Long-Term Impact of Strength Training on Muscle Strength Characteristics in Older Adults

Archives of Physical Medicine and Rehabilitation journal homepage: www.archives-pmr.org Archives of Physical Medicine and Rehabilitation 2013;94:2054-60

ORIGINAL ARTICLE

Long-Term Impact of Strength Training on Muscle Strength Characteristics in Older Adults Eva Kennis, MS,a Sabine M. Verschueren, PhD,b An Bogaerts, PhD,a Evelien Van Roie, MS,a Steven Boonen, MD, PhD,c,y Christophe Delecluse, PhDa From the aPhysical Activity, Sports & Health Research Group, Department of Kinesiology, and the bResearch Group for Musculoskeletal Rehabilitation, Department of Rehabilitation Sciences, Faculty of Kinesiology and Rehabilitation Sciences, Katholieke Universiteit Leuven, Leuven; and cLeuven University Center for Metabolic Bone Disease and Division of Geriatric Medicine, Faculty of Medicine, Katholieke Universiteit Leuven, Leuven, Belgium.

Abstract Objective: To evaluate the long-term preventive impact of strength training on muscle performance in older adults. Design: A 7-year follow-up on a 1-year randomized controlled trial comparing the effects of combined resistance training and aerobic training and whole-body vibration training on muscle performance. Setting: University training center. Participants: Men and women (NZ83; control [CON] group, nZ27; strength-training intervention [INT] group, nZ56) between 60 and 80 years of age. Interventions: The INT group exercised 3 times weekly during 1 year, performing a combined resistance training and aerobic training program or a whole-body vibration training program. The former training program was designed according to American College of Sports Medicine guidelines. The whole-body vibration training program included unloaded static and dynamic leg exercises on a vibration platform. The CON group did not participate in any training program. Main Outcome Measures: Static strength (STAT), dynamic strength at 60 /s (DYN60) and at 240 /s (DYN240), speed of movement at 20% (S20). Results: From baseline to postintervention, muscle performance did not change in the CON group, except for S20 (þ6.55%2.88%, P<.001). One year of strength training increased (P.001) STAT (þ11.46%1.86%), DYN60 (þ6.96%1.65%), DYN240 (þ9.25%1.68%), and S20 (þ7.73%2.19%) in the INT group. Between baseline and follow-up, muscle performance decreased (P<.001) in both groups. However, STAT and DYN60 showed a significantly lower loss in the INT group (8.65%2.35% and 7.10%2.38%, respectively) compared with the CON group (16.47%2.69% and 15.08%2.27%, respectively). This positive impact might be due to the preservation of the training-induced gains, given the similar annual decline rates in both groups from postintervention to follow-up. Additionally, in trained participants, aging seems to impact velocity-dependent strength and power more compared with basic strength, as the total losses in DYN240 (CON, 15.93%2.64%; INT, 11.39%1.95%) and S20 (CON, 14.39%2.10%; INT, 13.16%1.72%) did not differ significantly between the groups. Conclusions: A 1-year strength-training intervention results in an improved muscle performance in older adults 7 years after their enrollment in the intervention. However, an extensive exercise program cannot attenuate the age-related decline once the intervention stops. Archives of Physical Medicine and Rehabilitation 2013;94:2054-60 ª 2013 by the American Congress of Rehabilitation Medicine

Human muscle strength and power are well known to decline with advancing age. A gradual decline of 1% to 1.5% per year in y

Deceased. This study was supported by the Flemish Government. No commercial party having a direct financial interest in the results of the research supporting this article has conferred or will confer a benefit on the authors or on any organization with which the authors are associated. Clinical Trial Registration No.: NCT01216917, NCT01682330.

muscle strength, or maximum force generation capacity, is observed after the fifth decade, and even more rapid losses, reaching 3% per year, are reported after the age of 65 years.1,2 In addition, aging seems to have a more detrimental impact on dynamic muscle strength compared with static muscle strength. Muscle power, defined as the ability to generate force quickly, declines earlier and more precipitously than muscle strength.1,3-5

0003-9993/13/$36 - see front matter ª 2013 by the American Congress of Rehabilitation Medicine http://dx.doi.org/10.1016/j.apmr.2013.06.018

Long-term impact of training These age-related degeneration processes are referred to as dynapenia and are considered a major contributing factor to the loss of functional independence in older adults, given the highly predictive value of muscle performance for falls and everyday function.6,7 Therefore, preserving muscle strength and power into late life is of high clinical importance for older adults attempting to decrease their risk of disability and enhance their quality of life. Progressive resistance training, especially performed at a high intensity of 60% to 80% of 1 repetition maximum, has been proven to be a very effective strategy in directly increasing muscle strength and power.8,9 For example, previous studies10-14 found gains in static knee extension strength ranging from 11.8% to 32.0% after 12 weeks to 1.5 years of resistance training in older adults. Additionally, leg power, measured by countermovement jump performance, increased by 9.8% to 12.9%.12,13 More recently, whole-body vibration (WBV) training has been promoted as an efficient, low-threshold alternative strengthtraining intervention for patients or older adults who are not attracted to or not able to perform standard strength exercises.12-16 WBV training includes unloaded stands and exercises on a vibrating platform and has been shown to be as equally effective in improving muscle strength and muscle power as conventional resistance training.12-16 Previous studies,12,13,15,16 including 10 weeks to 1.5 years of WBV training in older adults, reported significant increases of 8.0% to 38.8% in static knee extension strength. In addition, improvements in countermovement jump height were found to range from 7.9% to 19.4%.12-14 Taking into account an average annual loss of 3% in muscle strength characteristics and given the above-mentioned postintervention effects, a relatively short period of strength training in older adults should theoretically result in gains in muscle strength and muscle power equivalent to the age-related losses of 2.6 to 12.9 years. Although the ability of an extensive strength-training intervention to reverse dynapenia has been well established, its sustainability in an older population remains a challenge.17 Consequently, few data exist on the persistence of traininginduced gains in the longer term. More specifically, it is not clear whether muscle performance can be maintained at an elevated level in the absence of continued supervised and guided exercise programs. Also unclear is whether a strength-training intervention merely postpones or effectively attenuates the detrimental effects of the human aging process on muscle strength and muscle power. Therefore, the aim of the present study was to assess the longterm preventive impact of strength training on muscle strength characteristics in older adults. More specifically, we investigated how muscle strength and muscle power in participants of a 1-year

List of abbreviations: ACSM ANOVA CON DYN60 DYN240 FPACQ INT PAL RDA S20 STAT WBV

American College of Sports Medicine analysis of variance control (group) dynamic strength at 60 /s dynamic strength at 240 /s Flemish Physical Activity Computerized Questionnaire strength-training intervention (group) physical activity level resistance training and aerobic training speed of movement at 20% static strength whole-body vibration

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2055 strength-training intervention relate to the muscle strength characteristics of a nontrained control group, 7 years after their enrollment in the study.

Methods Procedure The present follow-up study builds on a randomized controlled trial comparing the effects of combined resistance training and aerobic training on the one hand and WBV training on the other hand.13 The participants of the initial intervention study were recalled after a mean follow-up period of 7.3 years (88.18.66mo) after their enrollment in the study. Originally, subjects had been randomly assigned to 1 of 2 training groupsda combined resistance training and aerobic training group (RþA) or a WBV groupdor to a control group (CON). The training programs are described briefly below. At the end of the intervention period, participants were encouraged to pursue a physically active lifestyle but received no further support in doing so. In addition, they were not informed about the recall for the present follow-up measurements. The study was approved by the university’s human ethics committee in accordance with the Declaration of Helsinki. All subjects gave written informed consent.

Participants The initial intervention study included 223 older adults (115 men, 108 women) between 60 and 80 years of age. Subjects were locally recruited from Leuven and its surrounding area and were not physically active at moderate intensity for more than 2 hours a week but were noninstitutionalized. Exclusion criteria were (1) disorders or medications known to affect exercise capacity, muscle performance, or bone metabolism; and (2) engagement in moderate- to high-intensity exercise programs, according to the American College of Sports Medicine (ACSM) guidelines,18 in the 2 years preceding the study. After a mean interval of 7.3 years after their enrollment in the intervention study, all participants were invited by telephone for the present follow-up measurements and when interested, received written information. A sample of 83 men and women (CON, nZ27; RþA, nZ22; WBV, nZ34) was recruited. The results of both the RþA and WBV groups were combined in a strength-training intervention group (INT, nZ56) for the statistical analyses. Two-sample t tests revealed that the present study sample (NZ83) did not differ significantly from the original study population (NZ223) with regard to the baseline characteristics.

Interventions Strength-training intervention group All participants of the INT group exercised 3 times weekly on nonconsecutive days over a period of 1 year. All sessions were held at the University Training Center and were guided and supervised by qualified health and fitness instructors. The training program for the RþA group was constructed following the ACSM guidelines for exercise prescription in older adults and included aerobic, resistance, balance, and flexibility exercises. The duration of each session increased from 60 minutes at the beginning to 90 minutes at the end of the study. Aerobic

2056 traininga consisted of walking or running on a treadmill, cycling on a cycle ergometer, or stepping. Training intensity varied between 75% and 85% of the individual heart rate reserve (Karvonen formula). Subsequently, total-body resistance exercisesa, including leg press and leg extension for the lower body, were performed. A moderate to high loading between 8 and 15 repetition maximum for 1 to 2 sets was used. Thereafter, balance was trained by standing on 1 or both legs with the eyes open or closed, on a firm or unstable surface. Finally, all participants performed stretching exercises. The WBV group performed unloaded static and dynamic leg exercises on a vibration platformb: squat, deep squat (knee angle 90 , hip angle 80 ), wide stance squat (feet apart, toes pointed slightly outward, knee angle 90 ), 1-legged squat (knee angle 125 , hip angle 140 ), lunge (front foot on platform, back foot on ground, front knee angle 90 ), toes-stand (high squat while standing on toes), toes-stand deep (low squat while standing on toes), and moving heels (high squat on toes while describing small circles with the heels). In static exercises, the subjects were instructed to hold their position; in dynamic exercise, subjects were asked to slowly move up (2s) and down (2s). Each session lasted up to 40 minutes, including warming up and cooling down. The training load was low at the beginning but increased gradually according to the overload principle. The training volume was extended by increasing the duration of 1 vibration session, the number of series of 1 exercise, or the number of different exercises. The training intensity was increased by shortening the rest periods or by increasing the amplitude or the frequency of the vibration stimuli. Control group Individuals in the CON group did not participate in any training program. They were advised not to change their lifestyle during the study and not to engage in any new type of physical activity.

Baseline characteristics Body weight and standing height were measured according to standard procedures.19 Body mass index (kg/m2) was calculated. Aerobic fitness status was determined according to a maximal exercise test on an electrically braked Lode Excalibur cycle ergometerc with gradually increasing intensity. The exercise test started at a load of 20W, which was increased with 20W every minute until volitional exhaustion. Oxygen consumption was measured using breath-by-breath respiratory gas exchange analysis with a Cortex Metalyser 3B analyzer.d Peak oxygen consumption (mL$kg1$min1), defined as the highest value during the exercise test, was used for statistical analyses.

E. Kennis et al were stabilized with safety belts. The rotational axis of the dynamometer was aligned with the transversal knee-joint axis and connected to the distal end of the tibia using a lengthadjustable rigid lever arm. Range of motion was set from a knee angle of 90 to 160 (a fully extended leg corresponds to a knee angle of 180 ). Subjects performed the following test protocol twice:  Isometric test: Static strength of the knee extensors was evaluated, performing 2 maximal static knee extensions at a knee joint angle of successively 120 and 90 . The peak torque (Nm) of both contractions in both knee joint angles was recorded. Only the peak torque in 120 (STAT) was withheld for further analyses, whereas maximal isometric strength in 90 was used to set the external resistance during the isotonic tests.  Isotonic test: The isotonic test included 3 maximal ballistic knee extensions against a constant load of 20% of the maximum isometric strength in a knee joint angle of 90 . The subjects were asked to extend their leg as fast as possible from a knee joint angle of 90 to 160 and then passively return the leg to the starting position (90 ). Three explosive contractions were performed. The speed of movement was recorded. The highest value of these 3 repetitions was defined as the maximum speed of movement at 20% (S20,  /s) and was used for further analyses.  Isokinetic test: Dynamic knee extension strength was assessed by conducting a series of maximal knee extension-flexion movements at a constant velocity. The subjects performed 4 repetitions at a low velocity of 60 /s and 6 repetitions at a high velocity of 240 /s. Peak torque (Nm) of the 4 repetitions at 60 /s (DYN60) and peak torque of the 6 repetitions at 240 /s (DYN240) were recorded and further analyzed.

Physical activity behavior The Flemish Physical Activity Computerized Questionnaire (FPACQ), adapted for seniors, was used to assess self-reported lifestyle and physical activity patterns. The FPACQ measures how much time each participants spent performing several physical activities, including leisure time activities, transportation, and household and garden chores during a normal week. The index used in the present study was the overall physical activity level (PAL) index (metabolic equivalents), which was calculated as the summed energy expenditure of all reported activities divided by 168, the number of hours per week. The FPACQ has been proven to be test-retest reliable in the assessment of physical activity in senior citizens (intraclass correlation coefficients, .57e.96).20

Statistical analyses Outcome measurements Muscle performance Unilateral measurements of static and dynamic knee extension strength as well as speed of movement of the knee extensors were conducted on the Biodex Medical System 3 dynamometere by a blinded, experienced investigator during both the intervention and follow-up period. Measurements were performed on the right side, unless there was a medical contraindication. Participants were seated with the hips fixed between 90 and 100 of flexion. The upper leg on the test side, the hips, and the shoulders

After the intervention, both the combined resistance and aerobic training intervention and WBV training intervention were found to be equally effective in improving muscle strength and muscle power.13 Since the purpose of the present study was to assess the long-term preventive impact of strength training overall, the results of both the RþA and WBV groups were combined in an INT group for the statistical analyses. The start of the initial intervention period was spread over 3 months, and the follow-up measurements were planned over a period of 12 months. Consequently, the follow-up period www.archives-pmr.org

Long-term impact of training between baseline and follow-up measurements varied between subjects from 80 to 95 months, with a mean of 88 months or 7.3 years. A 2-sample t test revealed no significant difference in the follow-up period between the CON and INT groups (PZ.648). We therefore used linear regression to estimate follow-up values for muscle strength characteristics and physical activity behavior 7 years after baseline for each subject. All further statistical analyses were performed using these estimated follow-up values. In addition, all results in the article (which also includes the tables and figures), regarding the follow-up measurements are based on these estimated values. Percentage changes in the outcome parameters were individualized. The total changes from baseline to the follow-up values were calculated, whereas the changes from postintervention to follow-up were normalized per year. Data were expressed as means  SEs. Equivalence between groups at baseline as well as between the original study population and the present study sample was assessed using 2-sample t tests. In addition, the changes in outcome parameters from baseline to follow-up were analyzed with repeated-measures analyses of variance (ANOVA). The significance of the between- and withingroup differences were determined by contrast analyses. Finally, the 2-sample t test was used to analyze the variation in annual changes per year from postintervention to follow-up between the CON and INT group. All analyses were performed using SPSS software version 19.f Statistical significance was accepted as P<.05, using 2-tailed tests.

Results Baseline characteristics The baseline characteristics of the subjects in the CON group and INT group of the original study population and the sample used in the current study are presented in table 1. In both study samples, no significant differences were detected between the CON and INT groups at baseline for age, blood pressure, body composition, aerobic fitness status, and muscle strength characteristics. However, baseline PAL index was significantly higher in the CON group compared with the INT group in both the original study population as well as the present study sample. When comparing the original study population with the sample used in the present study, we also noticed no significant differences for any outcome parameters at baseline.

Impact of training from baseline to 1 year postintervention Table 2 presents the values for muscle strength characteristics and physical activity behavior at baseline and postintervention in the CON and INT groups. The percentage changes in these outcome parameters after 1 year of intervention are also shown. Muscle performance In the CON group, no significant changes were found in static strength (STAT: þ.22%3.28%, PZ.544), dynamic strength at low speed (DYN60: þ.35%3.09%, PZ.496), or dynamic strength at high speed (DYN240: þ2.80%2.66%, PZ.585) over the course of 1 year. Surprisingly, speed of movement (S20) did significantly increase from baseline to postintervention in the www.archives-pmr.org

2057 Table 1 Subject characteristics at baseline for CON group and INT group of the original study population (original) and the current study sample (current) Subject Characteristics Age (y) Original Current Py BMI (kg/m2) Original Current Py VO2peak (mL$kg1$min1) Original Current Py STAT (Nm) Original Current Py DYN60 (Nm) Original Current Py DYN240 (Nm) Original Current Py S20 ( /s) Original Current Py PAL index (METs) Original Current Py

CON

INT

P*

68.200.66 67.410.85 .499

67.190.36 66.320.51 .200

.156 .249

26.770.44 27.470.64 .393

26.510.29 26.180.40 .539

.615 .080

22.320.69 21.631.17 .599

21.240.38 21.460.64 .776

.147 .889

137.365.60 138.848.54 .885

136.563.38 133.344.90 .608

.900 .552

130.405.16 124.767.40 .544

129.582.97 125.394.69 .457

.884 .942

69.002.85 66.403.84 .611

68.431.60 67.332.59 .719

.852 .842

322.906.83 321.0210.03 .879

332.583.75 332.306.87 .969

.184 .353

1.580.02 1.560.02 .652

1.530.01 1.490.02 .069

.028 .017

NOTE. Values are mean  SE or as otherwise indicated. Abbreviations: BMI, body mass index; METs, metabolic equivalents; VO2peak, peak oxygen consumption. * Results of independent samples t test, evaluating differences between the INT and CON group. y Results of independent samples t test, evaluating differences between the original study population and the current study sample.

CON group (þ6.55%2.88%, P<.0001). This increase was not significantly different (PZ.752) from the gain in the INT group (þ7.73%2.19%, PZ.001). In contrast, the effects of the 1-year training intervention on STAT (þ11.46%1.86%, P<.0001), DYN60 (þ6.96%1.65%, P<.0001), and DYN240 (þ9.25% 1.68%, P<.0001) in the INT group were significantly higher compared with the CON group (PZ.002, PZ.042, and PZ.039, respectively). Physical activity behavior As requested, the subjects in the CON group did not alter their lifestyle or physical activity (.44%1.76%, PZ.706). In the INT group, 1 year of training had a positive effect on physical activity (þ5.98%.96%, P<.0001). These changes were significantly different between both groups (PZ.001).

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Table 2 Outcome measures at baseline, postintervention, and follow-up as well as percentage changes in muscle strength characteristics and physical activity behavior from baseline to postintervention and from baseline to follow-up in the CON and INT groups Outcome Measures

Baseline

Postintervention

Follow-up

BaselineePostintervention (%)

BaselineeFollow-up (%)

STAT (Nm) CON INT

138.848.54 133.344.90

135.767.35 149.065.54

114.056.52 119.654.40 Py

0.223.28 11.461.86* .002

16.472.69* 8.652.35* .046

Py

0.353.09 6.961.65* .042

15.082.27* 7.102.38* .038

Py

2.802.66 9.251.68* .039

15.932.64* 11.391.95* .184

Py

6.552.88* 7.732.19* .752

14.392.10* 13.161.72* .668

Py

0.441.76 5.980.96* .001

0.301.79 1.541.06 .355

DYN60 (Nm) CON INT DYN240 (Nm) CON INT S20 ( /s) CON INT

124.767.40 125.394.69

66.403.84 67.332.59

321.0210.03 332.306.87

PAL index (METs) CON INT

1.560.02 1.490.02

120.336.09 132.784.81

65.763.47 72.902.75

335.595.96 351.745.12

1.550.02 1.560.02

106.567.12 114.754.44

56.804.26 59.252.50

271.937.97 285.635.33

1.550.03 1.510.02

NOTE. Values are mean  SE or as otherwise indicated. Abbreviation: METs, metabolic equivalents. * Results from paired t test, significant at P<.05 level. y Results from independent samples t test.

Impact of training from baseline to follow-up

Changes from postintervention to follow-up

The status of the participants at baseline and after 7 years of follow-up, as well as percentage changes between baseline and follow-up for all outcome parameters in the CON and INT groups, are presented in table 2. Results of repeated-measures ANOVA are described below.

Postintervention and follow-up values for all outcome parameters in the CON and INT groups are presented in table 2. Figures 1 to 3 show the declination slopes in muscle strength characteristics and physical activity behavior between postintervention and follow-up in the CON and INT groups.

Muscle performance Results of repeated-measures ANOVA indicated a significant effect of time for all measures of muscle performance (P<.0001). The group-by-time interaction was only significant for STAT (PZ.040), but not for DYN60 (PZ.077), DYN240 (PZ.510), and S20 (PZ.843). However, after 7 years of follow-up, the CON group lost significantly (PZ.046 and PZ.038, respectively) more of STAT and DYN60 (16.47%2.69% and 15.08%2.27%, respectively) compared with the INT group (8.65%2.35% and 7.10%2.38%, respectively). The total losses in DYN240 (CON, 15.93%2.64%; INT, 11.39%1.95%) and S20 (CON, 14.39%2.10%; INT, 13.16%1.72%) did not differ between the groups (PZ.184 and PZ.668, respectively). Physical activity behavior For the PAL index, no effect of time (PZ.740) was seen, and no interaction effect (PZ.388) was found. After 7 years of followup, the PAL index remained at baseline levels in the CON (.30%1.79%, PZ.723) and INT groups (þ1.54%1.06%, PZ.246).

Fig 1 Percent changes (SE) over time with respect to baseline values in basic strength, including (A) STAT and (B) DYN60 in the CON group and INT group.

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Long-term impact of training

Fig 2 Percent changes (SE) over time with respect to baseline values in velocity-dependent strength and power, including (A) DYN240 and (B) S20 in the CON group and INT group.

Muscle performance Overall, muscle performance decreased significantly (P<.0001) in the CON and INT groups. No significant differences were observed between both groups, given that the downward slopes of the curves parallel each other. More specifically, STAT decreased by 3.03%.52% per year, DYN60 by 2.86%.66% per year, DYN240 by 3.80%.78% per year, and S20 by 3.62%.47% per year in the CON group. In the INT group, similar annual decline rates of 3.53%.38%, 2.51%.35%, 3.82%.56%, and 3.44%.32% were found for STAT, DYN60, DYN240, and S20, respectively. Physical activity behavior The CON group subjects maintained their physical activity level between postintervention and follow-up (þ.28%.21%, PZ.186), whereas a significant decrease in the PAL index was found in the INT group (.61%.15%, PZ.002). These changes were significantly different between both groups (PZ.002).

Discussion The present study evaluated the long-term preventive impact of strength training on muscle strength characteristics in older adults between 60 and 80 years of age. Our results suggest that the

Fig 3 Percent changes (SE) over time with respect to baseline values in PAL index in the CON group and INT group.

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2059 increases in muscle strength and muscle power after a 1-year strength-training intervention theoretically can compensate for age-related losses over 3 to 5 years. Moreover, 7 years after their enrollment in the study, participants of the INT group experienced a significantly lower loss in basic strength compared with the CON group. However, our findings suggest that this positive impact might be due to preservation of the training-induced gains, rather than an attenuation of the age-related decline. First, strength training during 1 year increased basic strength, including static strength and dynamic strength at low speed, by 7% to 11%. Improvements in velocity-dependent strength and power, including dynamic strength at high speed and speed of movement, varied between 8% and 9%. These training-induced gains correspond with the age-related losses in the CON group equivalent to 3 to 5 years for both basic strength and velocitydependent strength and power. Moreover, after 7 years of follow-up, the present results showed a significantly lower loss in basic strength in the INT group compared with the CON group, suggesting that an extensive strength-training intervention of 1 year has a long-term impact on basic strength. However, to draw general conclusions, the changes in basic strength from postintervention to follow-up should be taken into account as well. From postintervention to follow-up, we found no significant differences in the annual rates of decline between the CON and INT groups, indicating that without supervised and guided training, aging has a similar impact on basic strength in both groups. Although the age-related decline is inevitable, it would seem therefore that a 1-year strength-training intervention in older adults can postpone detrimental outcomes by preserving the training-induced gains in basic strength. However, the physical activity behavior of both the CON and INT groups was not monitored during the follow-up period. One year of strength training did not show a long-term preventive effect on velocity-dependent strength and power, as the total estimated loss after 7 years of follow-up and the annual decline rates from postintervention to follow-up in dynamic strength at high speed and speed of movement were not significantly different between the CON and INT groups. Thus, in trained participants, the human aging process apparently impacts the movement velocity component of muscle strength loss more compared with the force component. This result is in line with previous research, indicating that with advancing age, muscle power declines earlier and more precipitously than muscle strength, mainly as a result of an age-related slowing of contraction speed.1,4,5 Given the importance of the ability to develop power for functional performance, training and rehabilitation programs in older adults should also focus on the continuation of higher-velocity training protocols. The CON group did not participate in any training program during the initial intervention study, and the PAL index showed no changes in general physical activity behavior over time. This allows the CON group to be used to study the effects of general aging. From postintervention to follow-up, significant declines in muscle strength characteristics between 2% and 3% per year were found in the CON group. These rates correspond well with annual losses reported in cross-sectional studies.1,2 Follow-up studies3,21-23 in older adults between 60 and 89 years of age showed similar annual declines in isometric knee extension strength (1.8% to 2.0% per year), isokinetic knee extension strength at 60 /s (1.98% to 3.6% per year), isokinetic knee extension strength at 240 /s (2.48% per year), and muscle power (3% to 4% per year).

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Study limitations Our study has limitations. First, the training effects were rather limited when compared with other studies using resistance training or WBV training, especially with regard to basic strength. However, the training intensity of our training programs was designed to improve, not to maximize muscle strength. Second, during the 1-year training intervention, the CON group did not show the expected age-related decrease in muscle strength. In addition, a rather unexpected increase in velocity-dependent power was found in the CON group. Together, these findings are likely to reflect the stimulating effect of the extensive health and fitness measurements at baseline and postintervention. Finally, the physical activity behavior of both groups was not monitored during the follow-up period. Although this was not elementary for the interpretation of the present results, we cannot ascribe the lower decline in basic strength in the INT group to the preservation of the training-induced gains 7 years before.

Conclusions We found that a 1-year strength-training intervention results in improved muscle strength characteristics in older adults 7 years after their enrollment in the intervention. However, an extensive exercise program cannot attenuate the age-related decline in strength characteristics, once the training intervention is stopped. In addition, older adults seem to experience larger decreases in velocity-dependent strength and power than in basic strength. These findings underline the importance of continued strength training and engagement in sufficiently intensive physical activities over the whole lifespan, to preserve muscle strength and power. Given the importance of the ability to develop power for functional performance, training and rehabilitation programs in older adults should also include higher-velocity training protocols.

Suppliers a. Technogym Systems, 20 Via Giorgio Perticari, 47035, Gambetolla, Italy. b. Power Plate International, 160 Jan van Gentstraat, 1171, Badhoevedorp, The Netherlands. c. Lode BV, 16 Zernikepark, 9747, Groningen, The Netherlands. d. Cortex Biophysic GmbH, 2d Walter-Ko¨hn-Straße, 04356, Leipzig, Germany. e. Biodex Medical Systems, 20 Ramsey Rd, Shirley, NY 11967. f. SPSS Inc, 233 S Wacker Dr, 11th Fl, Chicago, IL 60606.

Keywords Aging; Follow-up studies; Muscle contraction; Rehabilitation; Resistance training

Corresponding author Christophe Delecluse, PhD, Tervuursevest 101, 3001 Leuven, Belgium. E-mail address: [email protected].

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