Knee extensor strength exhibits potential to predict function in sporadic inclusion-body myositis

Abstract

Introduction: In this study we address the challenging issue of potential use of muscle strength to predict function in clinical trials. This has immediate relevance to translational studies that attempt to improve quadriceps strength in sporadic inclusion-body myositis (sIBM).

Methods: Maximum voluntary isometric contraction testing as a measure of muscle strength and a battery of functional outcomes were tested in 85 ambulatory subjects with sIBM.

Results: Marked quadriceps weakness was noted in all patients. Strength was correlated with distance walked at 2 and 6 minutes. Additional correlations were found with time to get up from a chair, climb stairs, and step up on curbs.

Conclusions: Quadriceps (knee extensor) strength correlated with performance in this large cohort of sIBM subjects, which demonstrated its potential to predict function in this disease. These data provide initial support for use of muscle strength as a surrogate for function, although validation in a clinical trial is required.

Copyright © 2011 Wiley Periodicals, Inc.

Figures

FIGURE 1

Subject positioned for maximum voluntary isometric contraction testing of knee flexion (hamstrings). The position of the transducer is reversed and comes from behind the leg for knee extension (quadriceps) testing.

FIGURE 2

Asymmetry in knee extension MVICT values between the legs. The y-axis represents the percentage of all subjects with the given degree of asymmetry indicated in the histogram. Each column represents a bin of 10% increments. The number of patients in each category is indicated above each column. Notably, over half of the subjects had >20% difference in strength between the two sides.

FIGURE 3

Correlations shown between quadriceps strength and 2MWT (r = 0.603, P ≤ 0.001) and 6MWT (r = 0.578, P ≤ 0.001).

FIGURE 4

Kilograms of force predict distance walked in 6MWT. Means and standard deviations of force generation by MVICT in kilograms of bilateral quadriceps muscles grouped by the number of complete laps (one lap = 50 meters) walked in 6 minutes. Red bars indicate a walking speed sufficient to safely cross a crosswalk. Green bar indicates typical community walking speed in the elderly (>1.3 meters/second). Variability reflects compensatory patterns used to complete functional tasks at every level of muscle strength.