Muscle capacity and physical function in older women:What are the impacts of resistance training?

Anne O.Brady*,Chad R.Straight

aDepartment of Kinesiology,The University of North Carolina at Greensboro,Greensboro,NC 27402,USA

bDepartment of Kinesiology,University of Georgia,Athens,GA 30602,USA

Muscle capacity and physical function in older women:What are the impacts of resistance training?

Anne O.Bradya,*,Chad R.Straightb

aDepartment of Kinesiology,The University of North Carolina at Greensboro,Greensboro,NC 27402,USA

bDepartment of Kinesiology,University of Georgia,Athens,GA 30602,USA

The numberofolderadults(individuals≥65 years),particularly women,in oursociety is increasing and understanding the impactofexercise on muscle capacity(e.g.,strength and power)and subsequently physical function is of utmost importance to prevent disability and maintain independence.Muscle capacity declines w ith age and this change negatively impacts physical function in older women.Exercise,specifically resistance training,is recommended to counteract these declines;however,the synergistic relationships between exercise,muscle capacity,and physical function are poorly understood.This review w illsummarize the literature regarding age-related changes in the aforementioned variables and review the research on the impact of resistance training interventions on muscle capacity and physical function in older women.Recommendations for future research in this area w ill be discussed.

Muscle capacity;Older women;Physical function;Resistance training

1.Introduction

Across the world a demographic shift is occurring;the number of older adults(individuals≥65 years)is expected to nearly triple from 2010 to 2050.1Consequently,for the fi rst time ever,the totalnumber of older adults in the world w illbe greater than the number of young children(≤5 years).1Moreover,it is predicted that women w ill continue to outnumber and outlive men.2During aging,men and women are more likely to experience physical lim itations(difficulty perform ing certain tasks,such as walking up 10 steps w ithout resting,stooping,bending,or kneeling,due to health problems)3and disability(impairments,activity lim itations, and participation restrictions).4Importantly,individuals w ith disabilities utilize more health-care services than those without disabilities,resulting in higher health-care costs.5In comparison to men,women tend to be at greater risk for disability.6Thus,while women generally live longer than men,they also experience a greater number of years living w ith physical disability in later adulthood.6Therefore,the aging phenomenon w ill likely result in a greaternumberofwomen living w ith physical disabilities,negatively impacting health-care systems across the world.

Factors contributing to declines in physical function are numerous and include increased adiposity,7—10as well as inadequate skeletal muscle mass,9,11—14strength,15,16and power.17,18Compared to age-matched males,older women tend to have higheradiposity,12,19,20loweramounts of skeletal muscle mass,20,21lower muscle density(reflecting greater muscle lipid infi ltration),22less muscle strength,23and lower muscle power,24placing them at increased risk for impaired physical function and disability.

Physical activity(PA)is often recommended to prevent disability and maintain physical function.25Specifically, resistance training has been recommended as an intervention strategy for improving muscle strength and muscle power,two factors known to impact physical function in older adults.25However,the 2009 position stand published by the American College of Sports Medicine(ACSM),Exercise and Physical Activity for Older Adults,25clearly states that despite much research highlighting the positive impactof resistance training on muscle strength and power in older adults,the effects of such exercise on physical function are notwell-understood.A review article presents an integrated conceptual model to aid in understanding the synergistic impacts of various factors on physical function in older adults.26Congruentw ith the ACSM position stand,Brady and colleagues26highlighted the need to better understand the interrelated factors that impact physical function in older adults,specifically exercise and measures of muscle capacity.

This review w ill summarize age-related changes in PA levels,muscle capacity(strength and power),and physical function.In addition,we w ill explore the literature regarding the impact of exercise,specifically resistance training,on muscle capacity measures and physical function in older women.Based on the available literature,recommendations for future research w ill be presented.

2.PA and physiological changes with aging

Declines in PA may further contribute to detriments in physical function via loss of muscle mass,strength,and power. Muscle strength(maximum force that can be exerted in one muscle contraction)and muscle power(product of muscle force and contraction velocity)are indicators of muscle function and w ill be referred to as measures of muscle capacity throughout this review.The maintenance of adequate muscle strength and muscle power is vital as both have been associated with physical function in older adults,17,23,27—29although there is currently no consensus as to which has a stronger contribution to overall physical function.26

2.1.PA

According to the Centers for Disease Control and Prevention,PA is defined as any bodily movement produced by skeletal muscle contractions that results in energy expenditure above an individual’s basal level.In contrast,exercise is defined as planned,structured,or repetitive PA performed to either maintain or improve one or more components of physical fi tness.30Advancing age is associated w ith declines in PA,31including total volume of PA,32—34intensity of PA,33,35,36and increases in sedentary time,35which is particularly evident in older women.35Furthermore,a recent crosssectional study among older adults reported that individuals 70—80 years are lessactive than individuals 60—69 years in all domains,including leisure-time activity,work-related,and housework.37

PA recommendations for older adults include both aerobic exercise and resistance training.However,statistics indicate that only 51.1%and 21.9%of older adults meet the aerobic and resistance training guidelines,respectively.38Moreover,a sex difference exists such thatolder men are more active than older women.39In 2004,the percentage of women aged 18—24 years who reported engaging in resistance training was 20.1%.However,among older women,the percentage decreased considerably to only 10.7%(compared to 14.1%for older males).40

Globally,longitudinal studies report confl icting results in the PA trends of older adults.Some studies have reported increases41—43while others have reported declines in PA.44,45In general,a review by Sun and colleagues39found thatamong older adults,there tends to be a rise in leisure-time PA,yet mostolderadults do notengage in a sufficientvolume of PA to promote health benefi ts.39Despite Sun’s conclusions,the percent of older men and women engaging in resistance training in the United States increased significantly between 1998 and 2004(11.0%to 14.1%for men and 6.8%to 10.7% for women).40In summary,older adults(especially women) are not meeting the recommended PA guidelines,particularly as they relate to resistance training.

2.2.Body composition

Though not the focus of this review,profound changes in body composition(sarcopenia and increased adiposity)are also present during the aging process.In both older men and women,there tends to be an age-related increase in overall adiposity,which has been reported as a leading cause of disability.8,10Moreover,there is a noticeable decline in skeletalmuscle mass at～45 years of age in both sexes,although the age-associated decrease is greater in men compared to women.46In comparison to younger women,older women have lower quadriceps muscle cross-sectional area(CSA)by 33%.47This is important,as multiple studies have observed a relationship between low muscle mass and impaired physical function in older adults.13,48The aging process has also been associated w ith increases in muscle lipid content,46,49,50an independent risk factor for mobility lim itations.46Notably, older women have significantly lower m id-thigh muscle attenuation(greatermuscle lipid infi ltration)than oldermen.22Moreover,there may be sex differences in the relative importance of body composition determ inants of physical function.For instance,an analysis from the Health,Aging,and Body Composition(Health ABC)study found that the strongest independentpredictor of physical function was totalbody fat in older women,whereas the most important body composition determinant in men was thigh muscle CSA.51Findings from other studies support the notion that excess adiposity has a stronger impact on physical function in older women relative to men.20,52,53Despite these results,it was recently reported that body mass index did not differentially impact the relationship between muscle quality and physical function in older women,54suggesting thatmuscle capacity is critical for function regardless of body size.In summary,olderwomen tend to gain adiposity and lose muscle mass as they age,and these changes in body composition(especially adiposity)can have a profound,negative impact on physical function.

2.3.Muscle strength

Compared to younger individuals,older adults have lower muscle strength23,55,56w ith older women having lower strength than age-matched males.23Specifically,data from the Health ABC study show that isokinetic quadriceps torque is 38.1% lower in older women compared to older men (81.85 Nmvs.132.15 Nm,respectively).56Even when muscle strength is normalized for muscle mass or fat free mass(e.g., muscle quality),there is a significantdifference between older men and women.56,57Furthermore,in comparison to younger women,older women have lower concentric quadriceps strength58,59by as much as 56%—78%,59as well as lower isometric quadriceps strength(35%).47

Moreover,longitudinal studies indicate an age-associated loss of muscle strength,termed dynapenia.60,61A longitudinal study including generally healthy older adults,reported a loss of quadriceps muscle strength of 3.6%and 2.8%annually in men and women,respectively.62Interestingly,the loss of muscle strength over a 5-year period in endurance trained older adults was even greater:3%—4%decline in knee flexion strength and 4%—5%decline in knee extension strength(no significant differences between men and women).61Thus, although older women have lower absolute muscle strength than men,the annual rate of decline may be lower,though additional studies are warranted.

In older women,muscle strength is related to physical function.For example,leg muscle strength is associated w ith maximum gait speed and suggested as a useful method for identifying those at functional thresholds.63Furthermore,it was recently reported that knee extension strength relative to body weight was significantly correlated w ith measures of physical function in older women.This ratio explained 9%, 12%,14%,and 15%of the variance in self-reported mobility function,repeated chair test score,and normal and fast gait speed,respectively.64Moreover,women in the lowest quartile of this ratio were 5.9,24.7,12.1,and 20.9 times more likely to present w ith impairments in self-reported activities,chair stand test,and normal and fast gait speed,respectively,in comparison w ith women in the highestquartile.64In summary, older women experience an age-related loss of muscle strength which can negatively impact physical function as these two variables are highly correlated.

2.4.Muscle power

Older adults have lower muscle power than younger adults.24Specifically,older women have lower concentric knee extensor peak torque(53%)65compared to their younger female counterparts;and muscle force is a critical determinant of power.Similar to the relationship between muscle strength and sex,older women also exhibit lower absolute muscle power than older men.24Cross-sectional data indicate that leg extensor power is 34%lower in women relative to men at 80 years of age,and this disparity increases to 46%at85 years.66Maintenance of leg extensor power may represent a particularly important target for intervention as ithas implications for ambulation in older adults.For instance,one study of older men and women reported that the m inimum leg extensor power necessary to maintain a maximal gait speed of 1.3—1.49 m/s was 4 W/kg.In order to maintain faster gait speeds of 1.5—1.99 m/s and＞2 m/s,the corresponding values for leg extensor power were 7 W/kg and 9.5 W/kg,respectively.66Other data have identified leg extensor power as a predictor of incidentmobility disability(inability to walk 1 km or ascend a fl ight of stairs)in older men and women.In particular,a recent study found that 47.2%of older women w ith leg extensor power＜64 W developed mobility disability over a 3-year period,compared w ith only 15.7%of those w ith leg extensor power≥64 W.67

While both muscle strength and power decrease w ith age, muscle power declines sooner and more rapidly;68,69the rate of decline in power is 3%—4%per year greater than for muscle strength.69Similar to muscle strength,the rate of decline in power is lower in older women compared to men (1.7%vs.3.0%,respectively).70In summary,muscle power declines with age and this relationship is particularly important to physical function in older women.

2.5.Physical function

It is well-established that increasing age is accompanied by a general decline in physical function,or the ability to complete everyday tasks.In the U.S.,23%of individuals 60—69 years of age report≥1 physical lim itations,defined as difficulty or inability to perform specific functional tasks(walking a 0.25-m ile,ascending 10 steps,stooping,bending,or kneeling,lifting and carrying 10 pounds).3The percentage of individuals reporting lim itations increases w ith age,from 31.4%for 70—79-year-olds to 42.9%for individuals 80 and older.3

Across allage groups,women are more likely than men to report physical lim itations,highlighting a grow ing disparity w ith increasing age.3Specifically,among adults aged 65—74, 75—84,and 85+years,the prevalence of lim itations in functionalactivities is substantially higher forwomen compared to age-matched males(31%vs.24%,46%vs.37%,and 66%vs.50%,respectively).6While declines in physical function can be attributed to a variety of factors,the relationship between muscle capacity measures and physical function is wellestablished.In older adults,muscle strength23,71and muscle power17,18,27—29,72are strongly associated w ith physical function.Importantly,although these factors are associated w ith physical function in both older men and women,studies have reported different relationships according to sex.23,29A study including community-dwelling older adults aged 75—90 years reported thatmuscle contraction velocity was related to gait speed and physical function in both men and women. However,muscle strength was only related to gait speed andphysical function in men.29In contrast,data from the National Health and Nutrition Exam ination Survey(NHANES)indicate that the relationship between muscle strength and physical function in older men and women grouped by age(55—64, 65—74,75+years)is similar.However,the factor loading was significantly less in women aged 65—74 years.23Thus,older women and men may rely on different strategies,and subsequently different measures of muscle capacity,to complete physical function tasks.

A number of factors have been suggested to account for sex-related differences observed in physical performance between men and women.A recent analysis using the Health ABC cohort reported significant differences in a composite measure of physical performance between men and women aged 70—79 years.19However,statisticaladjustments for total body fat and thigh muscle CSA fully accounted for the differences in overallperformance between sexes.Moreover,in a separate regression model,adjusting for measures of thigh body composition(thigh muscle CSA,muscle density,subcutaneous fat,and intermuscular adipose tissue)fully explained the difference in performance between men and women.Thus,lower physical function among older women is partially explained by poorer body composition,which underscores the importance of exercise interventions for reducing adiposity and increasing skeletal muscle mass. However,additionalstudies should attempt to determine other variables that help explain the gender gap in physical performance between older men and women.In summary,in comparison to age-matched males,older women are more likely to experience physical lim itations which in part may be explained by poorer body composition.

2.6.Interactive effects of PA,body composition,and muscle capacity on physical function

As discussed previously,there is interplay between PA, body composition,and muscle capacity,and these may independently and synergistically affect physical function in older adults.In older adults,physical inactivity has been associated w ith obesity52,73and sarcopenic obesity.73Subsequently,unfavorable body composition,in combination w ith inadequate muscle capacity,can maxim ize the likelihood of impaired physical function in older women.Thus,the interaction of body composition w ith muscle capacity should be noted.It is possible that low muscle strength(dynapenia),in the presence of obesity,has a more detrimental impacton physical function than obesity alone in older women.Indeed,a publication using the NHANES cohort found that physical function was generally poorer among older women w ith dynapenic-obesity, relative to those women with obesity alone.15Likewise, Stenholm etal.16found thatgaitspeed for an average 65-yearold participant w ith obesity and low muscle strength declined from 1.03 m/s at baseline to 0.85 m/s over a 6-year period. This change represented a 17%decline in gait speed,which was greater than the declines observed for adults w ith only obesity(8%),only low muscle strength(4%),and neither obesity nor low muscle strength(2%).16These findings corroborated a previous report that found the prevalence of walking lim itations was markedly greater among older adults w ith high body fat and low handgrip strength relative to those adults w ith low body fatand high handgrip strength(61%vs.7%,respectively).74Thus,while studies have documented the negative impact of obesity(a measure of body composition) on physical function in older women,it is possible that its effects are exacerbated in the presence of dynapenia(a measure of muscle capacity),which highlights the integrative nature of the variables that impact physical function in older women.Thus,it is likely thatdeclines in PA,changes in body composition(increased adiposity and loss of skeletal muscle mass),and declines in muscle capacity,synergistically contribute to decrements in physical function experienced by older women.

3.Im pactof PA and exercise on body composition,muscle capacity,and physical function

As previously highlighted,PA,muscle capacity,and physical function decline w ith age,and it is likely that these factors are highly interactive.Due to a lack of studies exploring this phenomenon and each of its components,it remains difficult to determ ine the temporal sequence of these events in older adults.Rather,reductions in PA,alterations in body composition,declines in muscle capacity and physical function are commonly attributed to the general trajectory of aging.37Despite an incomplete understanding,resistance training exercise remains one of the most commonly prescribed intervention strategies for preserving physical function and preventing disability in older adults.

3.1.Lifetime PA

The impact of lifetime exercise,specifically resistance training,on muscle capacity and physical function is relatively unknown as longitudinal studies exploring these relationships are scarce.However,cohortstudies have exam ined the impact of leisure-time PA on health outcomes in older adults.For example,Cooper and colleagues75sought to explore the impact of leisure-time PA across adulthood on physical performance and strength in m idlife.In agreement w ith previous literature,female participants had lower muscle strength and poorer physical function at the last time point(age 53)in comparison to age-matched male participants.75After adjusting for potential confounders,it was reported that there appears to be a compounding effect of PA in adulthood, positively impacting physical function at age 53.Though the study did not continue into old age,the findings hold prom ise and indicate thatexercise across a lifetime may have positive effects on physical function in old age.However,recent evidence suggests there may be a sex difference w ith regard to the functional benefi ts of PA during m iddle adulthood.Strobl et al.76examined the relationship between mid-life leisuretime PA and late-life disability in men and women.Based on a mean follow-up of 18.0 years,the odds ratios for late-life disability w ith moderate and high activity were 0.67(0.51;0.88)and 0.62(0.44;0.88),respectively,in men.However,in women,the corresponding odds ratios for moderate and high activity were 0.90(0.70;1.15)and 0.82(0.58;1.16),respectively.Related to this,leisure-time PA may impact the risk for cause-specific mortality differently in men and women.During a median follow-up period of20.2 years,Wanneretal.77found that leisure-time PA was associated w ith cardiovascular disease mortality in women (adjusted hazard ratio 0.79 (0.69—0.94))but not men.In contrast,they observed a relationship between leisure-time PA and cancer mortality in men (0.63(0.47—0.86))but not women.Moreover,sport activity was associated w ith all-cause,cardiovascular disease,and cancer mortality in men;however,no relationships were evident in women.Thus,the protective effects of PA for disability and mortality may vary between men and women, and also may vary according to domain of activity,butadditional research is needed to better characterize these differences.

W ith a focus on women only,Kozakaietal.78exam ined the relationship between muscle strength and power w ith current leisure-time PA and pastadolescentexercise(12—20 years of age)in individuals aged 40—79.Itwas reported that67.1%of women currently engaged in leisure-time PA and 41.9% engaged in adolescent exercise.Women who engaged in adolescentexercise were more likely to reporthigher levels of leisure-time PA.Controlling for confounders,results indicate a significant relationship between leisure-time PA and adolescent exercise on both muscle strength and muscle power.Individuals who were more active had greatergrip strength,knee strength,and leg power.In this study,it is unclear if the beneficial effects of adolescent exercise on muscle capacity and physical function persisted across a lifetime or if it is simply that individuals who were active earlier in life maintained adequate levels of exercise,positively contributing to muscle function and performance.

In older resistance-trained athletes,the available literature indicates that these individuals generally have higher muscle mass79and are stronger79in comparison to sedentary counterparts.However,whether higher muscle mass and strength translates to better physical function remains unexplored in this population.

3.2.Body composition

In general,resistance training interventions including older adults report significant improvements in lean body mass.80,81A recent meta-analysis by Peterson et al.80found that men and women≥50 years experienced a significant gain in lean body mass(1.1 kg)follow ing an average of 20.5 weeks of resistance training.Likew ise,Leender etal.81reported similar gains in quadriceps muscle CSA in men and women(both 9% ±1%)after 6 months of resistance training,indicating that such training may equally benefi t both older men and women.

In older women,resistance training interventions have also been found to increase CSA of type II muscle fibers,81—83some by greater than 20%.81,82However,it should be noted thatsome studies have found that the training-induced increase in fat free mass of older women may be attenuated relative to the change in younger women.84,85For instance,Dionne etal.85found that the change in fat free mass was significantly lower in older women compared to younger women(+0.7 kgvs.+1.2 kg,respectively)follow ing 6 months of resistance training.A lthough the magnitude of change may be lower for older women,it is paramount to note that sarcopenia-related declines in muscle mass are present in older women,and thus even interventions that maintain muscle mass can be beneficial.Due to the established association of low muscle mass w ith physical function9,11,12and disability13,14in older adults,resistance training programs that maintain or increase skeletal muscle mass are clinically important.These findings are particularly noteworthy for older women who tend to have lower amounts of lean body mass20,21than older men.Furthermore,a recent study found that the ratio of leg m ineral-free lean mass to whole body mass impacted dynam ic physical function among older women,but not men.20Those women w ith greater leg lean mass to support totalbody weight had significantly better physical function than age-matched males with a lower ratio.In summary,training interventions that increase skeletal muscle mass,especially of the lowerbody,may be critical for maintaining physical function in the presence of age-related alterations in overall body composition.

3.3.Muscle strength

The relationship between muscle strength and physical function is more robust than the relationship between muscle mass and function.56Thus,the efficacy of resistance training inventions for improving muscle strength in older women is critical.Notably,studies involving older adults have consistently reported significant gains in muscle strength follow ing resistance training programs.83,86—90Previous studies have indicated that age-related declines in muscle mass and muscle strength are independent and may differentially impact physical function.Therefore,Peterson et al.91conducted a metaanalysis exploring the relationship between resistance training and muscle strength in men and women≥50 years. There were significant percentage changes for leg press (+29%)and knee extension(+33%)follow ing resistance training.Although explored,there was no relationship between sex and strength gains,indicating that both older men and older women achieved significant strength gains via resistance training.91Despite this,findings comparing the magnitude of improvements in muscle strength in older men and women follow ing resistance training are inconclusive; studies have reported that gains across sexes are sim ilar,92,93smaller in women,94or larger in women.95In accordance w ith previous studies,92,93it was recently reported that older men and women responded sim ilarly to a 6-month resistance training program;both groups experienced comparable gains in one-repetition maximum for knee extension strength(42% and 43%,respectively).81Moreover,Radaelliand colleagues89compared low-volume(1 set)and high-volume(3 sets)resistance training in older women.They reported both groups significantly improved one-repetition maximums in four different exercises,w ith no significant differences between training groups.However,w ith regard to intensity of training, higher intensity interventions are associated with a greater magnitude of improvement in muscle strength.91Thus,the efficacy of resistance training interventions for improving muscle strength in older adults is likely impacted by several training variables,including duration,volume,and intensity. In summary,extensive literature supports the significant, beneficial effects of resistance training on muscle strength in older adults.

3.4.Muscle power

As recent literature has concentrated on the importance of muscle power for physical function in older adults,24,72,96—99an increasing number of exercise interventions have specifically focused on improving muscle power through resistance training.These studies generally involve high-velocity resistance training for the major lower-extrem ity muscle groups as they are highly activated during ambulation and mobility. There is cogent evidence that resistance training involving explosive movements significantly increases muscle power in older adults,17,88,90,99—105w ith some studies reporting gains＞60%.88,100

Notably,power training studies involving older women have demonstrated significantgains in muscle power across a range of modalities,including traditional resistance equipment,100weighted vest exercises,106and body weight exercises.99Improvements have been observed across different populations as well,including mobility-limited100and community-dwelling older women.106In one study,older women performed functional tasks(e.g.,chair stands and toe raises)while wearing a weighted vest,and completed the concentric phase as quickly as possible.The weightof the vest was based on the individual’s body weight and progressively increased throughout the 12-week intervention.The training resulted in gains in bilateral leg press muscle power(12%—36%)over the range of 40%—90% of one-repetition maximum.106Recently,Pereira et al.99conducted a power training intervention in older women that utilized both traditional resistance equipmentand exercises(leg press and bench press)and power exercises(vertical jumps and medicine ball throw ing).A fter 12 weeks,the power training group significantly improved muscle strength(dynam ic and isometric),as well as vertical jump height(40.2%)and ball throw ing distance(17.2%).However,the controlgroup did notexperience significant changes in any outcome measures.Finally,Marsh et al.107recently found that 16 weeks of resistance training in older women significantly improved muscle power compared to a control group(between-group change=+29.3 W,p＜0.001);however,there was no difference between groups in isometric quadriceps strength(+7.6 Nm,p=0.12).In summary,it appears that multiple resistance training modalities are effective at increasing muscle power in older women.

3.5.Physical function

As previously reviewed,resistance training interventions can significantly improve muscle strength and muscle power in older adults;however,the ability of these interventions to confer improvements in physical function is paramount.A meta-analysis by Latham et al.108reported a modest improvement in some functional tasks(gait speed and chair rise time)after traditional resistance training interventions, despite significant strength gains.However,no effect of resistance training on physical disability was observed.The authors noted thatpoor methodologicalquality of the included studies posed challenges to draw ing general conclusions regarding the effectiveness of traditional resistance training in older adults.108More recently,Paterson and colleagues109conducted a systematic review to determine the impact of PA,as well as exercise interventions,on functional lim itations in older adults.Sim ilar to Latham’s conclusion,a number of studies reported an improvement in muscle strength,but had little to no impacton functional performance.For example,in a cohort of healthy,older women,12 weeks of progressive resistance training resulted in gains in knee extensor strength (27%)and leg power(18%),yet conferred minimal improvements in physical function.90

In opposition to traditional resistance training,highvelocity training interventions in older adults have shown positive benefi ts on physical function.A number of studies have observed improvements in balance100,101,103and stair climb time,17,88,100as well as overall physical function.17,101,106However,it should be noted that in some cases large gains in muscle power translated to small,butsignificant, gains in physical function.100In contrast,modest improvements in muscle power conveyed significant improvements in physical function.102Therefore,the magnitude of improvement in muscle power is not always proportional to the gains in physical function.One potential explanation for the disproportionate gains in muscle capacity and physical function in the aforementioned studies is the curvilinear relationship that appears to exist between these two parameters in older adults.17,27,110Low-functioning individuals may experience exponential gains in physical function as they improve muscle capacity,whereas high-functioning individuals may experience fewer benefi ts as they are already functioning near optimal levels(e.g.,ceiling effect).

Interestingly,a number of intervention studies lim ited to older women have significantly improved muscle capacity and physical function.Such results have been reported in a variety of populations including healthy community-dwelling,99breast-cancer survivors,111pre-frail,112chronic osteoarthritis,98and nonagenarian113older women.Furthermore, strength training interventions as short as 3—6 weeks have effectively increased muscle strength and physical function in older women.86,98In sedentary older women,6 weeks of strength training resulted in significant gains in muscle strength(23.5%)and quality(14.8%),and also improved physical function(30-s chair stand(23.8%)and 8-footup-andgo(-22.4%)).Furthermore,there was a significantassociationbetween changes in muscle quality and improvements in physical function tasks.86In summary,despite positive results from a few studies,additional research is needed to determ ine the correlation between change in muscle capacity and change in physical function follow ing a training intervention.

3.6.Strength training vs.high-velocity training

Numerous studies have compared the benefi ts of traditional strength trainingvs.high-velocity training in older adults. When compared directly,it has been reported that both methods significantly and similarly improve muscle strength and muscle power in healthy community-dwelling adults.88Contrastingly,it has also been reported that high-velocity training results in greater gains in muscle power in healthy older adults114and older women w ith self-reported functional lim itations.115Thus,no consensus has been reached w ith regard to which method has a greater impacton muscle capacity.

However,comparisons of traditional resistance training and high-velocity training in older adults largely indicate that high-velocity training may be more effective for improving physical function.100,102,106M iszko et al.101evaluated 16 weeks of resistance training in community-dwelling older adults.Individuals were random ly assigned to either the power training(high-velocity)group or the traditional strength training group.Both groups performed seven of the same exercises for three sets ofsix to eight repetitions,butw ith 40% and 80%of one-repetition maximums for power training and traditional strength training,respectively.For the power training group,the concentric and eccentric phases were 1 s and 2 s in duration,respectively.For the traditional strength training group,the concentric action was performed for 4 s and the eccentric action was performed in a slow and controlled manner.Follow ing the intervention,overall physical function in the power training group was significantly greater than in the strength training group(p=0.033),as well in specifi c domains of balance(p=0.013),endurance(p=0.026),and upper body flexibility(p=0.045).Interestingly,the change in physical function was not significantly correlated to the change in power(r=0.29)or strength(r=0.16).101Moreover,in older women w ith functional lim itations,high-velocity training and traditional resistance training significantly improved scores on the Short Physical Performance Battery and five chair-stand times,but only the high-velocity group significantly improved gait speed(p＜ 0.01)and unilateral stance(p=0.03).106Hence,improvements in physical function in older women follow ing an intervention may be dependent on type of resistance training as well as functional status.

It should also be acknow ledged that some studies comparing high-velocity training and traditional strength training in older adults reported that both types of resistance training improved muscle capacity but did not improve physical function.88,100,115Though resistance training is the focus of the current review,it is worth noting that greater amounts of PA,such as walking,are associated with higher physical function statuses in older adults.109Specifically, moderate-intensity PA(e.g.,normal walking or gardening) sustained for a moderate-to-high duration can reduce functional lim itations by 50%in older adults.109

4.Conclusion and future directions

In conclusion,decades of research have reported the positive impacts of resistance training on body composition,80,81muscle strength,83,86,88—91 muscle power,17,88,90,100—105 and subsequently,physical function,17,88,100,101,103,106in older adults.However,the exact mechanisms mediating these relationships have not been elucidated.Thus,even when improvements in physical function have been observed postintervention,it is difficult to understand the role of muscle strength and muscle power in facilitating this improvement. Future research should focus on exploring the relationship between changes in strength/power and physical function follow ing a training intervention,and also attempt to understand the mechanisms responsible for improving physical function in the absence of strength/power gains.A more clear understanding of how these factors are related can then be used to determ ine the most efficacious resistance training interventions to prevent disability in older women.Moreover, though itappears power training(high-velocity training)may be more beneficial than traditionalstrength training to improve physical function,there are currently no guidelines for this type of resistance training in older adults;additional research is needed to determ ine recommendations for training variables including frequency,intensity,and volume.Furthermore, power training interventions involving older adults w ith a variety of chronic conditions are warranted to understand how the response on muscle capacity and physical function may be differentially impacted.

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Received 4 February 2014;revised 7 April 2014;accepted 10 April 2014

*Corresponding author.

E-mail address:aobrady@uncg.edu(A.O.Brady)

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