Assessing skeletal muscle function using Tensiomyography - The suitability of the displacement rate as a measure of changes in contractile properties due to potentiation and fatigue

Tensiomyography is described as a valuable method for assessing skeletal muscle function, particularly in terms of muscular fatigue. Tensiomyography determines the contractile properties of a single skeletal muscle based on its radial displacement during an electrically stimulated twitch. The evidence regarding its diagnostic accuracy for assessing muscle fatigue, however, is contradictory, and evidence for assessing potentiation is lacking. Further, methodological problems exist using spatial and temporal tensiomyographic parameters to assess fatigue, prompting an increasing use of the displacement rate as an additional parameter. However, there is no standardized procedure for measuring the radial displacement or determining the displacement rate derived from it. Moreover, evidence on the reliability of the displacement rate and methodological factors, e.g., the inter-stimuli interval during electrical stimulation, is lacking. Therefore, the overall research question of this dissertation is whether tensiomyography, particularly the displacement rate, is a suitable measure to assess changes in muscle function due to potentiation and fatigue. To address this question, a systematic scoping review was conducted, providing a comprehensive overview of the concepts used to determine the displacement rate. Comparing the applied measurement protocols to identify the most commonly used procedures was not feasible due to incomplete reporting in the included studies. Instead, a new reporting guideline was proposed. Subsequently, an experimental study was conducted demonstrating adequate within- and between-day reliability of the five most frequently used displacement rate concepts in the rectus and biceps femoris muscles. This study further demonstrated that inter-stimuli intervals of 10 seconds are sufficient to prevent effects of potentiation or fatigue on tensiomyographic parameters during repeated stimulation. In a second experimental study, the twitch torque and displacement response of the rectus femoris muscle were compared during potentiation and fatigue induced by maximum voluntary isometric knee extensions. While significant changes in the reference marker twitch torque confirmed potentiation and fatigue effects, tensiomyographic parameters did not detect either. The insufficient diagnostic accuracy of tensiomyography regarding potentiation and fatigue was presumably due to a low signal-to-noise ratio at a knee flexion angle of 90° and the associated long muscle length of the rectus femoris. Thus, tensiomyography’s ability to detect potentiation or fatigue-induced changes in muscle function appears to be affected by joint angle and muscle length. Since evidence on the effect of joint angle, i.e., muscle length, on radial displacement is limited, further research into methodological factors is required to provide a valid answer regarding the suitability of tensiomyography in assessing changes in contractile properties due to potentiation and fatigue.