Influence of 120-Day Stimulated Microgravity with Countermeasures on Human Muscle Musculo-Tendinous Stiffness and Contractile Properties

Yuri A Koryak

doi:10.23937/2469-5718/1510006

Abstract

The effect of long-term (120-day) 5 ° head-down tilt (HDT) bed rest with countermeasures (physical training - PT) on the contractile properties and musculo-tendinous stiffness (MTS) of the human triceps surae muscle (TS) was studied in four healthy young women subjects. The training sessions were conducted for 60 min each day for 6-days a week for 14 weeks, and 30-40 min twice a day for 2 weeks under the experiment conditions. The maximal voluntary contraction (MVC), and isometric twitch contraction (P t ), and electrically evoked tetanic tension at 150 impulses. s -1 (Ро), time-to-peak tension (TPT), and half relaxation time (1/2 RT), and total contraction time (TCT) of the twitch were determined. The difference between Ро and MVC expressed as a percentage of Ро and referred to a force deficiency (FD). The MTS was determined according to the electromechanical delay (EMD) value during the explosive voluntary contraction. Surface electrodes sensed electromyographic activity (EMG) in the soleus muscle. A separate timer was used to determine total reaction time (TRT). Pre-motor time (PMT) was taken to be the time interval from the delivery of the signal to change in EMG. Electromechanical delay (EMD) was the time interval between the change in EMG and movement muscle force production. After HDT with PT the TPT, 1/2 RT and TCT of the TS decreased by 4%, 17%, and 19%, respectively. The МVС, Р t , and Р о decreased by 3% (р>0.05), and 14%, and 9%, respectively. The FD had decreased significantly by 10 % (р<0.02). The rate of increase of electrically evoked tetanic did not change significantly during HDT with PT, but the rate of increase in isometric voluntary tension development was increased. The EMD, PMT, and TRT decreased by 12.2% (p<0.05), 5.3% and 7.3% (p<0.01), respectively. The results obtained demonstrate that PT counteracted the negative effect of mechanical unloading, but the total physical load (mainly its intensity) was obviously insufficient to prevent changes to the mechanical properties of the muscular system, although both a nervous and muscular response to PT was observed.

Highlights

It is well known that the unloading of the musculoskeletal system by actual or simulated microgravity causes numerous changes in the musculoskeletal system, such as muscular atrophy and decreased contraction strength, both after relatively short-term (10-17) [1,2,3,4,5,6] and long-term (>5 weeks) periods of unloading [5,712]
This study considers the program of physical exercises carried out by the study subjects during 120-days of strict head-down tilt (HDT), while the International Space Station team members carry out a similar program over 6 months
We studied the advantages of this program in order to protect various contractile elements of the skeletal muscle

Summary

Introduction

It is well known that the unloading of the musculoskeletal system by actual or simulated microgravity causes numerous changes in the musculoskeletal system, such as muscular atrophy and decreased contraction strength, both after relatively short-term (10-17) [1,2,3,4,5,6] and long-term (>5 weeks) periods of unloading [5,712]. The deterioration of musculoskeletal function causes no direct health hazards and does not affect the capacity for work during short-term space missions. During long-term missions, serious problems can arise after returning back to Earth, unless the negative effects of microgravity are mitigated. Physical exercise (physical training - PT) has been proposed as a potential countermeasure (method) to microgravity-induced effects on the size (mass) and function of skeletal muscles [13,14]. High-resistance physical exercises are known to be effective in increasing the size (or cross-sectional area) or contractile force of muscles [15,16]. High-intensity resistance training may be successfully applied during unloading in order to counteract the deterioration of the contractile functions of the muscle

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