Muscle Stiffness Quantification With Novel Ultrasound Elastography Imaging During Fatiguing Contraction

Abstract

It is unknown how muscle stiffness changes during muscle fatigue with sustained contraction. Recent development of novel ultrasound elastography has opened the possibility for objectively quantifying muscle stiffness by imaging the internal propagation of muscle vibration with ultrafast ultrasound technology (Shinohara et al. Muscle & Nerve, 2010). PURPOSE: To examine the changes in muscle stiffness during sustained fatiguing contractions with the novel ultrasound elastography. METHODS: To induce fatigue in the biceps brachii muscle, six healthy young adults sustained isometric elbow flexion force corresponding to 30% of maximal voluntary contraction until they can no longer maintain it. The elbow joint angle was fixed at 90 degrees and the wrist was in the neutral position. Ultrasound elastography images of the biceps brachii muscle were obtained during the task. The alignment of ultrasound probe was alternated between longitudinal and transverse to the biceps brachii muscle. From these images, muscle stiffness in the biceps brachii was quantified in the units of kPa. Measurements in the longitudinal direction were used to assess changes in muscle stiffness due to fatigue of the muscle contractile elements whereas those in the transverse direction were used to assess changes in muscle stiffness due to factors that were not directly related to muscle contractile elements, such as a potential increase in muscle temperature. RESULTS: Subjects sustained the contraction for 11.0 ± 9.4 min. Muscle stiffness measured in the longitudinal alignment was 110.2 ± 11.4 kPa at the onset of contraction. It started to decrease at the 20% of the total duration and the steady decline continued until the 40% of the total duration. The average reduction in muscle stiffness was more rapid between 60% and 80% of the total duration. At the 80% of the total duration, muscle stiffness declined by > 30% to 75.0 ± 26.5 kPa (P < 0.05) compared with the onset. There were minimal changes in muscle stiffness with the transverse direction of the ultrasound probe. CONCLUSIONS: Muscle stiffness during muscle fatigue with sustained submaximal contraction showed a progressive decrease, most likely due to alterations in mechanical properties of the contractile elements, as quantified with the novel ultrasound elastography imaging.