Eur J Appl Physiol (1992) 64:467-470

.,u,.,'u'°'"Ao,p p l i e d Physiology and Occupational Phys/ology © Springer-Verlag1992

A comparison of strength and muscle endurance in strength-trained and untrained women Heidi A. Huczel and David H. Clarke Exercise Physiology Laboratory, Department of Kinesiology, University of Maryland, College Park, MD 20742, USA Accepted January 13, 1992

Summary. Muscular strength and fatig~/bility of strength-trained (ST) and untrained (UT) women were compared during a 6-min bout of maximal rhythmic exercise involving the elbow flexor muscles given at a rate of 30 contractions.min -1. Fifteen ST and 15 UT subjects, aged 18-34 years and pair-matched for body size, were tested for differences in initial strength, final strength, absolute endurance, relative endurance, and rate of fatigue. Results revealed a significant difference in initial strength, final strength, and absolute endurance in favor of ST subjects. No significant difference was found for relative endurance, and rates of fatigue were similar for both groups. It is concluded that muscular strength and endurance are enhanced in women engaged in a training program designed primarily to increase muscular strength and hypertrophy, but fatigability is not affected. Key words: Muscular strength - Muscular endurance Fatigability - W o m e n weight trainers

Introduction The development of muscular strength has long been attributed to progressive weight training in which the principle of overload is followed. Traditionally thought to be the restricted practice of men, it is currently practiced by women in growing numbers. There seems little doubt that strength is enhanced by such procedures, and the debate concerning the extent of gain in muscle hypertrophy in women favors the conclusion that hypertrophy is similar for both genders, especially when cross-sectional area is observed f r o m computed axial t o m o g r a p h y scans (Cureton et al. 1988). In fact, fast-twitch fibers seem to be affected more than the slow-twitch fibers in both men and women (Staron et al. 1984) by transforming type IIb fibers into type I I a fibers (Staron et al. 1990, 1991). Clarke and Stull (1970) and Stull and Clarke (1970) have Offprint requests to: D. H. Clarke

shown in men that both strength and endurance are developed equally f r o m either a low repetition and high load program or a high repetition and low load program. The same strength and endurance gains were achieved in both protocols. Several studies have provided data on strength differences between men and women (Clarke 1986; Heyward and McCreary 1977; Kroll 1971; Montoye and Lamphiear 1977; Wilmore 1974). Clarke (1986) examined the concept of sex differences in components of muscular strength, endurance, and fatigue and concluded that men are stronger and have greater absolute endurance than women, but women appear to have more relative endurance than men. What remains less clear is whether there are differences in these components when trained women are compared with their nontrained counterparts. Therefore, the present study compared measures of strength and fatigue of the elbow flexor muscles between strength-trained (ST) women bodybuilders and untrained (UT) females during a 6-min bout of rhythmic isometric muscular contractions. It is hypothesized that ST subjects will be stronger and have more absolute muscular endurance than UT subjects, but that they will experience greater rates of fatigue.

Methods Subjects. Thirty women volunteers between the ages of 18 and 34

years participated in this study after signing an informed consent document. ST (n = 15) and UT women (n = 15) were pair-matched using body mass index (BMI) which describes the relationship between height and weight according to the following formula: BMI = Wt (N)/ttt 2(m). ST had been engaged continuously in weight training for a minimum of 1 year at the time of testing. No subjects had ever taken androgenic-anabolic steroids, verified as far as possible through questionnaires and direct interviews. The UT were women who were not engaged in anything but what could be considered normal activity, and who had never engaged in weight training exercises. Strength and fatigue testing. The task was to undertake rhythmic maximal isometric contractions of the elbow flexor muscles at a

468 rate of 30 contractions.min 1. The duration of the exercise bout was 6 rain which exceeded by 1 min the length of time employed in previous studies using elbow flexion (Clarke and Stull 1970; Ordway et al. 1977; Stull and Clarke 1970), but preliminary data collected with weight-trained subjects suggested that the additional time was needed. The dominant arm was tested in all instances. The subject assumed a supine position on a padded table with knees and hips flexed, and the lower legs and upper body braced with a strap attached to the table. The elbow joint of the arm being tested was flexed at an angle of 2.01 rad as measured with a goniometer. A padded canvas strap was placed over the mid-forearm, equidistant from the elbow and wrist joints, and the tester braced the subject's arm at the shoulder to prevent extraneous movement. The strap was extended at a right angle from the forearm and was attached to a BLH SR-4 load cell, which in turn was connected to the support at the base of the testing table and placed directly in line with the effort of pull. The subject was instructed to pull maximally and then relax on alternate beats of a metronome, and the resultant force was transmitted to a dynagraph which recorded each contraction. The entire apparatus was calibrated with known weights periodically throughout the experiment.

Table 1. Descriptive statistics for physical characteristics of subjects* Trained

Age (years) Height (m) Weight (N) BMI (N.m -z)

Mean

SD

Mean

SD

23.9 1.61 562.5 217.0

4.6 0.09 74.5 28.8

22.7 1.63 591.6 222.7

3.6 0.07 85.4 21.7

* No significantdifferences (P> 0.05) were found between groups on any measure BMI, Body mass index

Table 2. Means and standard deviations for experimental variables Trained

Data reduction. The force exerted during the exercise bout was

determined by averaging the magnitude of three successive spikes beginning with the initial maximal voluntary contraction (MVC) and continuing every 30 s thereafter. Thus, a total of 13 average measures of peak tension was made for each test session. From this array of points the following variables were selected for analysis: initial strength, which was the average of the first three measured MVC; final strength, determined by averaging the last three averaged force values; absolute endurance, calculated as the sum of all the force values at each of the measured intervals (total envelope under the curve); and relative endurance, which was determined by subtracting final strength from each of the other values and obtaining their sum. This latter calculation provided a measure of endurance irrespective of maximal strength. The use of additional measures for final strength was to provide a more stable value, especially since the variability at the end of the experimental period is greater than at the beginning, and also since very little change in the fatigue level occurs beyond 5 min, given the contraction rate employed in the present experiment. Significance of the difference between mean values for the two groups was calculated by use of the t-test. Statistical significance was accepted at P < 0.05. Mathematical values for a single-component exponential curve were utilized to determine the rate of fatigue obtained from the average fatigue curves for the two groups. The mathematical expression that describes the force y at any time t is Yt=aoe-kt + c

where a is the amount of strength of the first contraction above the asymptote c, e is the Naperian log base (2.72), and k is the rate of fatigue constant. An asymptote was estimated and subtracted from the experimental mean data points. When plotted on a semilogarithmic graph, it was possible to calculate the half-time (tl/2) decline and compute the rate of fatigue constant.

Results Descriptive statistics for age, height, weight, a n d BMI are presented in T a b l e 1. There was n o significant difference between groups o n a n y of the physical characteristics ( P < 0 . 0 5 ) , indicating that the two groups were successfully p a i r - m a t c h e d . Tests of m u s c u l a r fatigue such as e m p l o y e d in the present experiment are k n o w n to be r o b u s t , b o t h for h a n d - g r i p p i n g exercise a n d elbow flexion (Clarke 1971,

Untrained

Initial strength (N) Final strength (N) Absolute endurance (N) Relative endurance (N)

Untrained

Mean

SD

Mean

SD

355.3 195.7 3028.7 485.2

98.2 49.8 678.9 263.5

256.6 41.0 153.2 36.3 2 3 5 1 . 5 420.2 360.3 198.1

1985). N o t only are the fatigue rate constants similar, b u t the correlations b e t w e e n strength tests are high. I n the present experiment, the correlation between the first two m e a s u r e d values of m a x i m a l strength for the experim e n t a l subjects was 0.958. W h e n the m e a s u r e d oddn u m b e r e d values t h r o u g h o u t the b o u t were correlated with the e v e n - n u m b e r e d points the correlation was 0.983. T h u s , the reliability of the criterion task is assured. The m e a n values for initial strength are given in Table 2, where it can be seen that the ST exceeded the U T subjects by 89.7 N, equivalent to a p p r o x i m a t e l y 34%. This difference was significant (t[28] = 3.27, P < 0 . 0 5 ) . F i n a l streflgth (Table 2) was also significantly higher for the ST subjects (t[28] = 2 . 6 7 , P < 0 . 0 5 ) , in this instance by approximately 28%. C o m p a r i s o n of the m e a n values for absolute endurance (Table 2) also revealed that the ST had significantly higher m e a n values t h a n the U T subjects b y some 29% (t[28] = 3 . 8 6 , P < 0 . 0 5 ) . However, when c o m p a r e d o n relative e n d u r a n c e , there was no significant difference b e t w e e n the two groups o f subjects (t[28] = 1.47, P > 0.05). The results of the semi-logarithmic plots are s h o w n in Fig. 1 for b o t h ST a n d U T groups. The goodness of fit is revealed by c o m p u t a t i o n of the s t a n d a r d deviation o f the deviation of the experimental values f r o m the linear t r e n d line. F o r the present data this was 3.04 N for the ST g r o u p a n d 3.63 N for the U T group. This has b e e n f o u n d to vary b e t w e e n 1.47 a n d 4 . 6 0 N for elbow flexion in adult male subjects (Clarke 1971; Clarke a n d Stull 1970; Stull a n d Clarke 1970), a n d thus assures a satisfactory curve fitting procedure. The various fatigue curve parameters are c o n t a i n e d in T a b l e 3. The value a,

469 500

Table 3. Exponential curve parameters

200

a (N)

c (N)

t,/z (s)

k (s ')

162.7 107.8

190.1 154.8

61 58

0.011361 0.011948

Trained Untrained

tOG

\\

k, Rate fatigue constant; a, amount of strength of first contraction above the asymptote c; c, asymptote; ta/2, half-time

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similar. The difference in values of tl/2 w a s only 3 s, and the corresponding value of k reflected this similarity. This is also noted in the parallel logarithmic plots of Fig. 1 which depict nearly identical strength decay rates. Smooth curves were plotted from the mathematical data on coordinate paper, and the experimental means were then superimposed. These curves are shown in Fig. 2, where it can be observed that the points all fall on or very near the mathematical curves, verifying the goodness of fit and providing evidence that a single-component curve was the appropriate model for these data.

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Discussion

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The adaptation to exercise is related to the type of training stimulus. For example, heavy-resistance, low-repetition exercise, such as occurs in the typiCal weight-training program, results in increases in strength and muscle cell hypertrophy (Gollnick et al. 1981; Thorstensson et al. 1976). Early work by DeLorme and Watkins (1948) revealed that such a training regimen would enhance muscular strength. The findings of the present study reveal a clear superiority in muscular strength for the ST subjects. This was evident in the initial MVC and carried through to the final value at the end of the fatigue test. At the same time, there is a controversy as to whether such procedures will enhance muscular endurance. Dudley and Djamil (1985) and Hickson (1980) have concluded that an incompatibility exists between strength and endurance (low-resistance, high-repetition) training.

(S)

Fig. I. Semi-log plot of force-time data for trained ( x - - × ) and

untrained (O---©) subjects fatigable strength, is analogous to initial strength, and it will be seen that the difference (56.8 N) was in favor of the ST subjects. This would be expected, since initial strength also was greater for these subjects. Similarly, the asymptote c favored the stronger subjects, which would correspond in a general way with the values for final strength. Thus, if the asymptotes are added to the values for a the result is approximately the same as initial strength, and provides evidence of the goodness of fit of the fatigue curves. The results of the exponential curve analysis showed the rates of fatigue for the two groups to be remarkably

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Fig. 2. Fatigue curves for trained ( x - ×) and untrained (O---G) subjects

470 However, the findings of Clarke and Stull (1970) and Stull and Clarke (1970) for adult male subjects found that both ST and endurance-training regimens caused increases to occur not only in strength but in muscular endurance as well, and both to about the same extent. The present study is in agreement that strength and absolute endurance are associated with ST activities. Muscle force will decline when the demands on the organism for energy are not met by the rate and supply of adenosine triphosphate. When the metabolites created by muscle contraction, such as adenosine diphosphate, inorganic phosphates and hydrogen ions, adversely affect the contractile mechanism, muscle fatigue will ensue (Edwards and Gibson 1991). Since maximaleffort work requires subjects to exert muscular force at similar relative intensities, it would follow that the rate of energy depletion would proceed in the same manner, especially early in the fatigue period when phosphagen supply would be at its greatest level. The results of the present study bear this out, since no signific~int difference was found for relative endurance between the ST and UT subjects, and the rates of fatigue were essentially the same for the two groups. Prior training apparently provided no advantage in delaying fatigue in subjects exposed to local muscular Work of maximal intensity. It should also be pointed out that not only were the rates of fatigue for the two groups similar to each other but they were also in general agreement with those reported in the literature for male subjects performing elbow flexion (Clarke 1971; Clarke and Stull 1970; Stull and Clarke 1970). All of these studies found the tl/z values to vary between approximately 45 and 60 s, which compares with 58 and 61 s in the present investigation for ST and UT women, respectively. It is concluded that muscular strength and muscular endurance are enhanced in ST women, but that no change is likely to result in the amount of strength lost to fatigue during rhythmic isometric contractions of maximal intensity, nor in the rate at which this strength is depleted.

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CO, Humphrey JM (eds) Current selected research in exercise physiology, vol 1. AMS Press, New York, pp 117-127 Clarke DH (1986) Sex differences in strength and fatigability. Res Q Exerc Sport 57 : 144-149 Clarke DH, Stull GA (1970) Endurance training as a determinant of strength and fatigability. Res Q 41 : 19-26 Cureton KJ, Collins MA, Hill DW, McElhannon JR (1988) Muscle hypertrophy in men and women. Med Sci Sports Exerc 20:338-344 DeLorme T, Watkins A (1948) Technics of progressive resistance exercise. Arch Phys Med Rehabil 29:263-273 Dudley GA, Djamil R (1985) Incompatibility of endurance- and strength-training modes of exercise. J Appl Physiol 59:14461451 Edwards RHT, Gibson H (1991) Perspectives in the study of normal and pathological skeletal muscle. In: Atlan G, Beliveau L, Bouissou P (eds) Muscle fatigue: biochemical and physiological aspects. Masson, Paris, pp 3-15 Gollnick PD, Timson BF, Moore RL, Riedy M (1981) Muscular enlargement and number of fibers in skeletal muscles of rats. J Appl Physiol 40:936-943 Heyward V, McCreary L (1977) Analysis of the static strength and relative endurance of women athletes. Res Q Exerc Sport 48:703-710 Hickson RC (1980) Interference of strength development by simultaneously training for strength and endurance. Eur J Appl Physiol 45 : 255-263 Kroll W (1971) Isometric strength fatigue patterns in female subjects. Res Q 42 : 286-298 Montoye HJ, Lamphiear DE (1977) Grip and arm strength in males and females, age 10 to 69. Res Q 48 : 109-120 Ordway GA, Kearney JT, Stull GA (1977) Rhythmic isometric fatigue patterns of the elbow flexors and knee extensors. Res Q 48 : 734-740 Staron RS, Hikida RS, Hagerman FC, Dudley GA, Murray TF (1984) Human skeletal muscle fiber type adaptability to various workloads. J Histochem Cytochem 32:146-152 Staron RS, Malicky ES, Leonardi MJ, Falkel JE, Hagerman FC, Dudley GA (1990) Muscle hypertrophy and fast fiber type conversions in heavy resistance-trained women. Eur J Appl Physiol 60: 71-79 Staron RS, Leonardi M J, Karapondo DL, Malicky ES, Falkel JE, Hagerman FC, Hikida RS (1991) Strength and skeletal muscle adaptations in heavy-resistance-trained women. J Appl Physiol 70: 631-640 Stull GA, Clarke DH (1970) High-resistance, low-repetition training as a determiner of strength and fatigability. Res Q 41 : 189193 Thorstensson A, Hulten B, Dobeln W von, Karlsson J (1976) Effect of strength training on enzyme activities and fibre characteristics in human skeletal muscle. Acta Physiol Scand 96 : 392398 Wilmore JH (1974) Alterations in strength, body composition, and anthropometric measurements consequent to a 10-week weight training program. Med Sci Sport 6:133-138