Short term bed rest reduces conduction velocity of individual motor units in leg muscles

Cescon, Corrado and Gazzoni, Marco and Merletti, Roberto (2008) Short term bed rest reduces conduction velocity of individual motor units in leg muscles. In: Proc. of the XVII Congress of the International Society of Electrophysiology and Kinesiology, 18-21 June 2008, Niagara Falls, Ontario, Canada.

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INTRODUCTION Space flight duration will increase with the use of the International Space Station and possible future missions to Mars. Therefore, there is a need to anticipate the medical problems, which could develop during the flight and to prepare astronauts for their return to Earth. Space permanence simulations such as prolonged bed rest (Mulder et al. 2007) can provide study conditions that are more accessible with respect to space flights. A short term bed rest experiment was organized by the German Aerospace Center (DLR), Cologne using the Head Down Tilted Bed Rest at minus 6° model, to simulate the effects of weightlessness for studying the adaptation to this condition. The aim of this study was to assess the consequences of a simulated long duration flight. METHODS The study was part of a comprehensive international collaboration: Short Term Bed Rest – Salty Life Study 7 (STBR-SLS7), where specific examination periods were assigned to each study group. Eight healthy, male, sedentary subjects (mean ± SD: age 26.1 ± 3.9 years; body mass index 24.6 ± 0.8 kg/m2) were recruited. Volunteers participated to the study laying in bed for two continuous periods of 14 days separated by 5 months. The participating subjects were divided in 2 groups receiving different diets (low and high sodium content). Surface EMG signals were detected before and after the bed rest periods from Vastus Medialis (VM), Vastus Lateralis (VL) and Tibialis Anterior (TA) muscles. The study protocol was divided in two phases. In the first phase four subjects received a specifically prepared diet with low sodium content (50 mmol/ day) and four subjects received a high sodium content diet (550 mmol/day) during the bed rest. In the second phase the subject groups were switched. EMG recordings were performed 2 days before (pre) and 4 hours after (post) each bed rest. Two experimental set-ups (for knee extensor and ankle flexor muscles) were prepared to test the subjects. For each muscle EMG signals were detected during a 1 minute voluntary contraction at 20% MVC with a linear electrode arrays (8 electrodes, 5 mm IED) positioned between the innervation zone and the distal tendon. The joint angle was at 120°. Subcutaneous tissue layer thickness was measured with an ultrasound scanner. Single motor unit (MU) action potentials were identified from the multi-channel surface EMG signals with the method described by Gazzoni et al. (2004), which tracks single MU action potentials over time with template update in case of progressive shape changes. This technique does not detect superimposed action potentials, thus only an incomplete firing pattern was obtained (Gazzoni et al. 2004). This is not a limitation for the current application. Muscle fiber conduction velocity (CV) was estimated with a multichannel algorithm (Farina and Merletti, 2003) on double differential signals. RESULTS AND DISCUSSION Since the same contraction type was repeated on each subject with the electrode arrays in the same positions (for each of the three muscles) in 4 different experimental sessions (Phase 1 pre; Phase 1 post; Phase 2 pre; Phase 2 post), the EMG signals were analysed in order to identify the same motor unit in the 4 sessions. The overall number of MUs identified in all subjects in the 4 sessions were 12, 12 and 10 for the vastus medialis, vastus lateralis and tibialis anterior muscle respectively. Matched pairs test (Wilcoxon signed-rank test) showed statistically significant lower motor unit CV after the bed rest period for vastus medialis and tibialis anterior muscles (p<0.05). The same results were observed in global CV while the maximal force didn’t change after the bed rest period. No significant change was observed in the subcutaneous tissue layer thickness of each of the three muscles after the bed rest period. No significant variation of ARV and MNF after the bed rest was observed for either diet type. SUMMARY/CONCLUSIONS The main results of the present study can be summarized as follows: 1) it was possible to track the same motor unit in different conditions after replacement of the electrodes. 2) no differences were observed in Torque after the bed rest for either diet type; 3) no reduction of subcutaneous tissue layer thickness was observed after the bed rest for either diet type; 4) no variation of ARV and MNF after the bed rest for either diet type; 5) a reduction of global estimate of muscle fiber CV was observed after the bed rest period (around 10% for all the three muscles and all the contraction levels); 6) a reduction of single motor unit CV was observed after the bed rest period (around 10% for VM and TA muscles and around 5% for VL muscle). As a conclusion the present study showed that conduction velocity of motor units is sensitive to small changes in muscle properties which cannot be detected with sEMG amplitude or spectral parameters.

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