Menopause: The Journal of The North American Menopause Society Vol. 22, No. 11, pp. 1204-1211 DOI: 10.1097/GME.0000000000000447 ß 2015 by The North American Menopause Society
Association between accelerometer-measured physical activity and muscle capacity in middle-aged postmenopausal women Chad R. Straight, MS, Christie L. Ward-Ritacco, PhD, MS, and Ellen M. Evans, PhD, MS Abstract Objective: Greater muscle strength and power are associated with better physical function in middle-aged and older women. The aim of the present study was to determine whether accelerometer-measured physical activity was associated with muscle strength and power in middle-aged postmenopausal women. Methods: Postmenopausal women (N ¼ 60; mean [SD] age, 58.9 [3.9] y) were assessed for physical activity (step count and moderate to vigorous physical activity [MVPA]) via accelerometer, for body composition via dualenergy x-ray absorptiometry, for concentric isokinetic knee torque at 608/s and 1808/s using isokinetic dynamometry, and for leg extensor power with the Nottingham power rig. Results: In linear regression analysis, daily step count was independently associated with isokinetic knee torque at 608/s (standardized b ¼ 0.32, P ¼ 0.01), isokinetic knee torque at 1808/s (standardized b ¼ 0.32, P ¼ 0.01), and total leg extensor power (b ¼ 0.36, P ¼ 0.01) after adjustment for covariates. Daily MVPA had similar associations with isokinetic knee torque at 608/s (b ¼ 0.38, P < 0.01), isokinetic knee torque at 1808/s (b ¼ 0.41, P < 0.01), and leg power (b ¼ 0.31, P ¼ 0.02). Analysis of covariance indicated that women who engaged in MVPA for 30 minutes or more per day produced significantly greater isokinetic knee torque (608/s and 1808/s) and leg extensor power compared with women not meeting this guideline (all P < 0.05). Conclusions: These findings suggest that daily step count and MVPA are independently associated with muscle strength and power in middle-aged postmenopausal women. Future studies should determine whether interventions that increase habitual physical activity in middle-aged women are associated with concomitant improvements in muscle capacity. Key Words: Physical activity – Muscle strength – Muscle power – Postmenopausal women.
F
rom 2002 to 2012, the number of adults aged 45 to 64 years (representing those adults who will reach 65 y in the next two decades) who were living in the United States increased by 24%.1 The rapid growth in the population of middle-aged adults is likely to have major public health implications as the risk for physical limitations (eg, walking 1/4 mile, lifting or carrying 10 lb) increases with age.2 According to data from the Study of Women’s Health Across the Nation, approximately 20% of women aged 40 to 55 years had self-reported limitations in physical function, including 9.2% with substantial limitations.3 Moreover, physical limitations may develop during middle adulthood; data from the National Health Interview Survey indicate that 50% of older adults with functional limitations reported that the onset became evident between ages 40 and 55 years.4 As a result, it is probable that a growing percentage of middle-aged women will live for a greater number of years with physical
Received November 17, 2014; revised and accepted January 15, 2015. From the Department of Kinesiology, University of Georgia, Athens, GA. Funding/support: None. Financial disclosure/conflicts of interest: None reported. Address correspondence to: Chad R. Straight, MS, Department of Kinesiology, University of Georgia, 330 River Rd, Athens, GA 30602. E-mail: [email protected]
1204
limitations that persist into later adulthood. Thus, identifying modifiable factors that can lead to better physical function in middle-aged women after menopause is an important area of research. Muscle capacity, defined in this context as muscle strength or power, is a primary determinant of physical function in postmenopausal women.5-8 However, the aging process is accompanied by marked declines in lower-extremity muscle strength and power. For example, longitudinal data indicate that the rate of decline in muscle strength is 1.2% per year in young postmenopausal women (54 y at baseline),9 although the rate of decline seems to be accelerated (2.7% per year) during later adulthood (70 y).10 Likewise, lower-body muscle power is reduced with advancing age11,12; again, a more precipitous decline is evident after 70 years.13 Moreover, the onset of muscle weakness occurs earlier in women than in men,14 and it has been suggested that women at ages 60 to 65 years may be weaker than men at ages 80 to 85 years.15 Identifying behaviors that help attenuate these agerelated declines in strength and power in the major lowerextremity muscle groups, which are critically important for performing functional activities (eg, rising from a chair), in middle-aged postmenopausal women may have important clinical implications by potentially reducing the likelihood of future functional decline and physical disability. Notably,
Menopause, Vol. 22, No. 11, 2015
Copyright @ 2015 The North American Menopause Society. Unauthorized reproduction of this article is prohibited.
PHYSICAL ACTIVITY, MUSCLE CAPACITY IN WOMEN
physical activity is one behavior that likely has a beneficial impact on muscle capacity during aging. The American College of Sports Medicine (ACSM) recommends that all adults engage in 30 minutes or more of moderate-intensity cardiorespiratory exercise 5 days or more per week to achieve a myriad of physical and mental health benefits.16 It also recommends that all adults engage in muscle-strengthening activities on two nonconsecutive days each week to promote muscular fitness. Although resistance training studies conducted in laboratory-based settings have demonstrated improvements in muscle strength and power in postmenopausal women,17-22 the relation between habitual physical activity and muscle capacity is less clear.23-26 In addition, although other research studies have suggested that adults should accumulate 10,000 steps or more per day,27 whether this volume of activity results in better muscle strength and power in middle-aged women has not been well-characterized. The health benefits of physical activity are well-documented in ACSM position stands16,28; however, there has been limited research on whether this type of activity is associated with better muscle capacity in postmenopausal women, which is especially critical as there is a noticeable decline in skeletal muscle mass in women beginning at approximately 45 years of age.29 Recently, it was shown that greater self-reported physical activity was associated with greater isometric quadriceps strength in a sample of 36 older men and women with knee osteoarthritis (r ¼ 0.44).30 In addition, greater loading doses of light, moderate, and vigorous physical activity (assessed via accelerometry) were associated with higher knee extension torque in middle-aged women (Kendall’s t ¼ 0.30-0.45).25 In contrast to those studies, another study found no relation between accelerometermeasured physical activity and isometric knee extension or flexion strength in older women.26 Therefore, the relation between objectively measured physical activity and muscle capacity in middle-aged women is unclear for a number of reasons: (1) equivocal nature of previous findings, (2) lack of assessment of lower-body muscle power, (3) use of selfreported physical activity rather than objective measurement, and (4) samples that have not included middle-aged postmenopausal women. In this context, the purpose of the present study was to examine the association between physical activity (step count and duration of moderate to vigorous physical activity [MVPA]) and various measures of muscle capacity, including isokinetic muscle strength and muscle power, in middle-aged postmenopausal women. We hypothesized that both measures of physical activity would be associated with muscle strength and power and that women who met recommended guidelines for physical activity would have greater muscle capacity than those who did not. METHODS Participants Postmenopausal women aged 45 to 65 years (mean [SD] age, 58.9 [3.9] y) were recruited for participation in the
present study. Participants were recruited via advertisements in newsletters, guest lectures, and flyers targeting local organizations of women. Eligible participants met the following inclusion criteria: nonsmoker for two or more previous years; weight-stable (2 kg for the past 3 mo); has completed cancer-related treatment for at least 5 years before enrollment; and free of any uncontrolled cardiovascular, pulmonary, or metabolic conditions, symptomatic joint abnormalities, and symptomatic neurological disorders that would preclude ability to complete testing. Comorbidities, including arthritis, osteoporosis, asthma, chronic obstructive pulmonary disease, cardiovascular disease, peripheral arterial disease, diabetes, and degenerative disc disease, were assessed via self-report using a questionnaire, and the total number of comorbidities was calculated and included as a covariate in subsequent regression analyses. Overall, 191 women contacted our laboratory and 91 women qualified for participation. Women were excluded from participating in the study for the following reasons: did not respond to follow-up contact (n ¼ 41), unwilling to make time commitment (n ¼ 19), current smoker (n ¼ 2), not postmenopausal (n ¼ 17), did not meet criteria for age range (n ¼ 8), did not meet criteria for body mass index (n ¼ 5), injury that prevented physical function testing to address a different primary aim (n ¼ 2), refusal to undergo dual-energy x-ray absorptiometry scanning (n ¼ 2), and not weight-stable (n ¼ 4). However, of the 91 women eligible for participation, only those with complete data were included in the final analysis (N ¼ 60). All participants provided written informed consent prior to enrollment, and all procedures in this study were approved by the Institutional Review Board at the University of Georgia. Body composition Standing height and weight were measured, with participants wearing lightweight clothing and no shoes. Height was obtained using a stadiometer (model 242; Seca) and measured to the nearest 0.1 cm. Weight was measured on a calibrated digital scale (model WB-110A; Tanita). Whole-body and regional soft-tissue composition was assessed via dual-energy x-ray absorptiometry (iDXA; GE Healthcare-Luna, Madison, WI), which provides a valid31,32 and reliable33 measurement of body composition in adults. Regional analyses were performed according to the manufacturer’s guidelines and involved bisecting the femoral neck and patella to measure mineral-free lean mass (MFLM) of the upper leg. Lower-body MFLM was quantified as total MFLM below the superior border of the iliac crest. Physical activity An objective measurement of physical activity was obtained via NL-1000 pedometers, which use a piezoelectric accelerometer mechanism (New Lifestyles-1000, Barebones Pedometer; New Lifestyles Inc, Lees Summit, MO). The NLseries pedometers provide a valid assessment of physical activity34,35 and have been shown to be unaffected by differences in body mass index, waist circumference, and Menopause, Vol. 22, No. 11, 2015
1205
Copyright @ 2015 The North American Menopause Society. Unauthorized reproduction of this article is prohibited.
STRAIGHT ET AL
pedometer tilt.36 Participants wore the accelerometer on their nondominant hip during all waking hours for a period of 7 days. Ten hours or more of wear time was considered a valid wear day, and at least four valid wear days were required for inclusion in our analysis. Accelerometry yielded two measurements of physical activity: step count (steps/d) and duration of MVPA (min/d). Daily step count and MVPA were calculated as means determined from the number of valid wear days. In addition, current participation in resistance training exercise was ascertained in a subset of women (n ¼ 48) using a bonespecific physical activity questionnaire.37 Mean frequency of participation (d/wk) in resistance training was calculated and associations with muscle capacity were examined. Muscle strength and power Maximal concentric isokinetic knee extension and flexion torque was measured at 608/s and 1808/s via isokinetic dynamometry to provide two measurements of lower-body muscle strength (System 4 Pro; Biodex Medical Systems Inc, Shirley, NY). Participants performed two sets of four extension and flexion repetitions for the left and right legs. The greatest extension and flexion peak torque values for each leg were summed to calculate maximal isokinetic knee torque at 608/s and 1808/s. Unilateral leg extensor power was measured using the Nottingham power rig (model NG7 2UH; University of Nottingham Medical School, Nottingham, UK). Participants were seated on an adjustable seat and instructed to push against a foot pedal as hard and quickly as possible until their knee was extended. Ten trials were performed for each leg, and lower-extremity muscle power was determined by summing the peak power values of the left and right legs. The Nottingham power rig has been shown to be a valid and reliable measure of leg extensor power in older adults.38 Statistical analysis All statistical analyses were conducted using SPSS for Windows version 22.0 (SPSS Inc, Chicago, IL). Histograms and distribution statistics (skewness and kurtosis) were examined to screen data. Assumptions of normality were met as all variables had skewness and kurtosis values lower than j2.0j. Hierarchical linear regression analyses were conducted to determine the independent association between physical activity (daily step count and MVPA) and muscle capacity (strength and power). To determine whether the relation between physical activity and muscle capacity differed according to accelerometry measure, we conducted separate regression models for daily step count (steps/d) and MVPA (min/d). For each measure of muscle capacity, regression analyses were performed in the following order: step 1, age and comorbidities; step 2, physical activity; step 3, MFLM. Because isokinetic knee torque and leg extensor power recruited different musculature to perform these movements, upper-leg MFLM was included in models for isokinetic knee torque, and lower-body MFLM was controlled for in analyses of lower-extremity muscle power. To further examine the effects of physical activity on muscle capacity,
1206
Menopause, Vol. 22, No. 11, 2015
we performed multiple categorical analyses using physical activity guidelines as cutoff points. In one analysis, participants were stratified according to whether or not they accumulated 10,000 steps or more per day.27 A separate analysis was performed to examine the impact of engaging in MVPA for 30 minutes or more per day on muscle capacity.16 Analysis of covariance (ANCOVA), adjusted for age, comorbidities, and MFLM, was conducted to determine whether group differences existed. Power analysis was conducted to ascertain the sample size necessary to detect a relationship between physical activity and muscle capacity. To detect a bivariate association between daily step count and lower-extremity muscle strength of r ¼ 0.40, which is similar to previous studies in middle-aged women,25,30 assuming a ¼ 0.05 and power of 0.80, a sample size of 46 was necessary. Statistical significance was set at P < 0.05 for all tests. RESULTS Participant characteristics for the sample are presented in Table 1. The most common self-reported medical conditions in our cohort included arthritis (35% of the sample), depression (18%), degenerative disc disease (17%), and anxiety (15%). Nine participants (15%) reported current use of hormone replacement therapy; however, there were no statistical differences between users and nonusers in daily step count, duration of MVPA, isokinetic knee torque at 608/s and 1808/s, or leg extensor power (all P > 0.05). The mean (SD) daily step count for the sample was 8,998.9 (3,673.7) steps/day, and the mean (SD) duration of MVPA was 29.0 (18.4) min/day. As expected, measures of isokinetic knee torque and lower-extremity muscle power were related in bivariate analyses (r ¼ 0.53-0.63, P < 0.01). Mean daily step count was associated with isokinetic knee torque at 608/s (r ¼ 0.28, P ¼ 0.03), isokinetic knee torque at 1808/s (r ¼ 0.28, P ¼ 0.03), and lower-extremity muscle power (r ¼ 0.33, P ¼ 0.01). Likewise, mean MVPA was also related to isokinetic knee torque at 608/s (r ¼ 0.29, P ¼ 0.02) and TABLE 1. Participant characteristics (N ¼ 60) Characteristic
Mean (SD)
Age, y 58.9 Height, meters 1.63 Body mass, kg 69.8 26.1 Body mass index, kg/m2 a 1.8 Comorbidities (total) Body fat, % 38.8 Upper-leg MFLM, kg 9.5 Lower-body MFLM, kg 20.4 Physical activity, steps/d 8,998.9 MVPA, min/d 29.0 Isokinetic knee torque at 608/s, N m 254.9 Isokinetic knee torque at 1808/s, N m 172.2 Lower-extremity muscle power, W 235.9
(3.9) (0.06) (13.7) (4.7) (1.8) (6.3) (1.6) (2.8) (3,673.7) (18.4) (62.8) (52.9) (65.9)
Minimum-maximum 50.0-65.0 1.51-1.82 49.1-109.1 18.1-38.3 0-9 27.7-52.8 7.0-13.8 15.9-28.5 954.3-21,275.0 2.3-77.4 79.0-379.1 59.9-314.2 114.2-415.0
MFLM, mineral-free lean mass; MVPA, moderate to vigorous physical activity. a Total number of self-reported medical conditions, including arthritis, osteoporosis, asthma, chronic obstructive pulmonary disease, cardiovascular disease, peripheral arterial disease, diabetes, and degenerative disc disease. ß 2015 The North American Menopause Society
Copyright @ 2015 The North American Menopause Society. Unauthorized reproduction of this article is prohibited.
PHYSICAL ACTIVITY, MUSCLE CAPACITY IN WOMEN TABLE 2. Association between daily step count and muscle capacity in hierarchical linear regression analyses (N ¼ 60) Isokinetic knee torque at 608/s Step 1 2 3
2
Isokinetic knee torque at 1808/s 2
Lower-extremity muscle power
Predictor
b
@R
P
b
@R
P
b
@R2
P
Age Comorbidities Age Comorbidities Step count Age Comorbidities Step count MFLM
0.07 0.07 0.09 0.04 0.27 0.02 0.10 0.32 0.43
0.004 0.005 0.008 0.001 0.075 0.000 0.012 0.120 0.191
0.61 0.59 0.51 0.79 0.04 0.87 0.42 0.01
Association between accelerometer-measured physical activity and muscle capacity in middle-aged postmenopausal women Chad R. Straight, MS, Christie L. Ward-Ritacco, PhD, MS, and Ellen M. Evans, PhD, MS Abstract Objective: Greater muscle strength and power are associated with better physical function in middle-aged and older women. The aim of the present study was to determine whether accelerometer-measured physical activity was associated with muscle strength and power in middle-aged postmenopausal women. Methods: Postmenopausal women (N ¼ 60; mean [SD] age, 58.9 [3.9] y) were assessed for physical activity (step count and moderate to vigorous physical activity [MVPA]) via accelerometer, for body composition via dualenergy x-ray absorptiometry, for concentric isokinetic knee torque at 608/s and 1808/s using isokinetic dynamometry, and for leg extensor power with the Nottingham power rig. Results: In linear regression analysis, daily step count was independently associated with isokinetic knee torque at 608/s (standardized b ¼ 0.32, P ¼ 0.01), isokinetic knee torque at 1808/s (standardized b ¼ 0.32, P ¼ 0.01), and total leg extensor power (b ¼ 0.36, P ¼ 0.01) after adjustment for covariates. Daily MVPA had similar associations with isokinetic knee torque at 608/s (b ¼ 0.38, P < 0.01), isokinetic knee torque at 1808/s (b ¼ 0.41, P < 0.01), and leg power (b ¼ 0.31, P ¼ 0.02). Analysis of covariance indicated that women who engaged in MVPA for 30 minutes or more per day produced significantly greater isokinetic knee torque (608/s and 1808/s) and leg extensor power compared with women not meeting this guideline (all P < 0.05). Conclusions: These findings suggest that daily step count and MVPA are independently associated with muscle strength and power in middle-aged postmenopausal women. Future studies should determine whether interventions that increase habitual physical activity in middle-aged women are associated with concomitant improvements in muscle capacity. Key Words: Physical activity – Muscle strength – Muscle power – Postmenopausal women.
F
rom 2002 to 2012, the number of adults aged 45 to 64 years (representing those adults who will reach 65 y in the next two decades) who were living in the United States increased by 24%.1 The rapid growth in the population of middle-aged adults is likely to have major public health implications as the risk for physical limitations (eg, walking 1/4 mile, lifting or carrying 10 lb) increases with age.2 According to data from the Study of Women’s Health Across the Nation, approximately 20% of women aged 40 to 55 years had self-reported limitations in physical function, including 9.2% with substantial limitations.3 Moreover, physical limitations may develop during middle adulthood; data from the National Health Interview Survey indicate that 50% of older adults with functional limitations reported that the onset became evident between ages 40 and 55 years.4 As a result, it is probable that a growing percentage of middle-aged women will live for a greater number of years with physical
Received November 17, 2014; revised and accepted January 15, 2015. From the Department of Kinesiology, University of Georgia, Athens, GA. Funding/support: None. Financial disclosure/conflicts of interest: None reported. Address correspondence to: Chad R. Straight, MS, Department of Kinesiology, University of Georgia, 330 River Rd, Athens, GA 30602. E-mail: [email protected]
1204
limitations that persist into later adulthood. Thus, identifying modifiable factors that can lead to better physical function in middle-aged women after menopause is an important area of research. Muscle capacity, defined in this context as muscle strength or power, is a primary determinant of physical function in postmenopausal women.5-8 However, the aging process is accompanied by marked declines in lower-extremity muscle strength and power. For example, longitudinal data indicate that the rate of decline in muscle strength is 1.2% per year in young postmenopausal women (54 y at baseline),9 although the rate of decline seems to be accelerated (2.7% per year) during later adulthood (70 y).10 Likewise, lower-body muscle power is reduced with advancing age11,12; again, a more precipitous decline is evident after 70 years.13 Moreover, the onset of muscle weakness occurs earlier in women than in men,14 and it has been suggested that women at ages 60 to 65 years may be weaker than men at ages 80 to 85 years.15 Identifying behaviors that help attenuate these agerelated declines in strength and power in the major lowerextremity muscle groups, which are critically important for performing functional activities (eg, rising from a chair), in middle-aged postmenopausal women may have important clinical implications by potentially reducing the likelihood of future functional decline and physical disability. Notably,
Menopause, Vol. 22, No. 11, 2015
Copyright @ 2015 The North American Menopause Society. Unauthorized reproduction of this article is prohibited.
PHYSICAL ACTIVITY, MUSCLE CAPACITY IN WOMEN
physical activity is one behavior that likely has a beneficial impact on muscle capacity during aging. The American College of Sports Medicine (ACSM) recommends that all adults engage in 30 minutes or more of moderate-intensity cardiorespiratory exercise 5 days or more per week to achieve a myriad of physical and mental health benefits.16 It also recommends that all adults engage in muscle-strengthening activities on two nonconsecutive days each week to promote muscular fitness. Although resistance training studies conducted in laboratory-based settings have demonstrated improvements in muscle strength and power in postmenopausal women,17-22 the relation between habitual physical activity and muscle capacity is less clear.23-26 In addition, although other research studies have suggested that adults should accumulate 10,000 steps or more per day,27 whether this volume of activity results in better muscle strength and power in middle-aged women has not been well-characterized. The health benefits of physical activity are well-documented in ACSM position stands16,28; however, there has been limited research on whether this type of activity is associated with better muscle capacity in postmenopausal women, which is especially critical as there is a noticeable decline in skeletal muscle mass in women beginning at approximately 45 years of age.29 Recently, it was shown that greater self-reported physical activity was associated with greater isometric quadriceps strength in a sample of 36 older men and women with knee osteoarthritis (r ¼ 0.44).30 In addition, greater loading doses of light, moderate, and vigorous physical activity (assessed via accelerometry) were associated with higher knee extension torque in middle-aged women (Kendall’s t ¼ 0.30-0.45).25 In contrast to those studies, another study found no relation between accelerometermeasured physical activity and isometric knee extension or flexion strength in older women.26 Therefore, the relation between objectively measured physical activity and muscle capacity in middle-aged women is unclear for a number of reasons: (1) equivocal nature of previous findings, (2) lack of assessment of lower-body muscle power, (3) use of selfreported physical activity rather than objective measurement, and (4) samples that have not included middle-aged postmenopausal women. In this context, the purpose of the present study was to examine the association between physical activity (step count and duration of moderate to vigorous physical activity [MVPA]) and various measures of muscle capacity, including isokinetic muscle strength and muscle power, in middle-aged postmenopausal women. We hypothesized that both measures of physical activity would be associated with muscle strength and power and that women who met recommended guidelines for physical activity would have greater muscle capacity than those who did not. METHODS Participants Postmenopausal women aged 45 to 65 years (mean [SD] age, 58.9 [3.9] y) were recruited for participation in the
present study. Participants were recruited via advertisements in newsletters, guest lectures, and flyers targeting local organizations of women. Eligible participants met the following inclusion criteria: nonsmoker for two or more previous years; weight-stable (2 kg for the past 3 mo); has completed cancer-related treatment for at least 5 years before enrollment; and free of any uncontrolled cardiovascular, pulmonary, or metabolic conditions, symptomatic joint abnormalities, and symptomatic neurological disorders that would preclude ability to complete testing. Comorbidities, including arthritis, osteoporosis, asthma, chronic obstructive pulmonary disease, cardiovascular disease, peripheral arterial disease, diabetes, and degenerative disc disease, were assessed via self-report using a questionnaire, and the total number of comorbidities was calculated and included as a covariate in subsequent regression analyses. Overall, 191 women contacted our laboratory and 91 women qualified for participation. Women were excluded from participating in the study for the following reasons: did not respond to follow-up contact (n ¼ 41), unwilling to make time commitment (n ¼ 19), current smoker (n ¼ 2), not postmenopausal (n ¼ 17), did not meet criteria for age range (n ¼ 8), did not meet criteria for body mass index (n ¼ 5), injury that prevented physical function testing to address a different primary aim (n ¼ 2), refusal to undergo dual-energy x-ray absorptiometry scanning (n ¼ 2), and not weight-stable (n ¼ 4). However, of the 91 women eligible for participation, only those with complete data were included in the final analysis (N ¼ 60). All participants provided written informed consent prior to enrollment, and all procedures in this study were approved by the Institutional Review Board at the University of Georgia. Body composition Standing height and weight were measured, with participants wearing lightweight clothing and no shoes. Height was obtained using a stadiometer (model 242; Seca) and measured to the nearest 0.1 cm. Weight was measured on a calibrated digital scale (model WB-110A; Tanita). Whole-body and regional soft-tissue composition was assessed via dual-energy x-ray absorptiometry (iDXA; GE Healthcare-Luna, Madison, WI), which provides a valid31,32 and reliable33 measurement of body composition in adults. Regional analyses were performed according to the manufacturer’s guidelines and involved bisecting the femoral neck and patella to measure mineral-free lean mass (MFLM) of the upper leg. Lower-body MFLM was quantified as total MFLM below the superior border of the iliac crest. Physical activity An objective measurement of physical activity was obtained via NL-1000 pedometers, which use a piezoelectric accelerometer mechanism (New Lifestyles-1000, Barebones Pedometer; New Lifestyles Inc, Lees Summit, MO). The NLseries pedometers provide a valid assessment of physical activity34,35 and have been shown to be unaffected by differences in body mass index, waist circumference, and Menopause, Vol. 22, No. 11, 2015
1205
Copyright @ 2015 The North American Menopause Society. Unauthorized reproduction of this article is prohibited.
STRAIGHT ET AL
pedometer tilt.36 Participants wore the accelerometer on their nondominant hip during all waking hours for a period of 7 days. Ten hours or more of wear time was considered a valid wear day, and at least four valid wear days were required for inclusion in our analysis. Accelerometry yielded two measurements of physical activity: step count (steps/d) and duration of MVPA (min/d). Daily step count and MVPA were calculated as means determined from the number of valid wear days. In addition, current participation in resistance training exercise was ascertained in a subset of women (n ¼ 48) using a bonespecific physical activity questionnaire.37 Mean frequency of participation (d/wk) in resistance training was calculated and associations with muscle capacity were examined. Muscle strength and power Maximal concentric isokinetic knee extension and flexion torque was measured at 608/s and 1808/s via isokinetic dynamometry to provide two measurements of lower-body muscle strength (System 4 Pro; Biodex Medical Systems Inc, Shirley, NY). Participants performed two sets of four extension and flexion repetitions for the left and right legs. The greatest extension and flexion peak torque values for each leg were summed to calculate maximal isokinetic knee torque at 608/s and 1808/s. Unilateral leg extensor power was measured using the Nottingham power rig (model NG7 2UH; University of Nottingham Medical School, Nottingham, UK). Participants were seated on an adjustable seat and instructed to push against a foot pedal as hard and quickly as possible until their knee was extended. Ten trials were performed for each leg, and lower-extremity muscle power was determined by summing the peak power values of the left and right legs. The Nottingham power rig has been shown to be a valid and reliable measure of leg extensor power in older adults.38 Statistical analysis All statistical analyses were conducted using SPSS for Windows version 22.0 (SPSS Inc, Chicago, IL). Histograms and distribution statistics (skewness and kurtosis) were examined to screen data. Assumptions of normality were met as all variables had skewness and kurtosis values lower than j2.0j. Hierarchical linear regression analyses were conducted to determine the independent association between physical activity (daily step count and MVPA) and muscle capacity (strength and power). To determine whether the relation between physical activity and muscle capacity differed according to accelerometry measure, we conducted separate regression models for daily step count (steps/d) and MVPA (min/d). For each measure of muscle capacity, regression analyses were performed in the following order: step 1, age and comorbidities; step 2, physical activity; step 3, MFLM. Because isokinetic knee torque and leg extensor power recruited different musculature to perform these movements, upper-leg MFLM was included in models for isokinetic knee torque, and lower-body MFLM was controlled for in analyses of lower-extremity muscle power. To further examine the effects of physical activity on muscle capacity,
1206
Menopause, Vol. 22, No. 11, 2015
we performed multiple categorical analyses using physical activity guidelines as cutoff points. In one analysis, participants were stratified according to whether or not they accumulated 10,000 steps or more per day.27 A separate analysis was performed to examine the impact of engaging in MVPA for 30 minutes or more per day on muscle capacity.16 Analysis of covariance (ANCOVA), adjusted for age, comorbidities, and MFLM, was conducted to determine whether group differences existed. Power analysis was conducted to ascertain the sample size necessary to detect a relationship between physical activity and muscle capacity. To detect a bivariate association between daily step count and lower-extremity muscle strength of r ¼ 0.40, which is similar to previous studies in middle-aged women,25,30 assuming a ¼ 0.05 and power of 0.80, a sample size of 46 was necessary. Statistical significance was set at P < 0.05 for all tests. RESULTS Participant characteristics for the sample are presented in Table 1. The most common self-reported medical conditions in our cohort included arthritis (35% of the sample), depression (18%), degenerative disc disease (17%), and anxiety (15%). Nine participants (15%) reported current use of hormone replacement therapy; however, there were no statistical differences between users and nonusers in daily step count, duration of MVPA, isokinetic knee torque at 608/s and 1808/s, or leg extensor power (all P > 0.05). The mean (SD) daily step count for the sample was 8,998.9 (3,673.7) steps/day, and the mean (SD) duration of MVPA was 29.0 (18.4) min/day. As expected, measures of isokinetic knee torque and lower-extremity muscle power were related in bivariate analyses (r ¼ 0.53-0.63, P < 0.01). Mean daily step count was associated with isokinetic knee torque at 608/s (r ¼ 0.28, P ¼ 0.03), isokinetic knee torque at 1808/s (r ¼ 0.28, P ¼ 0.03), and lower-extremity muscle power (r ¼ 0.33, P ¼ 0.01). Likewise, mean MVPA was also related to isokinetic knee torque at 608/s (r ¼ 0.29, P ¼ 0.02) and TABLE 1. Participant characteristics (N ¼ 60) Characteristic
Mean (SD)
Age, y 58.9 Height, meters 1.63 Body mass, kg 69.8 26.1 Body mass index, kg/m2 a 1.8 Comorbidities (total) Body fat, % 38.8 Upper-leg MFLM, kg 9.5 Lower-body MFLM, kg 20.4 Physical activity, steps/d 8,998.9 MVPA, min/d 29.0 Isokinetic knee torque at 608/s, N m 254.9 Isokinetic knee torque at 1808/s, N m 172.2 Lower-extremity muscle power, W 235.9
(3.9) (0.06) (13.7) (4.7) (1.8) (6.3) (1.6) (2.8) (3,673.7) (18.4) (62.8) (52.9) (65.9)
Minimum-maximum 50.0-65.0 1.51-1.82 49.1-109.1 18.1-38.3 0-9 27.7-52.8 7.0-13.8 15.9-28.5 954.3-21,275.0 2.3-77.4 79.0-379.1 59.9-314.2 114.2-415.0
MFLM, mineral-free lean mass; MVPA, moderate to vigorous physical activity. a Total number of self-reported medical conditions, including arthritis, osteoporosis, asthma, chronic obstructive pulmonary disease, cardiovascular disease, peripheral arterial disease, diabetes, and degenerative disc disease. ß 2015 The North American Menopause Society
Copyright @ 2015 The North American Menopause Society. Unauthorized reproduction of this article is prohibited.
PHYSICAL ACTIVITY, MUSCLE CAPACITY IN WOMEN TABLE 2. Association between daily step count and muscle capacity in hierarchical linear regression analyses (N ¼ 60) Isokinetic knee torque at 608/s Step 1 2 3
2
Isokinetic knee torque at 1808/s 2
Lower-extremity muscle power
Predictor
b
@R
P
b
@R
P
b
@R2
P
Age Comorbidities Age Comorbidities Step count Age Comorbidities Step count MFLM
0.07 0.07 0.09 0.04 0.27 0.02 0.10 0.32 0.43
0.004 0.005 0.008 0.001 0.075 0.000 0.012 0.120 0.191
0.61 0.59 0.51 0.79 0.04 0.87 0.42 0.01
