The Association Between Knee Extensor Force Steadiness, Force Accuracy, and Mobility in Older Adults Who Have Fallen

Abstract

Background: Older adults often experience limited mobility, lower extremity muscle weakness, and increased fall risk. Furthermore, when older adults perform tasks that require control of submaximal force, impairments in their ability to maintain steady and accurate force output have been reported. Such problems may be related to deteriorating levels of mobility, particularly in older adults who have fallen.

Purpose: The purpose of this study was to determine whether an association exists between muscle force steadiness (MFS) or muscle force accuracy (MFA) of the knee extensors and mobility in older adults who have fallen.

Methods: Twenty older adults ((Equation is included in full-text article.)= 77.5 ± 7 years, 5 males and 15 females) with 2 or more comorbid conditions and who experienced a fall in the past year underwent assessment of maximal voluntary isometric contraction of the knee extensors. A submaximal target force of 50% of their maximal voluntary isometric contraction was used to determine concentric and eccentric (ECC) steadiness (the fluctuations in force production) and accuracy (the average distance of the mean force from the target force) measures. Mobility was indicated by the 6-minute walk test, the Timed Up and Go, stair ascent, and stair descent tests. Correlation analysis was used to assess the relationship between measures of muscle force control and mobility.

Results: The correlations between muscle force steadiness and mobility were not significant (P > .05) for either contraction type. However, MFA during ECC contractions only was correlated significantly with all measures of mobility-6 minute walk test (r = -0.48; P = .03), Timed Up and Go (r = 0.68; P = .01), stair ascent (r = 0.60; P = .01), and stair descent (r = 0.75; P < .01).

Conclusion: The identification of the relationship between ECC MFA and mobility in older adults who have fallen is novel. Although the correlations are not causal, these relationships suggest that inaccurate force output during ECC contractions of the knee extensors is linked to impaired mobility.

Figure 1. Example of muscle force steadiness and muscle force accuracy measures

The force trace represents a muscle force steadiness (MFS) trial over a four second time interval recorded during a muscle contraction (either concentric or eccentric). Muscle force accuracy (MFA) was also recorded as the force output relative to a target force set as 50% of the subject’s maximal voluntary isometric contraction (MVIC). The middle 2 second interval (enlarged) was used to calculate MFS expressed as CV of force, and MFA.

Figure 2. A–D Scatterplots, correlations between eccentric muscle force accuracy and mobility tests

The scatterplots represent moderate to strong relationships between eccentric muscle force accuracy (MFA) (expressed as the percentage of target force) and mobility test scores: A) Eccentric muscle force accuracy and 6 minute walk r = −0.48, P<0.05; B) Eccentric muscle force accuracy and timed up and go test r = 0.68, P<0.01; C) Eccentric muscle force accuracy and stair ascent r = 0.60, P<0.01; D) Eccentric muscle force accuracy and stair descent r = 0.75, p<0.01.

Figure 2. A–D Scatterplots, correlations between eccentric muscle force accuracy and mobility tests

The scatterplots represent moderate to strong relationships between eccentric muscle force accuracy (MFA) (expressed as the percentage of target force) and mobility test scores: A) Eccentric muscle force accuracy and 6 minute walk r = −0.48, P<0.05; B) Eccentric muscle force accuracy and timed up and go test r = 0.68, P<0.01; C) Eccentric muscle force accuracy and stair ascent r = 0.60, P<0.01; D) Eccentric muscle force accuracy and stair descent r = 0.75, p<0.01.

Figure 2. A–D Scatterplots, correlations between eccentric muscle force accuracy and mobility tests

The scatterplots represent moderate to strong relationships between eccentric muscle force accuracy (MFA) (expressed as the percentage of target force) and mobility test scores: A) Eccentric muscle force accuracy and 6 minute walk r = −0.48, P<0.05; B) Eccentric muscle force accuracy and timed up and go test r = 0.68, P<0.01; C) Eccentric muscle force accuracy and stair ascent r = 0.60, P<0.01; D) Eccentric muscle force accuracy and stair descent r = 0.75, p<0.01.

Figure 2. A–D Scatterplots, correlations between eccentric muscle force accuracy and mobility tests

The scatterplots represent moderate to strong relationships between eccentric muscle force accuracy (MFA) (expressed as the percentage of target force) and mobility test scores: A) Eccentric muscle force accuracy and 6 minute walk r = −0.48, P<0.05; B) Eccentric muscle force accuracy and timed up and go test r = 0.68, P<0.01; C) Eccentric muscle force accuracy and stair ascent r = 0.60, P<0.01; D) Eccentric muscle force accuracy and stair descent r = 0.75, p<0.01.