Abstract
OBJECTIVE:
To describe and compare decreases in grip and knee extension strength accompanying aging.
METHOD:
Eighty-five community-dwelling women (20 to 79 years of age) participating in the validation phase of the NIH Toolbox for the Assessment of Neurological and Behavioral Function contributed data to this study. Their strength was measured bilaterally-grip with a hand dynamometer and isometric knee extension with an isokinetic dynamometer. Grip and knee extension strength measures of the left and right sides were combined and analyzed.
RESULTS:
Both combined grip strength and combined knee extension strength decreased with age. The decreases were significant between the 60–79 year age group and the 20–39 year age group. Strength of the 60–79 year age group, described as a percentage of the 20–39 year age group, was significantly less for knee extension than for grip.
CONCLUSION:
Grip strength declines less than knee extension strength with age. Grip strength, therefore, may not fully capture the effect of age on muscle strength.
Introduction
Muscle strength declines with age [1] and has consequences for aging adults. For example, older adults who are weaker are more likely to demonstrate cognitive impairments [2] and mobility limitations [3, 4, 5]. They are also more likely to die prematurely, use more health-care resources, and have a lower quality of life [5, 6].
Muscle strength can be characterized using objective measurements obtained from any of a number of muscle groups [7]. However, measurements of grip strength obtained with a hand dynamometer [8] and knee extension strength obtained with an isokinetic dynamometer [9] are probably the most highly recommended [10].
Given the association between grip and knee extension strength [11] and the greater practicality of hand-grip dynamometry, the use of grip strength as a surrogate for overall strength makes sense. That noted, it is uncertain whether grip and knee extension strength decline at a similar rate with aging [12, 13]. If knee extension strength declines at a faster rate than grip strength, reliance on grip strength as an indicator of overall strength may result in an underestimation of the strength declines associated with aging. This study, therefore, was undertaken to describe and compare decreases in grip and knee extension strength accompanying aging.
Methods
This study involved the secondary analysis of str-ength data from the motor domain of the validation phase of the NIH Toolbox for the Assessment of Neurological and Behavioral Function [14]. Two sites, the University of Connecticut and Rehabilitation of Chicago, contributed strength data. The institutional review boards of each site approved the study.
Participants
The database for the validation phase of the NIH Toolbox includes data obtained from consenting, ambulatory males and females between the ages of 3 and 85 years. The present study, however, was restricted to women 20 to 79 years. The restriction was based on the purpose of the present study and the availability of sufficient participants in relevant age-group strata. All participants provided written consent.
Procedures
The present study involved item level data provided by the NIH Toolbox. In the present study we extracted basic demographic and anthropometric data from the Toolbox database. We also extracted grip strength and knee extension strength data. Grip strength was measured as described by the American Society of Hand Therapists [15]. The best measure from each hand was summed to obtain a combined grip strength measurement which was converted to Newtons. Isometric knee extension strength was measured (knee at 90 degrees) with a Biodex isokinetic dynamometer. The best measure from each side was summed to obtain a combined knee extension strength measurement expressed in Newton meters.
Statistical analysis
Prior to analysis, data were stratified into three groups based on age (20–39, 40–59, and 60–79 years). Strength data were then summarized for the entire sample and for each age group. Additionally, strength data for the two older groups were normalized against (divided by) the mean strength of the 20–39 year group. Analysis of variance (ANOVA) with polynomial contrasts was used to compare actual strength across all three age groups. Where differences were found, Dunnett post hoc tests were performed. T-tests were used to compare normalized grip and knee extension strength within each of the two older age groups. The statistical Package for the Social Sciences (SPSS version 19.0) was used for all analyses.
Summary data (mean [standard deviation]) for study variables
Summary data (mean [standard deviation]) for study variables
In all, 85 women contributed data to the present study; 31 were 20-39 years, 20 were 40–59 years, and 34 were 60–79 years. Table 1 summarizes their demographic, anthropometric, and strength data.
The ANOVAs showed that both grip strength and knee extension strength decreased significantly and linearly with age (
Discussion
The present study confirmed the well-known fact [1] that strength decreases with age. The decreases, however, could only be ascribed to women of 60 or more years. Relative to the youngest age group, the oldest age group demonstrated a better preservation of grip strength than knee extension strength. This finding is consistent with that of Samuel et al. [12] who reported “a greater loss of quadriceps than grip strength,” but different from that of Xue et al. [13] who found grip strength to decline more rapidly than knee extension strength, at least between 70 to 75 years of age. Notably, Samuel et al.’s study was cross-sectional and involved both men and women, whereas Xue et al.’s study was longitudinal and involved women who were 70 to 79 years of age at the time of enrollment. In a previous cross-sectional study Bohannon reported values for knee extension strength that suggest a larger decline among 60 to 79 year old women (about 45%) [1] than in the present study (about 39%). In the previous study the strength of knee extension declined more than the strength of the five other muscle actions tested. Grip strength was not tested in that study.
A faster age-related decline in the strength of the knee extensors makes sense. The knee extensors are challenged by weightbearing activities (eg, sit-to-stand, walking, stair negotiation) [16] in a way that many other muscles, particularly those of the upper limbs, are not. As participation in these activities decrease with age [17, 18, 19], a relatively greater decline in knee extension strength might be expected.
This study has a few weaknesses. First, a convenience sample was used. Consequently, the sample may or may not be representativeness of adults in the United States. Second, the number of men and adults 80 years of age or more in the Toolbox data base was insufficient to justify their inclusion in the present study. Finally, the present study was cross-sectional. A longitudinal study in which the muscle strength of a cohort is followed over time would be more informative.
Conflict of interest
None to report.