Muscle Strength Assessment

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The human body has over 600 muscles that are used for work, for play, and to accomplish activities of daily living (ADL). Skeletal muscle across the age continuum has the ability to adapt to the circumstances of its use and disuse (76). This ability to adapt (muscle plasticity) can be assessed through changes in muscle size, muscle architecture, enzyme activity, or isoform expression as well as organelle and extracellular structures (76).

Health-related physical fitness includes the characteristics of functional capacity and is affected by the physical activity level and other lifestyle factors (10,13,14,37,43–45,100,102,103,109,117). Maintaining an appropriate level of health-related physical fitness is necessary for a person to meet emergencies, reduce the risk of disease and injury (3,6,8,11,18,19,23,25,28,32,37,46,47,63,87,105,109,115), work efficiently (15,31), and participate and enjoy physical activity (4,5,15,21,24,26–28,30,39,40,66,84,86,110,112,122), (sports, recreation, leisure). A high health-related physical fitness level focuses on optimum health and prevents the onset of disease and problems associated with inactivity at all ages (15,40,42,87,90,92,102,103,108,112,119).

Minimal levels of muscular strength, muscular endurance, and muscular power (muscular fitness) are required to perform ADL, maintain functional independence during aging, and be able to participate in leisure time physical activities without undue fatigue or risk of injury (49,50,54,55,77,79,91,101,111,116,121). Musculoskeletal fitness requires muscle strength and endurance as well as bone strength (having the necessary bone mineral content and density to withstand repeated loads placed upon muscle insertion) (12,55,63). Adequate levels of muscular fitness may reduce the risk of low back problems, osteoporotic fractures, and musculoskeletal injuries (49,55,91,116).

Resistance training is performed with various exercise modalities including exercise machines, free weights, or the use of gravity acting on the participant’s body mass. Most resistance training (strength) programs are based on a system of exercise to a one repetition maximum (1-RM) as presented in the mid-1940s by Delorme (33) for use in physical medicine and rehabilitation. Every time the athlete/participant performs a particular exercise, the bout (or set) is performed for the maximum number of repetitions possible (repetition maximum or RM), and this number is recorded along with the mass lifted or opposing force imposed by an exercise machine (a range from 1-RM to 15-RM). The 1-RM is commonly utilized to accurately determine maximum strength for a specific movement, and this value is used to calculate percentages of one’s 1-RM depending on the desired exercise prescription goals. For example, calculating 50% of the 1-RM would give the exercise professional a target weight to incorporate into an exercise prescription for muscular endurance. Determination of the 1-RM is explained in more detail in Box 5.1 (69).

Box 5.1	One Repetition Maximum Testing Procedure
1. Tester demonstrates and explains the proper mechanics of the intended movement using only the bar (if applicable). Provide as much detail as is necessary based on previous resistance training experience by the lifter.
2. Practice the intended movement (e.g., the bench press, utilize a hooked-thumb grip) with the bar only until movement technique is correct according to the technician.
3. Two warm-up trials are recommended in obtaining a safe and accurate 1-RM.
4. For the first warm-up trial, the tester instructs the lifter to complete 5–10 repetitions with a load that the lifter reports as “easy” using correct form and movement velocity, followed by a 1-minute rest (51). The 1-minute rest interval should have the lifter perform 30 seconds of active flexibility movements within his or her full, available range of motion for the muscle group being assessed.
5. The second warm-up trial requires the lifter to complete three to five repetitions of the exercise movement at 60%–80% of the estimated 1-RM load and is followed by a 2-minute rest interval of active recovery inclusive of flexibility. For example, if the estimated 1-RM was 100 lb, then the second warm-up trial should be completed using 60–80 lb.
6. The third warm-up trial requires the lifter to complete two to three repetitions of the exercise movement at ~90%–95% of the estimated 1-RM. For example, if the estimated 1-RM was 100 lb, then the third warm-up trial should be completed using 90–95 lb. The lifter attempts two to three repetitions with correct form. Upon completion, the lifter rests 2–4 minutes or longer if necessary with active recovery inclusive of flexibility (51).
7. The technician increases the load equivalent to the lifter’s estimated 1-RM determined by the following 1-RM prediction equation: 1-RM = 100 ∙ rep wt / (102.78 − 2.78 ∙ reps) (22)
8. The lifter attempts only one repetition with this load. A successful or unsuccessful lift will require the lifter to rest 2–4 minutes to allow for a full recovery before attempting another 1-RM effort at the next higher or lower weight increment, respectively (51). For example, if the 100 lb was successfully lifted, an additional 2.5%–5% load should be added for the next 1-RM trial. If the 1-RM attempt was unsuccessful, the load should be reduced 2.5%–5% (51).
9. Continue the process described in step 8 until the participant fails to complete the exercise movement in the correct form. The 1-RM is generally achieved in three to five trials.
10. Record the 1-RM value as the maximum weight lifted with correct form.

From Abadie BR, Altorfer GL, Schuler PB. Does a regression equation to predict maximal strength in untrained lifters remain valid when the subjects are technique trained? J Strength Cond Res. 1999;13(3):259–63; Beam WC, Adams GM. Exercise Physiology Laboratory Manual. New York (NY): McGraw-Hill; 2011. 320 p.; Haff GG, Triplett NT, editors. Essentials of Strength Training and Conditioning. 4th ed. Champaign (IL): Human Kinetics; 2016. 752 p.; Heyward VH, Gibson AL. Advanced Fitness Assessment and Exercise Prescription. 7th ed. Champaign (IL): Human Kinetics; 2014. 552 p.; Kraemer WJ, Fry AC. Strength testing: development and evaluation of methodology. In: Maud PJ, Foster C, editors. Physiological Assessment of Human Fitness. Champaign (IL): Human Kinetics; 1995. p. 115–38; Levinger I, Goodman C, Hare DL, Jerums G, Toia D, Selig S. The reliability of the 1RM strength test for untrained middle-aged individuals. J Sci Med Sport. 2009;12(2):310–6; and Rontu JP, Hannula MI, Leskinen S, Linnamo V, Salmi JA. One-repetition maximum bench press performance estimated with a new accelerometer method. J Strength Cond Res. 2010;24(8):2018–25.

Muscular strength assessment has many purposes and has been safely administered across the age continuum (2,6,8,15,16,18,19,21,25–28,30,32,34–36,38–40,43–45,47,50,51,57,63,70). Muscle strength data collected in children and adolescents provide meaningful information and understanding of growth, maturation, effects of acute and regular response to physical activity, trainability, variation within normal patterns, and secular trends (27,30,36,39,40,51,56,69,77,82,84,89,105,107). Muscular strength assessment allows the exercise physiologist to identify strengths and weaknesses (baseline) and to set specific attainable goals (12,55,63). Furthermore, muscular strength assessment allows for tracking strength changes (improvements/decrements) over time allowing for adjustments in the exercise prescription that will bring about a positive physiological response (15,17,45,47,57,66,70,87,110,122).

Aging is associated with a decrease in skeletal muscle mass and muscle strength (sarcopenia) (19,20,37) and power (77) which may lead to lower physical function in ADL (chair stand, ascending stairs) (43–45,48,50,102,103,109). Low physical performance (strength and cardiorespiratory) has been demonstrated to predict both mobility and disability (96), nursing home admission, and mortality in community-dwelling older adults (50,109).

Muscular fitness testing can be conducted with persons of all ages, levels of physical conditioning, and training experience; however, protocols and techniques should be individualized to accommodate each patient’s levels of experience and strength (2,15,38–40,43–45,48,50,52,64,65,67,70,73,102–104). The evaluation must consider various musculoskeletal fitness parameters and be age-appropriate in order to ensure that valid and reliable measurements are obtained (15,38–40,102,103).

The goal of the strength or endurance test selected is for the assessment to be reliable (similar results twice), valid (truly measures what test claims to measure), and objective (correct results no matter who the tester). Furthermore, metabolic energy system (64,68,70,72,101,103,111,116) and biomechanical movement pattern specificity (64,68,70,99,106) are essential components of test selection because they simulate the energy demands and movement patterns of the participant’s ADL.

Dynamic Strength (Isotonic)

Dynamic exercise involves both concentric and eccentric muscle actions when the muscles are shortened or lengthened (12). The force-generating capacity of the muscle varies throughout the movement during dynamic movements. Dynamic strength is limited to the load that can be lifted by the weakest point within the specific joint system range of motion (66). Common free- and machine-weight exercises utilized as dynamic strength tests are the bench press and the back squat/leg press. Depending on the controlled or standardized conditions of these two tests, one could classify them as a field or laboratory tests (12).

Free-weight and constant-resistance weight machines are recommended for muscular fitness testing. Free-weight testing will require more neuromuscular coordination to both stabilize and maintain body balance by the participant and thus may be used with individuals with more resistance training experience. Utilization of machines for muscular fitness testing reduces the need for spotting (guarding) during the test and may be more appropriate with individuals who have limited resistance training experience or are unfamiliar with resistance training movements. However, use of such machine devices may limit the range of motion and the direction of the movement. Additional limitations of constant-resistance machines include weight increments that are too large and the inability to accommodate various body sizes (55). A common field test for lower extremity power is the vertical jump test (12). Furthermore, Faigenbaum and colleagues (40) have conducted similar research with children and found such muscular fitness assessment to be safe and appropriate; however, they had specialized equipment sized to youth participants. Beam and Adams (12) suggest that one of the most operational definitions of dynamic strength is expressed as the 1-RM for the specific movement tested (bench press, leg press, and back squat are commonly assessed): The highest load that can be lifted through a full range of motion utilizing correct form is the 1-RM. The 1-RM can be directly measured (see Box 5.1) through some trial and error or indirectly estimated from a submaximal effort for a summary of both test procedures. The 1-RM may be estimated by determining the number of submaximal repetitions completed to fatigue. Brzycki (22) and others (59,60,65,82) suggest that estimations of 1-RM using prediction equations are best suited using a load that allows no more than 10 repetitions to fatigue (~75% 1-RM). A sample 1-RM prediction equation is included in Box 5.1.

Muscular Endurance Strength

A component of muscular strength, muscular endurance, is the ability of a muscle group to execute repeated muscle contractions (58,63,97). Muscular endurance is important for many ADL including numerous household activities such as house cleaning, painting, lawn mowing (rotary push mower), and snow shoveling. Muscular endurance tests such as the 1-minute sit-up test and the push-up tests have traditionally been components of physical fitness assessments of youth (83,97), the military (U.S. Army), and other professionals to either compare with normative data previously collected (Aerobics Institute, Dallas, TX) or determine one’s readiness for a particular job’s specific task. There is no functional utility in the timed sit-up or push-up test; however, they are commonly used as measures of physical fitness. These movements do not comprise specific movement patterns that are used in ADL or in the workplace. The 10th edition of the American College of Sports Medicine (ACSM) guidelines has recently eliminated the timed 1-minute sit-up test. One can make a case that such assessments serve little functional utility and that other functional assessments found later in this chapter are more useful.

Isometric (Static) Strength–Handgrip Strength

The functional and clinical importance of isometric handgrip strength cannot be underestimated. The utility of this test spans the age continuum (7,46,52,62–64,68,71,77,85,88,119,123). Adequate handgrip strength represents the muscles of the forearm and has been associated with lower mortality (46,63,100), reduced risk of falling, and other ADL. Moreover, Ortega et al. (94) suggested that a low level of muscular strength in late adolescence, as measured by knee extension and handgrip strength tests, was associated with all-cause premature mortality to a similar extent as classic risk factors such as body mass index or blood pressure indicating the need for strength assessment in adolescents (52,94). Wind et al. (123) found that grip strength was strongly correlated with total muscle strength (r = .736–.890), suggesting that grip strength could be used as a general indicator for overall muscle strength.

Handgrip strength (r = .60) is related to muscle mass in older adults (63), and others have found higher correlations (53,80).

One of the common handgrip dynamometers, the Jamar (manufactured by Jamar Performance Products, Lafayette, IN), utilizes a sealed hydraulic system to activate its force indicator during static muscle action. Strength is often measured in units of force or torque. Isometric dynamometry expresses force in newtons (N); however, kilograms and pounds are commonly found on the dials of many handgrip dynamometers (12).

Isometric strength assessment may be made on many muscles using a cable tensiometer, but only the isometric handgrip strength test is described in Box 5.2.

Box 5.2	Procedures for Isometric Handgrip Dynamometer
1. The participant is in the standing position.
2. The participant’s head is in the mid-position (facing straight ahead).
3. The grip size may need to be adjusted so the third digit’s second phalanx is approximately at a right angle.
a. Grip adjustments of 1.3 cm (~0.5 in) on the Jamar dynamometer are made by removing the moveable handle and repositioning grip into one of the five manufactured slots; slot 1 = innermost position for the smallest grip size; slot 5 = slot at the outside position for the largest grip size.
b. Grip adjustments up to 4 cm for other dynamometers (Lafayette) are possible.
4. The grip setting (1–5 for Jamar) is recorded by the technician.
5. The participant’s forearm is placed at any angle between 90° and 180° of the upper arm (right angle to straight).
6. The participant’s wrist and forearm are positioned in the mid-prone position.
7. The participant is instructed to exert maximally and quickly after hearing the technician’s following instructions:
a. “Are you ready?”
b. “Squeeze as hard as you can.” As the participant begins the technician says, “Harder! . . . Harder! . . . Relax.”
8. The participant will be asked to complete two or three trials, alternatively with each hand.
9. The participant will be given a 30-second or up to a 1-minute rest between trials for the same hand.
10. The technician records the force in kilogram and then converts the circled best score to newtons by multiplying the kilogram value by 9.8066.
11. The technician resets the dynamometer’s pointer to 0 after each trial.