Reliability And Validity Of A Hand-held Myotonometer For Assessing Skeletal Muscle Stiffness

Abstract

Active stiffness of the muscles surrounding a joint has been shown to be an essential component for maintaining joint stability. To date, methods for measuring active muscle stiffness have utilized complicated computer algorithms and sophisticated laboratory hardware, making accurate assessment of muscle stiffness for the clinician nearly impossible. PURPOSE: To examine the reliability and validity of a hand-held device for assessing skeletal muscle stiffness. METHODS: Active muscle stiffness of the rectus femoris muscle was collected on 20 subjects (21.88 ± 2.86 yrs, 169.96 ± 9.29 cm, 71.17 ± 11.82 kg) over loads of 10, 20, 30, 40, and 50% of their maximum voluntary isometric contraction. Subjects were seated in an isokinetic dynamometer with their knee flexed to 60 degrees. Measurements were collected from the mid-bellies of the rectus femoris with a myotonometer that exerted a local impact on the tissue by means of a brief mechanical impulse. Five trials were measured in each condition. Intraclass correlations (ICC(2,1)) were performed across the five trials in each load condition to assess trial reliability. RESULTS: Mean stiffness values for each of the load conditions (10% = 314.52 ± 69.11, 20% = 344.74 ± 85.96, 30% = 362.26 ± 99.28, 40% = 371.76 ± 99.11, 50% = 386.79 ± 120.40 N/rad) were consistent with previous literature. Post-hoc analysis revealed that stiffness values at 10% load were significantly different than those at 30, 40, and 50% loads, and that 20% was significantly different than stiffness levels at 50% MVIC. Trial reliability was excellent for each load condition with ICCs ranging from 0.86-0.96 with SEMs ranging from 19.38-46.78 N/rad. CONCLUSION: The results suggest that a hand-held myotonometer is a reliable tool for measuring muscle stiffness over a range of contraction levels. Stiffness levels increased with the proportional increase in force output, which agrees with previous literature showing an increase in active muscle stiffness values with an increase in contraction level. While we did not find a direct linear relationship between level of contraction and measured stiffness values, the data trend suggests the myotonometer measurements are also valid.