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Definitions of Strength

This is an excerpt from Science and Development of Muscular Strength by Timothy J Suchomel.

Strength has been defined in a variety of ways within the strength and conditioning field. While many of these definitions focus on the maximum weight an athlete can lift during a specific exercise, others focus on the performance of a specific task. The following paragraphs will provide readers with different definitions of strength, highlight the context in which they are used, and discuss how they may be useful in sport and the physical development of athletes. Although general information regarding the assessment of these qualities will be mentioned in each section, a thorough overview of testing muscular strength is provided in chapter 9.

ABSOLUTE STRENGTH

Humans are competitive and frequently compare themselves to others in a variety of ways, including appearance, personality, athletic ability, and so on. In fact, social media and fitness influencers have capitalized on the question, “How much do you bench?” While this question may not be viewed seriously by many within the strength and conditioning field, the basis of the question is still rooted in human competition. In this light, absolute strength refers to the maximum weight an athlete can lift for either a single or given number of repetitions. This measure of strength capacity is quite general in that it does not account for athlete body weight, size, or body composition. Although this type of comparison may be applicable when comparing athletes who play the same sport or position within a sport (e.g., American football linemen, rugby props), comparisons between athletes who differ significantly in body weight are less applicable and do not provide strength and conditioning professionals with additional context about an athlete’s abilities. In general, heavier athletes have an absolute strength advantage over lighter athletes due to potentially greater magnitudes of muscle volume and, by extension, greater muscle cross-sectional area (34, 35, 70).

RELATIVE OR SCALED STRENGTH

Because heavier athletes may have an advantage in maximal strength, it is difficult to directly compare them to lighter athletes. To reduce the competitive advantage of heavier athletes, certain sports (e.g., weightlifting, judo, wrestling) have created weight classes in which athletes that are similar in size can compete with one another. Within these sports, those who are stronger hold a competitive advantage; in fact, the relative strength of an athlete may directly determine the final standing in competition in some sports (e.g., weightlifting and powerlifting). Because not all sports include weight classes, strength and conditioning professionals may require a method to directly compare the strength characteristics of athletes of different body sizes. This may be accomplished by calculating an athlete’s relative strength, which refers to the maximum weight an athlete can lift for either a single repetition or multiple repetitions divided by their body mass (load lifted / body mass). It should be noted that there are several scaling methods, including ratio scaling (described previously), allometric scaling (load lifted or force produced / body mass^0.67), and group-specific allometric scaling (load lifted or force produced / body mass^a; a = fitted allometric scaling exponent derived from a log-log transformation specific to the population assessed) (13, 34, 35, 70, 95); however, it is important that individuals measuring performance ensure that the appropriate assumptions are met before using a given method (95).

CONCENTRIC STRENGTH

Concentric muscle actions are characterized by a shortening of the muscle fibers resulting from greater contractile forces produced over resistive forces. Therefore, concentric strength refers to the maximum amount of weight an individual can lift during the concentric phase of an exercise (e.g., upward phase of a squat, bench press). Although concentric-only 1 repetition maximum (1RM) values have been assessed previously (27, 32, 97, 98, 99, 103), the concentric phase of a traditional exercise that includes both eccentric and concentric muscle actions limits the athlete’s strength potential in that lift because athletes are weaker during concentric actions compared to eccentric actions (72). Thus, strength and conditioning professionals likely use paired muscle actions when assessing an athlete’s concentric strength characteristics (e.g., back squat, bench press). It should be noted that some sporting events such as cycling (7) and swimming consist almost exclusively of concentric muscle actions, which may provide support for performing concentric-only maximal strength tests with these athletes.

ECCENTRIC STRENGTH

While concentric actions shorten muscle length, eccentric actions are characterized by a lengthening of the muscle fibers due to greater resistive forces compared to contractile forces. Eccentric strength has been shown to be 20% to 60% (29) or approximately 40% (72) greater than concentric strength. Like concentric-only strength assessments, researchers have also used eccentric-only 1RMs to assess the eccentric strength characteristics of participants (29, 84). It should be noted that regarding different tasks performed in sport, eccentric muscle actions rarely occur in isolation. In fact, eccentric and concentric muscle actions are often performed in succession during actions such as jumping, sprinting, and change-of-direction tasks, a pairing termed the stretch–shortening cycle. This had led some researchers to assess eccentric strength qualities during stretch–shortening cycle actions (12); however, further research needs to be completed to determine whether this is a valid method.

ISOMETRIC STRENGTH

Isometric muscle actions are characterized by no demonstrable changes in muscle fiber length due to equal contractile and resistive forces. Although isometric muscle actions may occur in specific sporting situations (e.g., rugby scrum, competitive climbing, wrestling), these actions, however brief, also occur between the eccentric and concentric actions as part of the stretch–shortening cycle (termed the amortization phase within plyometric exercises). In general, an athlete’s isometric strength will be greater than their concentric strength but less than their eccentric strength (18). Whereas concentric strength and eccentric strength are typically measured in pounds or kilograms, isometric strength is quantified in Newtons or kilograms of force and may be assessed during both single-joint (e.g., knee extension or flexion) (73, 77) and multi-joint (e.g., mid-thigh pull, squat, bench press) tests (4, 14, 68). Although isometric strength may not seem to relate to dynamic strength based on appearance (i.e., no dynamic movement), researchers have shown positive relationships between isometric and dynamic strength (58, 59).

STRENGTH–ENDURANCE

Strength–endurance refers to the ability of an athlete to perform a large number of exercise repetitions at a given weight or percentage of their maximum. Although they are related to an athlete’s endurance performance, it is important to note that the terms strength–endurance, muscular endurance, and cardiorespiratory endurance are not synonymous. For example, strength–endurance may be defined as performing 8 to 12 repetitions using moderate to moderately heavy loads, with the goal of improving work capacity; muscular endurance involves performing a large number of repetitions (≥12) using a submaximal load that may not contribute to maximal strength adaptations; cardiorespiratory endurance is an aerobic characteristic that refers to the ability to use oxygen for energy production during longer-duration, submaximal activity (e.g., running, cycling). Although it is not ideal due to a wide spectrum of body sizes and relative strength, the National Football League (NFL) Combine requires athletes to perform bench press repetitions at 225 lb (102 kg) until failure (i.e., referred to as the NFL-225 test) (56). As of 2023, the official NFL Combine record is 49 repetitions, which ultimately makes this a test of muscle endurance rather than maximal strength for many athletes. This notion is supported by the fact that the NFL-225 bench press test may not serve as a good test for tracking maximal strength in collegiate football players (55).

STRENGTH–SPEED

Strength–speed is a characterization of specific training methods or exercises that focus on the intent to move heavy loads quickly. Examples of exercises that may be classified as strength–speed exercises are weightlifting movements and their derivatives (e.g., clean, clean grip hang pull, snatch pull from the floor) (89) as well as heavily loaded jumps (e.g., hexagonal barbell jumps) (91, 92). While the prescription of strength–speed exercises may vary based on the individual needs of each athlete, these exercises may be featured in both general and absolute strength phases but also during strength–power and taper phases to enhance early rapid force production characteristics (93).

SPEED–STRENGTH

In contrast to strength–speed, speed–strength training methods and exercises are characterized by the intent to move light loads quickly. Exercises that fall under the speed–strength category are typically ballistic in nature and are implemented using relatively light loads, with the goal of improving rapid force production and power output characteristics (e.g., plyometric exercises, unloaded and lightly loaded jumps, bench press medicine ball throws) (91-93). During strength–power phases of training, strength and conditioning professionals may also pair strength exercises with speed–strength exercises to form a potentiation complex (see chapter 6 for more detail) and potentially augment the training stimulus an athlete receives (8, 80, 90, 100). However, like strength–speed exercises, speed–strength exercises should be prescribed based on the needs of the athlete during phases that complement their overall training goals.

REACTIVE STRENGTH

The reactive strength characteristics of an athlete refer to their ability to quickly transition between eccentric and concentric muscle actions (111). Researchers have indicated that reactive strength is strongly associated with maximal strength, especially eccentric strength (5). The reactive strength characteristics of an individual are often assessed during plyometric exercises such as a drop jump and are quantified as a reactive strength index (jump height / ground contact time) (63). While researchers have examined the modified reactive strength index during a countermovement jump (jump height / time to takeoff) (19, 60, 88), it should be noted that each variant may reflect different athlete qualities (50). This is likely due to the differences in braking and propulsion phase duration between drop jumps and countermovement jumps, because the latter is less reactive in nature. It should be noted that although upper-body drop “jumps” can be performed (96), further research is needed to determine whether these actions are truly reactive based on their phase durations.

ISOKINETIC STRENGTH

Isokinetic muscle actions are those in which the velocity of the movement is constant. The isokinetic strength qualities of an athlete are typically assessed using an isokinetic dynamometer and are quantified as a torque (Newtons × meters). Beyond some correlation research (57, 77, 101), isokinetic strength qualities are typically not assessed in healthy athletes. For example, athletes returning from an anterior cruciate ligament tear may complete isokinetic testing to determine the torque produced from both the knee flexors and extensors (102). The ability to test injured athletes in this manner may provide rehabilitation professionals and strength and conditioning professionals with information regarding the strength characteristics of an athlete performed under controlled conditions (i.e., velocity) as part of a return-to-play protocol. However, isokinetic testing may not be frequently completed in real-world scenarios with healthy athletes, owing to the need for specialized equipment, testing space, tester competency, and assessment of joints in isolation. Regarding isolated joint assessment, researchers have shown that changes in dynamic (1RM) and isokinetic strength are not equivalent (24); thus, the applicability of isokinetic strength and performance may be limited in some capacity.

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