Age-Related Differences in Isometric and Dynamic Strength and Endurance

TORI JOHNSON

To measure age-related skeletal muscle changes, 30 healthy, moderately active women performed isometric and dynamic strength and endurance tests of their left quadriceps femoris muscle groups. Fifteen of the subjects were ages 20 to 29 years and 15 were ages 50 to 80 years. A Cybex® II isokinetic dynamometer was used to measure the torque output isometrically with the knee at 90 degrees of flexion on three trials. In addition, each subject performed three trials from 90 degrees of knee flexion to full knee extension and back to 90 degrees of knee flexion with the velocity of the isokinetic dynamometer set at 10 rpms. Endurance time was calculated to be the length of time a torque output of at least 50 percent of the maximal strength could be maintained. The t tests done revealed a significant difference between the two groups on isometric and dynamic strength, while no significant difference was found on isometric or dynamic endurance. Statistically correcting for the extraneous variables, height and weight, through a partial correlation revealed a significant negative corre­lation between age and strength. The younger subjects had higher torque outputs, both isometrically and dynamically, than the older subjects. The same partial correlation between isometric endurance and age and dynamic endurance and age demonstrated no significant correlation. The fact that endurance did not change in this older population while strength did may be the result of fast-twitch and slow-twitch muscle fibers changing with age.

Key Words: Age factors, Energy metabolism, Exercise tests, Exertion.

With increasing age, muscle function, as well as physical activity patterns, changes. As an increasing proportion of the population lives longer and remains physically active longer, it becomes more important to obtain detailed knowledge and understanding of muscle function in older populations. Skeletal muscle strength and endurance are two possibly significant age-related changes. The purpose of this study was to quantify possible age-related changes in skeletal mus­cle strength and endurance.

REVIEW OF LITERATURE

In 1975, Petrovsky and Lind investigated isometric handgrip strength, endurance, and cardiovascular re­sponses with special reference to age and body fat

content in 100 men, 22 to 62 years old.1 They found an increase in age was associated with an increase in endurance, and an increase in the body-weight factor was associated with a decrease in isometric endur­ance. Therefore, as age increased isometric endurance also increased, and as body weight increased isometric endurance decreased.

In 1977, Larsson and Karlsson investigated quad­riceps femoris muscular endurance in several age groups from a population of sedentary men, 22 to 65 years of age.2 Their results showed a slight increase in both isometric and dynamic endurance with age; however, neither increase was statistically significant.

In 1978, Aniansson et al3 found an increase in static endurance with increasing age in their subjects (60 men and 60 women, 70 years old) as compared to the subjects of Larsson and associates.4 No difference in dynamic endurance was shown between men and women or when compared to the younger subjects of Larsson and associates.

Because Larsson and Karlsson used subjects who had participated in an earlier study dealing with strength and histochemical and biochemical changes with aging (studies by Larsson and associates4), they

Ms. Johnson is the staff physical therapist at C Bar V Ranches, PO Box 240, Wilson, WY 83014 (USA).

This article is adapted from a paper written in partial fulfillment of the requirement for the Master of Science Degree, Duke Univer­sity, Durham, NC.

This article was submitted September 8,1980, and accepted Novem­ber 20,1981.

Volume 62 / Number 7, July 1982 9 8 5

had available to them fiber-type proportions and enzyme activity data on each subject. Larsson and associates found (in these same subjects) that the fiber-type distribution shifted toward a lower per­centage of type II fibers (fast-twitch) and a higher relative percentage of type I fibers (slow-twitch) with age. Both fast-twitch glycolytic and fast-twitch oxi­dative fibers decreased in average cross-sectional area, while slow-twitch oxidative fibers showed no signifi­cant change. These data are consistent with an earlier study investigating the decreased proportion and pref­erential atrophy of type II fibers with age.5

In 1977, Larsson and Karlsson used the data on fiber type and enzymatic-activity changes with aging from a previous study (Larsson and associates4) to hypothesize reasons for their results.2 In previous research, the proportion of type II A and B fibers to type I fibers was found to decrease with age,4,5 and the area of type II fibers decreased with age while the area of type I remained unchanged.4 Larsson and Karlsson found that isometric endurance decreased with an increasing type II fiber area; this correlation was statistically significant.2

Hulten and colleagues also studied the relationship between isometric endurance and muscle fiber types in 19 physical education students.6 They found that an increase in isometric endurance at 50 percent of maximal voluntary isometric contraction was related to the individual fiber-type composition; the increase in endurance was found where there was a higher percentage of slow-twitch fibers than fast-twitch fi­bers.

Larsson and Karlsson's research on enzymatic ac­tivity data demonstrated decreased anaerobic lactate dehydrogenase (LDH) activity with aging and no change in the aerobic LDH activity.2 Therefore, the ratio between anerobic and aerobic LDH activity decreased with aging. In 1978, Tesch and co-workers suggested that dynamic endurance, as measured here, is limited by lactate accumulation.7 The accumulation occurs preferentially in the type II fibers, and a lactate translocation from type II to type I fibers is postu­lated. Larsson and Karlsson found an increase in dynamic endurance with an increasing relative area of type I fibers. As the relative area of type II fibers decreased, the amount of lactate accumulation also decreased, and as a consequence dynamic endurance was improved.2 Larsson and Karlsson's results sup­ported past hypotheses that dynamic endurance changes with age.

Thorstensson and Karlsson studied the develop­ment of fatigue and fiber composition in skeletal muscle performing fast dynamic maximal contrac­tions in 10 men, 25 to 40 years old.8 The results showed a positive correlation between fatigability with rapid maximal voluntary isokinetic contractions and the proportion of type II fibers in the contracting quadriceps femoris muscle.8

It has been well-documented that skeletal muscle strength, both isometric and dynamic, decreases with increasing age. No conclusive evidence has been pre­sented in the literature about the effects of increasing age on isometric and dynamic endurance. The follow­ing investigation compared isometric and dynamic strength and endurance of the left quadriceps femoris muscle between a young population (20 to 29 years) and an older population (50 to 80 years) of healthy, moderately active (Saltin and Grimby's Activity Level III,9 see Appendix) women. This investigation paralleled Larsson and Karlsson's measurements of isometric strength and endurance of the left quadri­ceps femoris muscle groups of sedentary Scandina­vian men using a Cybex® II.*2 It was anticipated that strength of the left quadriceps femoris muscle group would decrease with increasing age while skeletal muscle endurance would not change in these moder­ately active women.

METHOD

Subjects

To study isometric and dynamic endurance changes with age, samples were drawn from two populations of women who were moderately active (Level III) at the time of the study, according to Saltin and Grimby's9 defined activity levels (Appendix). The first sample consisted of 15 graduate student volunteers, ages 20 to 29, and the second sample consisted of 15 volunteers from the community, ages 50 to 80. The subjects were not familiar with the Cybex® II. None of the subjects had a history of any neuromuscular or skeletal disorder. None of the sub­jects had evidence of high blood pressure (160/90 mmHg) as measured at the onset of the study. Sub­jects were monitored for signs of excessive fatigue (flushing of the face, difficulty breathing, complaints of pain, rapid pulse) during the experiment.

Procedure

Subjects were positioned on the testing device with a backrest, the hips at 60 degrees of flexion, and no wedge under the knee; this enhanced knee extension force, as demonstrated in previous studies.10-12 The subjects' knees were positioned at 90 degrees of flex­ion, consistent with the knee position of subjects in Larsson and Karlsson's study.2 The left knee joints were aligned with the fulcrum of the Cybex® II lever arm, and straps were used to stabilize the hips and left lower limbs at the thigh and leg. The lever arm was placed in a position of comfort for the patient,

* Cybex Division of Lumex, 2100 Smithtown Ave, Ronkonkoma, NY 11779.

986 PHYSICAL THERAPY

which was usually about 70 percent of the distance between the medial joint line and the medial malleo­lus.

Height and weight measurements were obtained on each subject. Maximal dynamic strength, maximal isometric strength, dynamic endurance, and isometric endurance of the left quadriceps femoris muscles for each subject were measured as torque output (units in foot-pounds). The Cybex® II testing protocol* was used for each of the four measurements.

Isometric strength and endurance were measured with the velocity of the Cybex® II set at zero. Maxi­mal isometric strength was measured with subjects pressing maximally against a pad located distally on the lever arm. The highest value for peak torque from three trials of 5 seconds each, with a 10-second recov­ery period between contractions, was calculated to be the value for maximum isometric strength. After a three-minute rest, isometric endurance was measured. Subjects were instructed to repeat a maximal isomet­ric contraction of the quadriceps femoris muscle and continue doing so until asked to stop. Isometric en­durance time was measured as the maximal amount of time a torque of at least 50 percent of the maximal isometric strength could be maintained. Following the measurement of isometric endurance, subjects rested for 15 minutes.

Maximal dynamic strength and dynamic endur­ance were measured with the Cybex® II velocity set at 10 rpms. Subjects were instructed to exert maximal effort through the range of motion, from 90 degrees of knee flexion to full knee extension, and back to 90 degrees of knee flexion. The highest value for peak torque from three trials was calculated to be the value for maximal dynamic strength. After a three-minute rest, subjects were instructed to repeat the procedure and continue doing so until asked to stop. Dynamic endurance time was measured as the maximal amount of time a torque of at least 50 percent of the maximal dynamic strength was maintained.

Each subject was tested in the same order: first isometric strength, next isometric endurance, then dynamic strength, and finally dynamic endurance. The testing order could not be randomized because it was necessary to know the isometric and dynamic strength of each subject in order to measure endur­ance as defined here. (Endurance was calculated to be the length of time a subject was able to maintain a torque output of at least 50 percent of her own maximum.)

Statistical Analysis

Pearson's correlations were done between each of the four variables (height, weight, isometric strength and endurance, and dynamic strength and endurance) within the two age groups. Four t tests were done to

TABLE Mean and Standard Deviation of Strength and

Endurance of Left Quadriceps Femoris Muscle Groups of Two Age Groups

Variable

Strength (ft-lb) Isometric Dynamic

Endurance (sec) Isometric Dynamic

Group 1

s

113.5 76.3

4.5 14.9

32.7 19.5

1.1 11.2

Group 2

68.2 44.7

5.1 11.7

s

23.3 19.5

2.3 8.8

compare the differences between the means of the two age groups for the four variables. Four partial correlations were done, each statistically controlling for the extraneous variance of height and weight. Age was correlated with isometric strength and endurance and with dynamic strength and endurance.

RESULTS

Significant correlations were found in Age Group 1 (20 to 29 years) between height and weight (r = .74,. p < .002) and between height and isometric and dynamic strength (r = .68, p < .006 and r = .63, p < .01, respectively). Weight was also found to correlate significantly with isometric strength and endurance (r = .73, p < .002 and r = .55, p < .03, respectively). Isometric and dynamic strength remain correlated (r = .67, p < .006), and isometric and dynamic endur­ance correlated (r = .53, p < .04). In Age Group 2 (50 to 80 years), a significant correlation was found be­tween isometric and dynamic strength (r = .83, p < .002). These were the only Pearson's correlations found to be significant at the p < .05 level.

The means and standard deviations of the two strength measurements and two endurance measure­ments by age group are presented in the Table. The t tests (df = 28) demonstrated a highly significant difference between the two age groups on isometric and dynamic strength (F values 1.97 and 1.00, and t values 4.37 and 4.46, respectively). For both strength measurements, the significance level was less than .01. No significant difference was found between the two age groups on isometric or dynamic endurance (F values 4.34 and 1.62, and t values -0.88 and 0.88, respectively).

When the extraneous variables of height and weight were statistically controlled through a partial correlation, isometric and dynamic strength nega­tively correlated with age (df = 26, p < .01 and p < .06, and r = -.46 and r = -.05, respectively). The same partial correlation between isometric endurance and age (df = 26, p < .06, r = -.36) and between

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dynamic endurance and age (df = 26, p < .33, r = -.19) demonstrated no significant negative correla­tion, although the former approached significance.

DISCUSSION

Pearson's correlation tests revealed several signifi­cantly correlated variables, most within Age Group 1 (20 to 29 years). As would be expected, height and weight significantly positively correlated; therefore, as height increased, weight increased and vice versa. In this group, height also significantly positively cor­related with both isometric (p < .01) and dynamic (p < .05) strength, indicating that as height increased, so did isometric and dynamic strength of the quadri­ceps femoris muscle group. Weight positively corre­lated (p < .05) with isometric strength and endurance in this group of women aged 20 to 29.

In Age Group 1, isometric and dynamic strength significantly positively correlated (p < .05), as did isometric and dynamic endurance (p < .05). There­fore, the 20- to 29-year-old women who demonstrated a relatively long endurance time of the knee extensor muscles isometrically also did so dynamically. The former was also true in Age Group 2; that is, isometric and dynamic strength correlated (p < .01). It is interesting to note that neither the two isometric variables nor the two dynamic variables significantly correlated in either age group.

A t-test analysis also revealed no significant differ­ence between the two age groups on isometric and dynamic endurance times of the quadriceps femoris muscles. In other words, the 50- to 80-year-old women performed equally well on isometric and dynamic endurance tests compared to the 20- to 29-year-old women. These results are also consistent with previ­ous findings in the literature.1-5

The partial correlation analyses were done to con­trol for any effects of the extraneous variables of height and weight on strength and endurance. Iso­metric and dynamic strength remain negatively cor­related with age (p < .05). This implies that the decrease in strength observed with increasing age was not due to height or weight differences.

Isometric endurance and age were not significantly correlated, nor were dynamic endurance and age when height and weight were controlled in these correlations. No significant increase or decrease in endurance time of the quadriceps femoris muscle groups was found in the 50- to 80-year-old women when compared to the 20- to 29-year-old women.

Several recent studies have been published dealing with skeletal muscle strength changes related to ag­ing.2,3, 13 All of these investigators agree that a de­crease in strength, both isometric and dynamic, occurs with increasing age, at least in the quadriceps femoris muscle group. Muscular endurance changes with ag­

ing are not well-documented in the literature; few studies were found dealing with skeletal muscle en­durance changes with aging-2-5

The studies in the literature that have dealt with skeletal muscle endurance changes with aging have been based on the same sample of sedentary Scandi­navian men.2-5 Our study used moderately-active women to measure skeletal muscle strength and en­durance. This study also presents a cultural differ­ence: previous studies were based on Scandinavians while this study is based on Americans. Our study along with the others mentioned all indicate that skeletal muscle endurance does not necessarily follow the pattern of strength decline with aging.

In order to explain these results, it is necessary to understand changes in muscle fiber type with aging. Through muscle biopsy studies, several researchers have concluded there exists a preferential atrophy and decreasing percentage of fast-twitch fibers, both glycolytic and oxidative, with increasing age.1-3,5,6

Slow-twitch oxidative fibers do not significantly change in number or size with increasing age. These authors also agree on the existence of a positive correlation between muscle fatigability (decreasing isometric or dynamic endurance time, or both) and the proportion of fast-twitch fibers.

Some authors have further suggested that skeletal muscle endurance, especially dynamic, is limited by lactate accumulation. Such accumulation occurs pref­erentially in fast-twitch muscle fibers, and a lactate translocation from fast- to slow-twitch fibers may also occur. Lactate accumulation appears to decrease en­durance.2,7,8 According to this hypothesis, as the relative area of fast-twitch fibers decreases, the amount of lactate accumulation also decreases, and as a consequence, muscular endurance may not be as limited.

The mechanisms occurring with aging, both in muscle fiber type changes and at a cellular level with lactate accumulation, are not completely understood. Further studies are necessary to research muscle func­tion changes with aging. Greater functional potential than previously thought may exist in aging skeletal muscles.

CONCLUSION

In two populations of healthy, moderately active women, one group of 15 subjects, aged 20 to 29 years, and the second group of 15 subjects, aged 50 to 80 years, isometric and dynamic strength and endurance measurements were obtained on a Cybex® II. Statis­tical analysis revealed a significant difference in both isometric and dynamic strength and no significant difference in isometric or dynamic endurance in the older population as compared to the younger popu­lation. These results may be caused by changes in

988 PHYSICAL THERAPY

RESEARCH

muscle fiber type with aging, specifically a decrease in the proportion and a preferential atrophy of fast-twitch muscle fibers, both glycolytic and oxidative types. This may decrease the amount of lactate ac­cumulation in the involved muscles and so decrease limitations on muscular endurance.

Acknowledgments. Thanks to John Riebel, Art Buehler, Paul Koisch, and Mary Tyrey for their as­sistance in finding subjects for my study and to Elia Villanueva for her knowledge and support through­out my study and for her suggestions in the prepara­tion of the final manuscript.

APPENDIX9

Activity Level

OCCUPATIONAL ACTIVITY I. Predominantly sedentary, sitting: desk worker, watch­

maker, sitting assembly-line worker (light worker). II. Sitting or standing, some walking: cashier, general office

worker, light tool and machinery worker, foreman. III. Walking, some handling of material: mailman, waiter,

construction worker, heavy tool and machinery worker. IV. Heavy manual work: lumberjack, dock worker, stone­

mason, farm worker, ditch digger.

SPARE-TIME PHYSICAL ACTIVITY I. Almost completely inactive: watching TV and movies,

reading. II. Some physical activity during at least 4 hours a week:

riding a bicycle or walking to work, walking or skiing with the family, gardening.

III. Regular activity: heavy gardening, running, calisthenics, tennis.

IV. Regular hard activity several times a week: physical training for competition in running events, soccer, rac­ing, European handball.

REFERENCES

1. Petrovsky JS, Lind AR: Isometric strength, endurance, and the blood pressure and heart rate responses during isometric exercise in healthy men and women, with special reference to age and body fat content. Pfluegers Arch 360:49-61, 1975

2. Larsson L, Karlsson J: Isometric and dynamic endurance as a function of age and skeletal muscle characteristics. Acta Physiol Scand 104:129-136, 1978

3. Aniansson A, Grimby G, Hedberg M, et al: Muscle function in old age. Scan J Rehabil Med 56:43-49, 1978

4. Larsson L, Grimby G, Karlsson J: Muscle strength and speed of contraction in relation to age and muscle morphology. J Appl Physiol 46(3):451-456, 1979

5. Larsson L, Sjödin B, Karlsson J: Histochemical and biochem­ical changes in human skeletal muscle with age in sedentary males, age 22-65 years. Acta Physiol Scand 103:31-39, 1978

6. Hültén B, Thorstensson A, Sjödin B, et al: Relationship be­tween isometric endurance and fibre types in human leg muscles. Acta Physiol Scand 93:135-138, 1975

7. Tesch P, Sjödin B, Thorstensson A, et al: Muscle fatigue and its relation to lactate accumulation and LDH activity in man. Acta Physiol Scand 103:(1) 40-46, 1978

8. Thorstensson A, Karlsson J: Fatigability and fibre composi­tion of human skeletal muscle. Acta Physiol Scand 98:318-322, 1976

9. Saltin B, Grimby G: Physiological analysis of middle-aged and old former athletes. Circulation 38:1104-1115, 1968

10. Currier DP: Positioning for knee strengthening exercises. Phys Ther 57:148-152, 1977

11. Currier DP: Evaluation of the use of a wedge in quadriceps strengthening. Phys Ther 55:870-874, 1975

12. Richard G, Currier DP: Back stabilization during knee strengthening exercise. Phys Ther 57:1013-1015, 1977

13. Murray MP, Gardner GM, Mollinger LA, et al: Strength of isometric and isokinetic contractions. Phys Ther 60:412-419, 1980

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