Abstract
The human resting muscle tone (HRMT) system provides structural and functional support to skeletal muscle and associated myofascial structures (tendons, fascia) in normal life. Little information is available on changes to the HRMT in bed rest. A set of dynamic oscillation mechanosignals ([Hz], [N/m], log decrement, [ms]) collected and computed by a hand-held digital palpation device (MyotonPRO) were used to study changes in tone and in key biomechanical and viscoelastic properties in global and postural skeletal muscle tendons and fascia from a non-exercise control (CTR) and an exercise (JUMP) group performing reactive jumps on a customized sledge system during a 60 days head-down tilt bed rest (RSL Study 2015–2016). A set of baseline and differential natural oscillation signal patterns were identified as key determinants in resting muscle and myofascial structures from back, thigh, calf, patellar and Achilles tendon, and plantar fascia. The greatest changes were found in thigh and calf muscle and tendon, with little change in the shoulder muscles. Functional tests (one leg jumps, electromyography) showed only trends in relevant leg muscle groups. Increased anti-Collagen-I immunoreactivity found in CTR soleus biopsy cryosections was absent from JUMP. Results allow for a muscle health status definition after chronic disuse in bed rest without and with countermeasure, and following reconditioning. Findings improve our understanding of structural and functional responses of the HRMT to disuse and exercise, may help to guide treatment in various clinical settings (e.g., muscle tone disorders, neuro-rehabilitation), and promote monitoring of muscle health and training status in personalized sport and space medicine.
Highlights
In physiological conditions, the resting muscle tone constitutes only about 1% of maximally voluntary contraction force (MVC) ability (Woledge et al, 2003) but appears to be sufficient for body stabilization whether being reclined, sitting or relaxed standing in quiet gravity-neutral body positions (Gurfinkel et al, 2006; Loram et al, 2007)
The significant parameter changes notably found in either Measure points (MPs) or between groups (CTR vs. JUMP) at various time point intervals (∼10 days intervals from head-down-tilt bed rest (HDT)+10 to HDT+59) during the bed rest period are highlighted in a separate figure (Figure 12) as additional reference
Recordings were obtained from a bed rest only group (CTR) and a bed rest exercise group (JUMP) performing reactive jumps as a countermeasure, at the start of bed rest, during, and 10 days after bed rest (60 days reactive jump on a sledge (RSL) study 2015–2016)
Summary
The resting muscle tone constitutes only about 1% of MVC ability (Woledge et al, 2003) but appears to be sufficient for body stabilization whether being reclined, sitting or relaxed standing in quiet gravity-neutral body positions (Gurfinkel et al, 2006; Loram et al, 2007). Myofascial elasticity and tone of the so called HRMT system provides postural stability via anti-gravity forces inherent to intrinsic passive structural body elements such as fascia, tendon and skeletal muscle (Masi and Hannon, 2008). These structures are linked to myofascial kinematic chains traversing through the whole human body to support, for example, lumbopelvic stability and static posture during habitual performance (Willard et al, 2012; Wilke et al, 2016) and to allow optimal force transmission adjustment during exercise (Fitts and Widrick, 1996; Findley et al, 2015). The state of tension of a fully relaxed muscle in the HRMT is reflected by the passive tone (inherent tension) of supposedly resting muscle earlier referred to as rhythmically micro-movements or microvibration (Rohracher, 1962), or as viscoelastic tone (specific tone), a vital property of striated muscle at low tension resistent to strain but different from contractile activity such as EMGsilent contractures, normal contraction, or even pathological electrogenic spasms (Simons, 2008)
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