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Original Research

Associations of Maximal Strength and Muscular Endurance Test Scores with Cardiorespiratory Fitness and Body Composition

Vaara, Jani P.1,2; Kyröläinen, Heikki1,2; Niemi, Jaakko1; Ohrankämmen, Olli3; Häkkinen, Arja4,5; Kocay, Sheila2; Häkkinen, Keijo2

Author Information
Journal of Strength and Conditioning Research: August 2012 - Volume 26 - Issue 8 - p 2078-2086
doi: 10.1519/JSC.0b013e31823b06ff
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Abstract

Introduction

Fitness testing is important for monitoring chronic training adaptations and evaluating the level of physical performance at both a population level and in more distinct groups, such as athletes. The most commonly used physical performance tests in fitness testing are the evaluation of maximal aerobic capacity and muscular fitness of which the latter can be divided to muscular endurance and maximal strength (41). The ability of a muscle or group of muscles to exert external forces repeatedly over a period of time describes muscular endurance, whereas the maximal strength is the ability to produce maximal force against resistance exerted by a muscle or group of muscles in a single maximal voluntary contraction (22,23).

Both maximal strength and muscular endurance can be determined in both laboratory and field conditions. The field-based tests are commonly used in epidemiological studies, fitness assessments, and in school and military settings (5,22). Advantages of the field-based tests are time efficiency, low cost, and no requirements of any specific equipment. In addition, they can be administered to a large number of participants simultaneously (5). Widely used muscular endurance tests include push-ups and pull-ups for upper extremities, repeated squats for lower extremities, and sit-ups and static back extension for trunk muscles (9,32). Maximal strength tests include dynamic (mainly concentric), isokinetic, or isometric performances for the upper or lower extremities and for trunk muscles using various dynamometers (18,25). Given the popularity of these muscular fitness tests, the information available on their associations, especially those of maximal strength and muscular endurance with cardiorespiratory fitness and for body composition is surprisingly incomplete.

Previous studies have shown moderate positive relationships between lean body or fat-free mass (FFM) and maximal strength (21,26). In addition, body fat (BF) indices have repeatedly been inversely associated with muscular endurance (7–9,25). Inverse associations between BF and maximal strength, and muscular endurance and maximal oxygen uptake (V[Combining Dot Above]O2max) have also been observed (6,9,31,42). These previous studies have well established the relationships between body composition, maximal oxygen uptake, and muscular fitness (7–9,21,25,26,31,43). However, the relationship between muscular endurance and maximal strength tests is far less studied. Therefore, this study was designed to investigate the relationship between maximal strength and muscular endurance test scores additionally to previously studied associations with body composition and maximal aerobic capacity.

Methods

Experimental Approach to the Problem

For the purpose of the study, anthropometric and body composition variables and physical fitness tests were measured in 846 young men while they were participating in military refresher training organized by the Finnish Defence Forces. To address the study objectives, the associations of maximal strength tests, muscular endurance tests, maximal aerobic capacity, and body composition were examined. Detailed information is given in the following sections.

Subjects

One thousand one hundred and fifty-five Finnish men were invited for military refresher training organized by the Finnish Defence Forces. Out of these 1155, 920 men participated in the training course (92%). The most typical reasons for nonparticipation were work-, study-, or health-related issues. Out of the 920 men, 846 men (age 25.1 ± 4.6 years, range 18–48 years) volunteered to take part in the study (73.2%). Not all the participants took part in each fitness test, so the final study sample consisted of 773–846 young Finnish men (Table 1). The study sample was compared with the corresponding cohorts of 20- to 30-year-old Finnish men in the national register data (Statistics Finland) from 2007 to 2008 for the following variables: age, education, and place of residence. Based on these comparisons, it can be concluded that the study sample is representative of an average Finnish male between the ages of 20 and 30 years, with the exclusion of the Northern-part of Finland (underrepresented in this study sample) (Statistics Finland). The subjects were carefully informed about the design of the study, and they signed an informed consent before voluntary participation in the study. This study was approved by the Ethical Committee of the Central Finland Health Care District, The Scientific Advisory Board for Defence, Finland, and the University of Jyväskylä, Finland. The amount of leisure time physical activity (LTPA) of the subjects was determined by a questionnaire. Self-reported LTPA of the subjects varied largely: 12.7% reported no LTPA at all and 38.0% some LTPA without feeling out of breath or sweating or with feeling out of breath or sweating once a week. Of the subjects, 19.4% reported LTPA while feeling out of breath or sweating twice a week and 29.9% LTPA while feeling out of breath or sweating a week ≥3 times per week.

T1-10
Table 1:
Descriptive characteristics of body composition variables and the results of physical fitness tests.

Procedures

The measurements were carried out in 8 different sessions during 2008 (March–November). The participants arrived at the garrisons latest by 14:00 o'clock. They were divided into groups of 10 persons for measurements, which started in the next morning at 5:50 after a night of sleep with an overnight fast with only water consumed ad libitum. At first, the body composition was measured, and afterward the participants ate a light breakfast and drank a maximum of 1 cup of coffee or tea. After the breakfast, grip strength and bilateral maximal isometric leg extension and bench press were measured. Finally, an indirect graded cycle ergometer test and tests of muscular endurance (push-ups, sit-ups, repeated squats) were performed.

Maximal Strength

Bilateral maximal isometric leg extension and arm extension forces were measured using dynamometers for leg press and bench press, respectively. The knee angle was set to 107° with a goniometer, and hands were placed on handle grips for the leg extension test (17). During the maximal bench press protocol, the participants were in a supine position with their back flat on a bench and feet flat on the floor, with elbows and shoulders positioned at an angle of 90°. Warm-up included at least 2 submaximal trials before maximal trials. Three maximal trials were performed with 30 seconds of recovery between trials. The best performance was included for further analysis. The participants were instructed on the testing protocol and saw a demonstration of the correct technique for each test before testing. They were also instructed to produce maximal strength as fast as possible and to maintain it for 3 seconds. Furthermore, the testing personnel encouraged them during the maximal efforts. Maximal force and force production time were collected with an AD converter (CED power 1401, Cambridge Electronic Design, Ltd., Cambridge, United Kingdom) at a frequency of 1 KHz on a computer. Data were analyzed with Signal (2.16) software. The repeatability has been reported to be high in maximal isometric strength tests (r = 0.98, coefficient of variation = 4.1%) (37). Maximal isometric grip strength was measured in a sitting position (Saehan Corporation, Masan, South Korea). Grip width was adjusted individually by 2 handle options. Elbow angle was maintained at 90° during the performance. The test was executed twice for each hand, alternating hands between trials. For the analysis, the best result of each hand was averaged for the determination of the overall grip strength result. The test-retest reliability of the measurement has been reported to be high in young adults (intraclass correlation coefficient [ICC] = 0.94) (35). A maximal strength index (MSI) was calculated by transforming the results of the 3 maximal isometric strength test scores to z-scores. Thereafter, the z-scores of maximal strength tests were averaged to obtain the MSI.

Muscular Endurance

Tests for muscular endurance consisted of push-ups, sit-ups, and repeated squat tests. The push-up test measures performance of arm and shoulder extensor muscles, and trunk muscles to stabilize the trunk during the performance (8–10). The sit-up test is a measure of the abdominal and hip flexor muscles (29), whereas the repeated squat test measures the performance of the knee flexor and extensor muscles (19). The participants were instructed to perform as many repetitions as they were able to during 60 seconds. Between each test, a recovery period of 5 minutes was allotted. Only repetitions completed with the correct technique were counted. During the test performances, test personnel observed the participants' technique. The test-retest reliability of the push-up, sit-up, and repeated squat tests has been reported to be high among young adults and middle-aged adults (ICC = 0.93–0.95, ICC = 0.83–0.93, r = 0.95, respectively) (1,2). A muscular endurance index (MEI) was calculated by transforming the results of the 3 muscular endurance test scores to z-scores. Thereafter, the z-scores of muscular endurance tests were averaged to form the MEI.

Maximal Aerobic Capacity

Maximal aerobic capacity (V[Combining Dot Above]O2max) was indirectly determined using a graded cycle ergometer test (Ergoline 800S, Ergoselect 100K, Ergoselect 200K, Bitz, Germany). A progressive protocol was used with an initial power output of 50 W increased by 25 W every 2 minutes until exhaustion (volitional fatigue or inability to maintain a pedalling cadence of at least 60 rpm). The heart rate was continuously recorded during the test using heart rate monitors (Polar Vantage NV, Polar Electro, Kempele, Finland).

Body Composition

Body composition was determined using bioelectrical impedance analysis (BIA) (Inbody 720, Seoul, South Korea) after an overnight fast, to determine fat mass (FM), %BF, and FFM. The BIA estimates of body composition have shown to highly correlate with the dual-energy x-ray absorptiometry (DXA) method (r = 0.82–0.95) (4). Body mass and height were measured to the closest 0.1 kg and 0.1 cm, respectively on a commercial scale. Body mass index (BMI) was calculated, and waist circumference (WC) was measured at the level of the iliac crest after exhaling by a cloth tape measure.

Statistical Analyses

Analyses were carried out using PASW software (PASW for Windows 18.0.1). Descriptive statistics as means and SDs and minimum and maximum were calculated for anthropometry, body composition, and physical fitness tests of the study sample. After verifying the normality, the associations of maximal strength tests, muscular endurance tests, maximal aerobic capacity, and body composition were examined using Pearson correlation coefficients and simple regression analyses. Further, because of the intercorrelated structure of muscular endurance and of maximal strength tests, multiple multivariate regressions were performed to calculate the standardized β-coefficients. First, muscular endurance tests were explained by maximal strength tests and body composition, and second, strength tests were explained by endurance tests and body composition. The level of significance was set at p ≤ 0.05.

Results

The anthropometry, body composition, and physical fitness test results are presented in Table 1. This shows that there was a large variation for all the physical fitness parameters.

The number of push-ups was correlated with maximal force in the bench press (r = 0.61, p < 0.001) and with V[Combining Dot Above]O2max (r = 0.46, p < 0.001) (Figure 1). In addition, the multiple multivariate regression revealed a strong association between push-ups and bench press (β = 0.67, p < 0.001) (Table 4). The correlation between maximal leg extension force and the number of repeated squats was weak (r = 0.23, p < 0.001), but a correlation was found between maximal leg extension force and V[Combining Dot Above]O2max (r = 0.55, p < 0.001) (Figure 2). In the regression model repeated squats as dependent variable, the standardized β–coefficient for leg extension was 0.19 and for V[Combining Dot Above]O2max 0.43 (p < 0.001), respectively (Table 4).

F1-10
Figure 1:
Pearson correlation coefficient for push-ups and bench press (A) and for push-ups and V[Combining Dot Above]O2max (B).
T2-10
Table 4:
The standardized β-coefficients from multiple multivariate regression model.*
F2-10
Figure 2:
Pearson correlation coefficient for repeated squats and leg extension (A) and for repeated squats and V[Combining Dot Above]O2max (B).

Maximal force in bench press was also correlated with the number of sit-ups (r = 0.37, p < 0.001) and maximal grip strength (r = 0.34, p < 0.001). The number of push-ups correlated with repeated squats (r = 0.55, p < 0.001) and sit-ups (r = 0.65, p < 0.001). In addition, sit-ups and MEI correlated with V[Combining Dot Above]O2max (r = 0.48–0.58, p < 0.001), respectively (Table 2).

T3-10
Table 2:
Pearson correlation coefficient for muscular endurance, muscular strength, and aerobic capacity.

Maximal force of the bench press and leg extension showed low correlations with BM and BMI, whereas correlations were moderate with FFM (Table 3). When explaining the leg extension in the multivariate approach, the standardized β-coefficient for FFM was 0.31. Similarly, in the same analysis, explaining the bench press, the FFM had the standardized β-coefficient of 0.36 (p < 0.001) (Table 4). In addition, WC was weakly correlated with maximal force of the leg extensors (r = 0.21, p < 0.001) (Table 3). Grip strength showed a low correlation with BM (r = 0.27, p < 0.001) and a moderate correlation with FFM (r = 0.44, p < 0.001) and the association in the regression model was moderate (β = 0.53, p < 0.001) (Tables 3 and 4). Table 3 demonstrates further that the MSI showed low correlations with BM, WC, BMI, and FM and a moderate one with FFM. Muscular endurance test results correlated negatively with BM, WC, BMI, FM, and %BF (r = −0.25 to −0.47, p < 0.001) but not with FFM (Table 5). The MEI was also negatively correlated with BM, WC, BMI, FM, and %BF (r = −0.33 to −0.52, p < 0.001).

T4-10
Table 3:
Pearson correlation coefficient between maximal strength and body composition variables.*
T5-10
Table 5:
Pearson correlation coefficient between muscular endurance and body composition variables.*

Discussion

The results of this study revealed that muscular endurance tests were positively associated with maximal oxygen consumption and negatively with BF content, whereas maximal strength was associated with FFM. However, the novel finding was that the relationships between muscular endurance and maximal strength test scores were found for the upper but not for the lower extremities.

In this study, maximal bench press was moderately associated with push-ups, whereas maximal leg extension and repeated squats were only weakly associated. These associations were further confirmed by the results of multiple multivariate regressions. The results suggest that the performance of repeated squats is mainly dependent on aerobic (and anaerobic) capacity rather than on maximal strength characteristics, whereas push-ups depend also on maximal force of the upper extremity muscles. The differences between muscular endurance tests of the lower and upper extremities and maximal strength may partly be explained by a higher relative load (body weight) in the push-up performance than in the repeated squat performance. Moreover, grip strength was moderately associated with maximal bench press and weakly with push-ups. This may partly be explained by the nature of grip strength test, which have been shown to measure muscular strength of the forearms (34), whereas maximal bench press and push-ups also measure shoulder and chest muscles (10,11,33).

The positive association between muscular endurance tests and maximal aerobic capacity is in line with that of a previous study (9). In addition, the moderate positive association between sit-up and push-up test scores observed in this study is congruent with the studies of Esco et al. (7,8). This association may derive from the activation of abdominal muscles during the push-up test to provide neutral and linear position during the performance (10,12). Furthermore, all muscular endurance tests were moderately associated, which may also be a reflection of participants' overall muscular endurance level.

Fat-free mass was positively associated with all maximal strength tests in this study supporting previous findings (21,26). Similarly, Huygens et al. (15) have reported that FFM was the main determinant accounting for the variance of knee, trunk, and elbow strength (21–45%). In this study, the relationships between maximal strength and body mass (BM) or BF content variables were weak. These relationships have shown to be similar (21,25) or stronger (21,25–27,39) in previous studies compared with that in this study. This may result from the differences in the participants' body composition and physical fitness level. In these previous studies, the participants were lean and fit men who may have had more muscle mass and less fat mass compared with that in this study, where the participants' variance of body composition and physical fitness was large. In addition, previous studies have also reported higher maximal strength and power in obese subjects compared with that in their lean counterparts (13,14,24), and this has been speculated to be because of a greater amount of FFM in obese than in lean subjects.

The inverse associations between muscular endurance performances and BF content are in concordance with those of previous studies (8,9). In practice, these findings may account that heavier individuals may perform worse in dynamic muscular fitness tests, such as the muscular endurance tests used in this study, when the performance is done against one's own body weight. Correspondingly, lighter individuals may perform better in the dynamic muscular strength tests and worse in tests where force production is made by exerting external force (20). This may partly refer to natural scaling laws as in muscular endurance tests a person is required to move the body against the gravity. A higher degree of fat mass (dead weight) may also increase the moment of inertia (9,13).

The associations of MEI and MSI with body composition and physical fitness tests revealed consistency compared with associations between each individual test. The MEI was closely associated with V[Combining Dot Above]O2max and BF content but not with FFM, whereas MSI was associated with the maximal strength of the upper and lower extremities and FFM. Hence, both MEI and MSI can be used to describe overall muscular endurance and maximal strength, respectively.

The strength of this study was the large and nationally representative study sample of young men with varying degrees of physical fitness. However, certain limitations need to be considered. The BIA method used to assess body composition in this study was chosen, because the reliability of BIA has been shown to be moderate or even high for %BF, fat mass, and FFM when DXA has been used as a reference method (r > 0.69−0.98) (4,30) and because it is a time-efficient method for large study samples. However, the BIA method may to some extent underestimate or overestimate body composition parameters (30,38). Furthermore, we were fully aware of the role of BM bias in health-related muscular fitness tests, such as the muscular endurance tests used in the current study (20,36). However, we opted not to use any correction formulas, because we provided the relationships between the physical fitness test scores and BM variables. Finally, because of the large number of subjects, we were not able to detect the important roles that the cross-sectional areas of the muscles and maximal voluntary neural capacity to activate the muscles have in maximal force production (16). It is also worth noting that, in the muscular endurance tests, dynamic contractions were used, whereas the muscular strength required isometric muscle action. This may have had a minor effect on the association of maximal strength and muscular endurance tests. However, strong correlations between isometric and dynamic muscular strength measures have previously been reported (3,28). In addition, maximal strength has reportedly been well assessed independent of the type of muscle action (isometric or dynamic) (18).

In conclusion, the primary findings of this study suggest that the relationships between muscular endurance and maximal strength test scores were found for the upper but not for the lower extremities. In addition, all muscular endurance tests were mostly associated with V[Combining Dot Above]O2max and, therefore, measure cardiorespiratory fitness and local muscular endurance. Moreover, content was negatively associated with muscular endurance tests, whereas FFM had a positive association with maximal strength test scores.

Practical Applications

At a practical level, this study results indicate that the 1-minute push-up test is an appropriate method with reasonable accuracy to estimate also maximal strength of the upper extremities, especially, when the test is needed to be conducted as a field-based test for a number of subjects simultaneously. Furthermore, repeated squats primarily measure the local muscular endurance. Thus, an additional test is required to estimate the maximal strength of the lower extremities. This could be conducted with an additional load either in a field-based test or in a more controlled laboratory condition.

Acknowledgments

The authors thank all the test personnel for their work in data collection and biostatistician (M.Sc.) Elina Kokkonen for statistical guidance.

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Keywords:

muscular strength; aerobic fitness; fitness testing

© 2012 National Strength and Conditioning Association