, Bellon CR, Stone MH|journal=Sports Med|] 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,[reference|url=https://pubmed.ncbi.nlm.nih.gov/26838985|title=The Importance of Muscular Strength in Athletic Performance|published=2016 Oct|authors=Suchomel TJ, Nimphius S, Stone MH|journal=Sports Med|] which includes concentric (muscle shortening) and eccentric (muscle lengthening) muscle actions. The most popular method is a 1-repetition maximum (1RM) test,[reference|url=https://pubmed.ncbi.nlm.nih.gov/24497158|title=A brief review of strength and ballistic assessment methodologies in sport.|published=2014-May|authors=McMaster DT, Gill N, Cronin J, McGuigan M|journal=Sports Med|] 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.[reference|url=https://pubmed.ncbi.nlm.nih.gov/26838985|title=The Importance of Muscular Strength in Athletic Performance|published=2016 Oct|authors=Suchomel TJ, Nimphius S, Stone MH|journal=Sports Med|]
Another option is an isometric strength test, which involves producing a maximal force against an immovable resistance.[reference|url=https://pubmed.ncbi.nlm.nih.gov/8819238|title=The use of isometric tests of muscular function in athletic assessment.|published=1996-Jul|authors=Wilson GJ, Murphy AJ|journal=Sports Med|] 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.[reference|url=https://pubmed.ncbi.nlm.nih.gov/24497158|title=A brief review of strength and ballistic assessment methodologies in sport.|published=2014-May|authors=McMaster DT, Gill N, Cronin J, McGuigan M|journal=Sports Med|]
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.[reference|url=https://pubmed.ncbi.nlm.nih.gov/35015560|title=Muscle hypertrophy and strength gains after resistance training with different volume-matched loads: a systematic review and meta-analysis.|published=2022-Apr|authors=Carvalho L, Junior RM, Barreira J, Schoenfeld BJ, Orazem J, Barroso R|journal=Appl Physiol Nutr Metab|][reference|url=https://pubmed.ncbi.nlm.nih.gov/33433148|title=Resistance Training Load Effects on Muscle Hypertrophy and Strength Gain: Systematic Review and Network Meta-analysis|published=2020 Dec 26|authors=Pedro Lopez, Regis Radaelli, Dennis R Taaffe, Robert U Newton, Daniel A Galvão, Gabriel S Trajano, Juliana Teodoro, William J Kraemer, Keijo Häkkinen, Ronei S Pinto|journal=Med Sci Sports Exerc|] 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.[reference|url=https://pubmed.ncbi.nlm.nih.gov/33497853|title=Effects of resistance training performed to repetition failure or non-failure on muscular strength and hypertrophy: A systematic review and meta-analysis.|published=2022-03|authors=Grgic J, Schoenfeld BJ, Orazem J, Sabol F|journal=J Sport Health Sci|] 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.[reference|url=https://pubmed.ncbi.nlm.nih.gov/35038063|title=The Effect of Load and Volume Autoregulation on Muscular Strength and Hypertrophy: A Systematic Review and Meta-Analysis.|published=2022-Jan-15|authors=Hickmott LM, Chilibeck PD, Shaw KA, Butcher SJ|journal=Sports Med Open|][reference|url=https://osf.io/preprints/sportrxiv/v3tr9/||title=The effects of different intra-set velocity loss thresholds on lower-limb adaptations to resistance training in young adults: A systematic review and meta-analysis|authors=Gantois et al|published=2021-06] Also, rest intervals between sets should be at least 3 minutes.[reference|url=https://pubmed.ncbi.nlm.nih.gov/36157335|title=Manipulating Resistance Training Variables to Induce Muscle Strength and Hypertrophy: A Brief Narrative Review.|published=2022|authors=DE Camargo JBB, Brigatto FA, Zaroni RS, Trindade TB, Germano MD, Junior ACT, DE Oliveira TP, Marchetti PH, Prestes J, Lopes CR|journal=Int J Exerc Sci|][reference|url=https://pubmed.ncbi.nlm.nih.gov/19204579|title=American College of Sports Medicine position stand. Progression models in resistance training for healthy adults|published=2009 Mar|authors=American College of Sports Medicine|journal=Med Sci Sports Exerc|]
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.[reference|url=https://journals.lww.com/nsca-scj/Abstract/2020/10000/Emerging_Nutritional_Supplements_for_Strength_and.7.aspx|title=Emerging Nutritional Supplements for Strength and Hypertrophy: An Update of the Current Literature|authors=Gonzalez et al\journal=Strength and Conditioning Journal|published=2020-10]
The most effective supplements for increasing muscle strength appear to be creatine,[reference|url=https://pubmed.ncbi.nlm.nih.gov/34199588|title=Creatine for Exercise and Sports Performance, with Recovery Considerations for Healthy Populations.|published=2021-Jun-02|authors=Wax B, Kerksick CM, Jagim AR, Mayo JJ, Lyons BC, Kreider RB|journal=Nutrients|] protein,[reference|url=https://pubmed.ncbi.nlm.nih.gov/28698222|title=A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults|published=2018 Mar|authors=Morton RW, Murphy KT, McKellar SR, Schoenfeld BJ, Henselmans M, Helms E, Aragon AA, Devries MC, Banfield L, Krieger JW, Phillips SM|journal=Br J Sports Med|] and caffeine.[reference|url=https://pubmed.ncbi.nlm.nih.gov/34291426|title=Effects of Caffeine on Resistance Exercise: A Review of Recent Research.|published=2021-11|authors=Grgic J|journal=Sports Med|] 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.[reference|url=https://pubmed.ncbi.nlm.nih.gov/21660839|title=Nutrition guidelines for strength sports: sprinting, weightlifting, throwing events, and bodybuilding.|published=2011|authors=Slater G, Phillips SM|journal=J Sports Sci|][reference|url=https://pubmed.ncbi.nlm.nih.gov/28698222|title=A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults|published=2018 Mar|authors=Morton RW, Murphy KT, McKellar SR, Schoenfeld BJ, Henselmans M, Helms E, Aragon AA, Devries MC, Banfield L, Krieger JW, Phillips SM|journal=Br J Sports Med|]
Muscle glycogen is a primary fuel source during resistance exercise,[reference|url=https://pubmed.ncbi.nlm.nih.gov/12076177|title=Fatigue during high-intensity intermittent exercise: application to bodybuilding.|published=2002|authors=Lambert CP, Flynn MG|journal=Sports Med|] and glycogen depletion is associated with muscle fatigue and impaired muscle contraction efficiency,[reference|url=https://pubmed.ncbi.nlm.nih.gov/33900579|title=Muscle Glycogen Metabolism and High-Intensity Exercise Performance: A Narrative Review|published=2021 Sep|authors=Jeppe F Vigh-Larsen, Niels Ørtenblad, Lawrence L Spriet, Kristian Overgaard, Magni Mohr|journal=Sports Med|] so consuming at least 3–5 grams of carbohydrate per kilogram of body weight per day is recommended to maximize strength gains.[reference|url=https://pubmed.ncbi.nlm.nih.gov/21660839|title=Nutrition guidelines for strength sports: sprinting, weightlifting, throwing events, and bodybuilding.|published=2011|authors=Slater G, Phillips SM|journal=J Sports Sci|]
With that said, many studies have not found differences in strength gains between higher- and lower-carbohydrate diets,[reference|url=https://pubmed.ncbi.nlm.nih.gov/35215506|title=The Effect of Carbohydrate Intake on Strength and Resistance Training Performance: A Systematic Review.|published=2022-Feb-18|authors=Henselmans M, Bjørnsen T, Hedderman R, Vårvik FT|journal=Nutrients|] 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,[reference|url=https://pubmed.ncbi.nlm.nih.gov/34617822|title=Behavior of motor units during submaximal isometric contractions in chronically strength-trained individuals.|published=2021-Nov-01|authors=Casolo A, Del Vecchio A, Balshaw TG, Maeo S, Lanza MB, Felici F, Folland JP, Farina D|journal=J Appl Physiol (1985)|][reference|url=https://pubmed.ncbi.nlm.nih.gov/32187154|title=Performance and Anthropometrics of Classic Powerlifters: Which Characteristics Matter?|published=2022-Apr-01|authors=Ferrari L, Colosio AL, Teso M, Pogliaghi S|journal=J Strength Cond Res|][reference|url=https://pubmed.ncbi.nlm.nih.gov/11990746|title=The role of FFM accumulation and skeletal muscle architecture in powerlifting performance.|published=2002-Feb|authors=Brechue WF, Abe T|journal=Eur J Appl Physiol|][reference|url=https://pubmed.ncbi.nlm.nih.gov/19153760|title=Anatomical predictors of maximum isometric and concentric knee extensor moment.|published=2009-Apr|authors=Blazevich AJ, Coleman DR, Horne S, Cannavan D|journal=Eur J Appl Physiol|][reference|url=https://sciendo.com/article/10.2478/v10078-010-0040-3|title=Correlations of Anthropometric and Body Composition Variables with the Performance of Young Elite Weightlifters|authors=Siahkouhian and Hedayatneja|journal=Journal of Human Kinetics|published=2009-01] which is supported by the mechanistic rationale that a larger muscle has greater force-generating capacity.[reference|url=https://pubmed.ncbi.nlm.nih.gov/31016546|title=Exercise-Induced Myofibrillar Hypertrophy is a Contributory Cause of Gains in Muscle Strength|published=2019 Jul|authors=Taber CB, Vigotsky A, Nuckols G, Haun CT|journal=Sports Med|] Other contributors to muscle strength include neural factors,[reference|url=https://pubmed.ncbi.nlm.nih.gov/29372481|title=The Importance of Muscular Strength: Training Considerations.|published=2018-Apr|authors=Suchomel TJ, Nimphius S, Bellon CR, Stone MH|journal=Sports Med|] such as the threshold at which motor units are recruited and the motor unit discharge rate, and genetics.[reference|url=https://pubmed.ncbi.nlm.nih.gov/34366884|title=Effects of Genetic Variation on Endurance Performance, Muscle Strength, and Injury Susceptibility in Sports: A Systematic Review.|published=2021|authors=Appel M, Zentgraf K, Krüger K, Alack K|journal=Front Physiol|] Additionally, simply practicing the test used to examine strength (e.g., a back squat 1-repetition maximum) can promote increases in strength.[reference|url=https://pubmed.ncbi.nlm.nih.gov/28463902|title=Practicing the Test Produces Strength Equivalent to Higher Volume Training.|published=2017-Sep|authors=Mattocks KT, Buckner SL, Jessee MB, Dankel SJ, Mouser JG, Loenneke JP|journal=Med Sci Sports Exerc|][reference|url=https://pubmed.ncbi.nlm.nih.gov/27875635|title=Muscle adaptations following 21 consecutive days of strength test familiarization compared with traditional training.|published=2017-Aug|authors=Dankel SJ, Counts BR, Barnett BE, Buckner SL, Abe T, Loenneke JP|journal=Muscle Nerve|]
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