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Relationship Between Measures of Balance and Strength in Middle-Aged Adults

Muehlbauer, Thomas1; Gollhofer, Albert2; Granacher, Urs1

Journal of Strength and Conditioning Research: September 2012 - Volume 26 - Issue 9 - p 2401–2407
doi: 10.1519/JSC.0b013e31823f8c41
Original Research
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Muehlbauer, T, Gollhofer, A, and Granacher, U. Relationship between measures of balance and strength in middle-aged adults. J Strength Cond Res 26(9): 2401–2407, 2012—The purpose of this study was to investigate the relationship between variables of static and dynamic postural control as well as between isometric and dynamic muscle strength. A single-group design was used. Thirty-two middle-aged healthy adults (mean age: 56 ± 4 years) performed measurements of static (unperturbed)/dynamic (perturbed) balance and of isometric (i.e., maximal isometric torque [MIT]; rate of torque development [RTD] of the plantar flexor)/dynamic (i.e., countermovement jump [CMJ] height and power) lower extremity muscle strength. No significant associations were observed between variables of static and dynamic postural control (r = +0.128–0.341, p > 0.05) and between measures of balance and strength (r = −0.189 to +0.316, p > 0.05). Significant positive correlations were detected between variables of isometric and dynamic strength ranging from r = +0.361 to +0.501 (p < 0.05). Further, simple regression analyses revealed that a 10% increase in the mean CMJ height (3.1 cm) was associated with 44.4 N·m and 118.4 N·m·s−1 better MIT and RTD, respectively. The nonsignificant correlations between static and dynamic balance measures and between balance and strength variables imply that static and dynamic postural control and balance and strength are independent of each other and may have to be tested and trained complementarily.

1Institute of Sport Science, Friedrich-Schiller-University Jena, Jena, Germany

2Institute of Sport and Sport Science, Albert-Ludwigs-University Freiburg, Freiburg, Germany

Address correspondence to Dr. Thomas Muehlbauer, t.muehlbauer@uni-jena.de.

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Introduction

Throughout the last 30 years, demographic change in terms of an increase in the proportion of elderly people and a decrease in that of younger people occurred in societies of western industrialized countries and is mirrored in an aging workforce (32). In the European Union, the proportion of 15- to 24-year-olds comprised about 18% of the workforce in 2005, whereas that of 50- to 64-year-olds amounted to 27% (17). Population projection indicate that the proportion of the middle-aged workforce (50- to 64-year-olds) will increase to 35% in 2025, whereas the proportion of the young workforce (15- to 24-year-olds) will nearly stay the same (17%) (17).

Further, there is evidence that absence from work because of injury and sickness is more than doubled in people aged 55–64 years compared with that in people aged 25–34 years (25). As a consequence, medical treatment and compensatory costs are higher for the older compared with the younger employee (4). Occupational injuries because of slips, trips, and falls lay a high financial burden on the public health care system (4). Further, workers aged 45 years and more are at higher risk for sustaining slip, trip, or fall accidents than are their younger counterparts (2,21).

The etiology of falls comprises extrinsic and environmental risk factors (e.g., lighting stairs, floor surfaces, obstructed walkways, inadequate handrails) and intrinsic/subject-related risk factors (e.g., impaired postural control, reduced levels of muscle strength) (22). Morfitt (23) reported that with increasing age, the number of falls caused by intrinsic factors increases. In addition, balance deficits in terms of larger postural sway and slower gait speed can already be found in middle-aged compared with in young healthy adults (1,3,27,29). With respect to strength performance, middle-aged adults showed lower levels of maximal strength and muscular power than did young adults (30,34).

From a therapists' or practitioners' point of view, a knowledge of the relationship between postural control and lower extremity muscle strength may be important for both the identification of persons with an increased injury/fall risk and the development of injury/fall-preventive training programs. More specifically, given the high incidence rate of fall-related injuries in middle-aged people (2,21), findings on potential associations between variables of balance and lower extremity muscle strength could provide scientific rationales to injury/fall risk assessment and to the development of specifically tailored injury/fall prevention and rehabilitation programs in middle-aged adults.

Associations between measures of static and dynamic balance and isometric and dynamic lower extremity strength have already been studied in healthy middle-aged adults. However, the reported correlations largely vary in size (low to high) and level of significance (nonsignificant to significant). For example, Izquierdo et al. (18) reported correlations between isometric (i.e., maximal isometric force [MIF], rate of force development [RFD]) and dynamic (i.e., countermovement jump [CMJ], squat jump [SJ], standing long jump [SLJ]) lower extremity strength measures ranging from r = +0.02 to +0.63. In the same study, associations between variables of strength and several balance capacities (e.g., bipedal stance) were reported ranging from r = −0.66 to +0.90. Further, Holviala et al. (15) assessed various balance (i.e., monopedal standing time, 10-m walk time, 10 steps climbing time) and lower extremity strength (i.e., MIF, 1-repetition maximum [1RM] strength) capacities and observed correlations ranging from r = −0.60 to −0.50. Lastly, Weirich et al. (33) reported that most measures of balance (i.e., monopedal stance, tandem walk, etc.) were only weak or moderately related to predictor variables such as quadriceps, hip extensors, and hamstrings strength in middle-aged people. The reason for this discrepancy in the literature is probably to be found in the differing research designs applied (i.e., large variety of methods for the assessment of balance and strength). In other words, further research is needed to better determine the relationship between balance and lower extremity muscle strength in healthy middle-aged adults. Therefore, the objectives of this study were to investigate the associations between variables of static and dynamic balance, isometric and dynamic lower extremity muscle strength and between measures of balance and strength in middle-aged adults.

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Methods

Experimental Approach to the Problem

Given that deficits in balance and lower extremity muscle strength represent important intrinsic fall/injury risk factors in middle-aged adults, we calculated associations between measures of static and dynamic balance and variables of static and dynamic muscle strength in a controlled cross-sectional study. As a measure of static and dynamic postural control, displacements of the center of pressure (CoP) in anterior-posterior (ap) and mediolateral (ml) directions were computed under unperturbed and perturbed conditions. We selected 2 dependent variables for isometric strength testing (i.e., maximal isometric torque [MIT] and rate of torque development [RTD] of the plantar flexors) and 2 for dynamic strength testing (i.e., CMJ height [CMJH] and power [CMJP]). Potential associations were calculated using Pearson's correlation coefficient. The expected results may contribute to the development and application of specifically tailored exercise programs for fall and sport injury prevention and rehabilitation in middle-aged adults.

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Subjects

Thirty-two middle-aged, healthy adults participated in the study after the experimental procedures were explained. The participants' characteristics are presented in Table 1. The participants were asked to fill in the validated “Freiburg questionnaire of physical activity©” (7) (Table 1). The test-retest reliability of the questionnaire is medium to large with r = 0.35–0.91 depending on the type of physical activity considered. In addition, an acceptable validity (r = 0.42) in adults aged 18–78 years has been shown (7). The mean physical activity level for people aged 50.0–59.9 years amounts to 11.0 h·wk−1. Appropriate informed consent was gained from the participants. Local ethical permission was given by the Ethikkommission beider Basel, and all the experiments were conducted according to the latest version of the declaration of Helsinki. The study was conducted from January to March 2009.

Table 1

Table 1

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Procedures

Static and Dynamic Balance

Test circumstances (e.g., room illumination, temperature, noise) were in accordance with recommendations for posturographic testing (20). Static and dynamic postural control was assessed by means of a balance platform (GKS 1000®, IMM, Mittweida, Germany). The balance platform consists of 4 uniaxial sensors measuring displacements of the CoP in the ml and ap directions. Under static (unperturbed) conditions, the balance platform was firmly fixed on the floor. For experimental testing, the participants were asked to stand on their dominant leg on the platform with their supported leg in 30° flexion, hands placed on hips and gaze fixated on a cross on the nearby wall. The dominant leg was determined according to the lateral preference inventory (6). The subjects were instructed to remain as stable as possible and to refrain from any voluntary movements during the trials. Before testing, the participants performed 2 practice trials on the balance platform. Thereafter, 3 test trials were conducted. The best trial (least CoP displacements) was used for further analysis. Data were acquired for 30 seconds at a sampling rate of 40 Hz (20). Displacements of the CoP in ap (CoPap_s in millimeters) and ml (CoPml_s in millimeters) directions were computed under static conditions and used as outcome measures.

Under dynamic (perturbed) conditions, the platform was placed into a cage that was mounted to 4 springs and free to move in the transversal, ml, and ap directions. Mediolateral perturbation impulses were applied to investigate dynamic postural control of the participants. Therefore, the platform was moved 2.5 cm from the neutral position in the ml direction, where it was magnetically fixed. The participants' test position was identical with that during the assessment of static postural control. Several trials helped the participants to get accustomed to the measuring device. After the investigators visually controlled the position of the subjects, the ml perturbation impulse was unexpectedly applied by detaching the magnet. The platform suddenly accelerated in the medial direction. The participants' task was to damp the oscillating platform by balancing unilaterally on the platform. Data were acquired for 10 seconds at a sampling rate of 40 Hz. If the participants did not accomplish the whole sampling duration, they were allowed to repeat. Three trials were performed. The best trial (least CoP displacements) was used for further analysis. Again, displacements of the CoP in ap (CoPap_d in milliliters) and ml (CoPml_d in millimeters) directions were computed under dynamic conditions and used as outcome measures. Intraclass correlation coefficients (ICCs) were calculated for total displacements of the CoP under static (CoPap_s: ICC = 0.87; CoPml_s: ICC = 0.89) and dynamic conditions (CoPap_d: ICC = 0.83; CoPml_d: ICC = 0.87). This protocol has recently been described in detail elsewhere (10,12).

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Isometric Strength

Maximal isometric torque of the plantar flexors was measured on an isokinetic system (Isomed 2000®, D & R Ferstl GmbH, Hemau, Germany). The maximum error of the torque sensor was <0.2%. The participants lay supine on the seat of the isokinetic device, with hip and knee angles in neutral position (180°) and the ankle angle at 100°. Straps attached to the isokinetic system firmly fixed the shoulders, the waist, the thigh, the shank, and the foot. In addition, the participants were asked to cross their arms in front of their chest. Thus, evasive movements of the upper and lower body were not possible. The exact position of each participant was documented and saved so that it was identical in the pre, post, and follow-up tests. Testing was performed with the dominant leg. Before the testing started, the participants warmed up by doing 3–5 submaximal isometric actions in the isokinetic system to get accustomed to the testing procedure. Thereafter, each subject performed 3 plantar flexor exercises with maximal voluntary effort. For each trial, the subjects were thoroughly instructed to act as forcefully and as fast as possible and to avoid forced respiration. The torque signal was sampled at 200 Hz. A digital fourth-order recursive Butterworth low-pass filter, with a cut-off frequency of 50 Hz filtered the torque signal. During offline analysis, the best trial in terms of maximal torque was selected and used for further data analyses. The MIT was defined as the maximal voluntary torque value of the torque-time curve, determined under isometric condition. Rate of torque development was defined as the mean slope of the torque-time curve between 20 and 80% of the individual maximal torque. The ICC was calculated for MIT (ICC = 0.97) and RTD (ICC = 0.93) of the plantar flexors. This protocol has recently been described in detail elsewhere (14).

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Dynamic Strength

The participants performed maximal vertical CMJs while standing on a 1-dimensional force platform (Kistler® type 9290AD, Winterthur, Switzerland). The vertical ground reaction force was sampled at 500 Hz. During the CMJ, the subjects stood in an upright position on the force platform and were instructed to begin the jump with a downward movement, which was immediately followed by a concentric upward movement, resulting in a maximal vertical jump. The subjects performed 3 CMJs with a resting period of 1 minute between jumps. The best trial in terms of maximal jumping height was taken for further data analysis. The ICC was calculated for the CMJ height (CMJH: ICC = 0.98) and CMJ power (CMJP: ICC = 0.98). This protocol has recently been described in detail elsewhere (13).

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Statistical Analyses

Data are presented as group mean values ± SDs. Associations of static and dynamic balance variables with isometric and dynamic strength variables were assessed using Pearson product-moment correlation coefficient. Associations are reported by their correlation coefficient r, level of significance, and the amount of variance explained (r2). Values of r = 0.10 indicate a small, r = 0.30 a medium, and r = 0.50 a large size of correlation (5). In addition simple linear regression models were calculated to determine the most robust predictors of the respective outcome variables. Total variance is reported by the coefficient of determination (R2) and the respective level of significance (p value). Further, parameter estimate (B), standard error (SE), standardized estimates (β coefficients), and t values are also provided. The significance level was set at α = 5%. All analyses were performed using Statistical Package for Social Sciences version 19.0.

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Results

Means and SDs are presented for all the variables in Table 1.

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Static and Dynamic Balance

No statistically significant correlations were detected between variables of static (unperturbed condition) and dynamic (perturbed condition) postural control. Respective r values ranged from +0.128 to +0.341 (Table 2). Based on r2, only a small proportion of variance could be explained (2–12%). An example for the association between static and dynamic balance performance is given in Figure 1A.

Table 2

Table 2

Figure 1

Figure 1

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Isometric and Dynamic Strength

Significant positive correlations were detected between variables of isometric and dynamic muscle strength with RTD of the plantar flexors showing higher correlations with variables of CMJ performance than MIT of the plantar flexors (Table 2). The r values ranged from +0.361 to +0.501 (p < 0.05). Values for r2 indicated an explained variance of 13–25%. An example for the association between isometric and dynamic lower extremity muscle strength is provided in Figure 1B.

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Balance and Strength

No significant correlations were observed between variables of postural control and muscle strength. Respective r values ranged between −0.189 and +0.316 (Table 2). Based on r2, only a small proportion of variance was explained (4–10%). An example for the association between static balance and isometric strength is given in Figure 1C.

Because statistically significant correlations were found only between isometric and dynamic lower extremity muscle strength and not between static and dynamic balance and between variables of balance and strength, we calculated linear regression models for measures of strength only. Results of the simple regression analysis for the variables of isometric and dynamic lower extremity muscle strength are shown in Table 3. The R2 values ranged between 0.131 and 0.655 (p < 0.05), explaining 13.1–65.5% of total variance of the respective strength parameters. The covariates sex, body mass, body height, and body mass index did not influence our result (data not shown). Furthermore, the regression formulas obtained for the association of CMJ height with MIT (y = 3.790x + 32.478) and RTD (y = 13.215x + 76.949) revealed that, for example, a 10% increase the in mean CMJ height (corresponds to 3.1 cm) was related to 44.4 N·m and 118.4 N·m·s−1 better MIT and RTD values, respectively.

Table 3

Table 3

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Discussion

Only scarce information is available regarding the association of static (unperturbed) and dynamic (perturbed) balance measures in healthy middle-aged adults, which is why results from different age groups have to be consulted. In contrast to our results, Hsiao-Wecksler et al. (16) were able to predict the behavior during a perturbed balance condition from performance during a unperturbed balance condition in young (mean age: 25 ± 3 years) and old healthy adults (mean age: 69 ± 2 years). Based on their findings, the authors suggested that the postural control system may use the same control mechanisms during quiet stance and mild perturbation conditions. The reason for the discrepancy between our results and the findings reported by Hsiao-Wecksler et al. may be the result of the differing methods applied for balance and strength assessment. Although we investigated associations between quiet stance and severe stance perturbations, Hsiao-Wecksler et al. (16) examined the relationship between quiet stance and mild stance perturbation. The mild perturbations in the study of Hsiao-Wecksler et al. were initiated by activating a mechanical trigger. The authors reported that the tug necessitated only a mild postural sway response. The severe perturbations in this study were applied by detaching a magnet. Thus, it can be speculated that in fact different neuromuscular mechanisms might be responsible for the regulation of mild and severe stance perturbation impulses. In other words, the compensation of mild stance perturbations might afford only minor magnitudes of reflex controlled muscle activations whereas severe stance perturbations demand large reflex activations of lower extremity muscles to successfully stabilize the center of gravity over the base of support. In fact, it was recently shown that reflex controlled muscle contractions are necessary to compensate for this type of severe perturbation impulse (11).

Our results may have functional implications for future directions in balance assessment and in planning and developing adequate balance training programs to counteract intrinsic injury/fall risk factors in middle-aged adults. Based on our findings, it can be hypothesized that different neuromuscular mechanisms are responsible for the regulation of static and dynamic postural control. Given that slips, trips, and fall accidents primarily occur during dynamic conditions (i.e., walking, stair climbing) in middle-aged adults (31), injury/fall risk assessment should particularly be carried out under dynamic conditions to identify potential balance problems. From an injury/fall preventive point of view, our results indicate that static and dynamic postural control are independent of each other and may have to be trained complementarily. In fact, this is reinforced by studies that proved the effectiveness of balance training in reducing fall risk among middle-aged adults (24,28). The authors reported that the applied training program involved static and dynamic exercises both in double and single-leg stance with the goal to maintain balance for as long as possible.

The present results are to large parts in accordance with those in the literature regarding the association between isometric and dynamic variables of lower extremity muscle strength. For example, Izquierdo et al. (18) compared isometric and dynamic leg strength measures in healthy middle-aged adults (mean age: 40 ± 2 years). For this purpose, the participants performed maximal isometric leg extensions and CMJs. The CMJ height showed statistically significant correlations with isometric leg test performance expressed by MIF (r = 0.63, p < 0.05) but not by RFD (r = 0.07, p > 0.05). In contrast to our results, Izquierdo et al. (18) did not detect a significant relationship between the CMJ height and RFD of the knee extensors. A potential explanation for this discrepancy is that we investigated RTD of the plantar flexors, whereas Izquierdo et al. (18) tested the RFD of the knee extensors. Therefore, the primary outcome measures of the 2 studies varied in terms of muscle group (plantar flexors vs. knee extensors) used for testing and physical parameter (RTD vs. RFD) used for analysis.

Given that in our study, a 10% increase in the mean CMJ height (e.g., achieved by a plyometric strength training) was associated with 44.4 N·m and 118.4 N·m·s−1 better MIT and RTD, respectively, it can be postulated that the ability to produce dynamic muscle contractions shares some structural and functional foundation with the ability to generate maximal isometric contractions of the plantar flexors.

From a practitioner's perspective, this indicates that training-induced strength adaptations are not necessarily task specific. In other words, gains in isometric muscle strength can be transferred at least to a certain extent to dynamic conditions and vice versa. In fact, a study conducted by Kalapotharakos et al. (19) indicated that resistance training significantly increased 1RM lower body strength, SJ height, and CMJ height in middle-aged adults (age > 53 years). This finding could be beneficial for therapists, educators, and coaches in terms of the development and application of effective resistance and plyometric training programs during rehabilitation of lower extremity injuries.

To date, there are only 3 studies available that investigated associations between static and dynamic balance and isometric and dynamic muscle strength in healthy middle-aged adults (15,18,33). In the first study, Izquierdo et al. (18) scrutinized the relationship between static balance (i.e., mono- and bipedal stance time/area), isometric (i.e., MIF, RFD), and dynamic (CMJ height, SJ height, SLJ height) strength in men aged 40 ± 2 years. The authors reported significant and nonsignificant correlations between balance and strength performances (r values ranged between −0.66 and +0.90). More recently, Holviala et al. (15) investigated the relationship between isometric strength (i.e., MIF), static (i.e., monopedal standing time), and dynamic (i.e., 10-m walk time, 10 steps climbing time) balance in women aged 53 ± 2 years. As a result, statistically significant correlations were found ranging from r = −0.50 to −0.60. In contrast to that, Weirich et al. (33) reported that most measures of static (i.e., monopedal stance) and dynamic (i.e., tandem walk) balance were only weakly or moderately related to quadriceps, hip extensor, and hamstring strength in middle-aged (age range: 35–45 years) and late middle-aged (age range: 55–64 years) women. Based on the large variations in the size of correlations and their level of significance, it remains unclear whether balance and leg muscle strength are independent or dependent neuromuscular capacities in healthy middle-aged adults. In the first case it could be argued that balance and strength capacities have to be tested and trained complementarily. In fact, 3 former studies from our laboratory also showed small and nonsignificant correlations for several balance and strength measures when using the same experimental approach but investigating prepubertal children (9), adolescents (8), and young adults (26). In the latter case, this indicates that gains in balance can be transferred at least to a certain extent to strength performances of the lower extremities and vice versa. This is reinforced by a recent study that investigated the effects of resistance training on muscle strength characteristics and balance in healthy middle-aged women (15). Twenty-one weeks of heavy resistance training resulted in significant improvements in 1RM strength, MIF, bipedal stance performance, and 10-m walking time at normal and maximal speeds.

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Practical Applications

Based on the results of this study, static and dynamic balance appears to be independent of each other in healthy middle-aged adults. Given that slips, trips, and fall accidents primarily occur during ambulation and thus during dynamic conditions in middle-aged adults (31), injury/fall risk assessment should particularly be carried out under dynamic conditions to identify potential balance deficits. From an injury/fall preventive point of view, our results indicate that static and dynamic exercises (e.g., bipedal, step, tandem, monopedal stance on firm vs. foam ground or under unperturbed vs. perturbed conditions) should be incorporated in injury/fall-preventive balance training programs. Furthermore, the strong associations between isometric and dynamic lower extremity muscle strength imply that gains made in 1 variable (e.g., MIT of the plantar flexors) after training may be associated with a change in performance in other strength variables (e.g., CMJ height). Thus, increases in isometric lower extremity muscle strength after resistance training can be transferred at least to a certain extent to an improved jumping performance. Further, improved jumping performance after plyometric training can be transferred in part to increases in isometric muscle strength. The observed lack of an association between balance and lower extremity muscle strength indicates that these 2 important neuromuscular capacities are unrelated and should therefore be trained complementarily for injury/fall-preventive purposes. However, because of somewhat contrary findings from other studies further research is needed to reveal whether measures of balance and strength are indeed independent of each other or whether there is a relationship between these 2 capacities.

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Keywords:

static and dynamic postural control; maximal isometric torque; rate of torque development; jumping height and power

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