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The interplay of central and peripheral factors in limiting maximal O 2 consumption in man after prolonged bed rest
Author(s) -
Ferretti Guido,
Antonutto Guglielmo,
Denis Christian,
Hoppeler Hans,
Minetti Alberto E.,
Narici Marco V.,
Desplanches Dominique
Publication year - 1997
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.1997.677bm.x
Subject(s) - bed rest , chemistry , skeletal muscle , muscle biopsy , head down tilt , vo2 max , medicine , cardiology , anatomy , endocrinology , biopsy , blood pressure , heart rate
1 The effects of bed rest on the cardiovascular and muscular parameters which affect maximal O 2 consumption ( V O2,max ) were studied. The fractional limitation of V O2,max imposed by these parameters after bed rest was analysed. 2 The V O2,max , by standard procedure, and the maximal cardiac output (Q̇ max ), by the pulse contour method, were measured during graded cyclo‐ergometric exercise on seven subjects before and after a 42‐day head‐down tilt bed rest. Blood haemoglobin concentration ([Hb]) and arterialized blood gas analysis were determined at the highest work load. 3 Muscle fibre types, oxidative enzyme activities, and capillary and mitochondrial densities were measured on biopsy samples from the vastus lateralis muscle before and at the end of bed rest. The measure of muscle cross‐sectional area (CSA) by NMR imaging at the level of biopsy site allowed computation of muscle oxidative capacity and capillary length. 4 The V O2max was reduced after bed rest (−16.6%). The concomitant decreases in Q̇ max (−30.8%), essentially due to a change in stroke volume, and in [Hb] led to a huge decrease in O 2 delivery (−39.7%). 5 Fibre type distribution was unaffected by bed rest. The decrease in fibre area corresponded to the significant reduction in muscle CSA (−17%). The volume density of mitochondria was reduced after bed rest (−16.6%), as were the oxidative enzyme activities (−11%). The total mitochondrial volume was reduced by 28.5%. Capillary density was unchanged. Total capillary length was 22.2% lower after bed rest, due to muscle atrophy. 6 The interaction between these muscular and cardiovascular changes led to a smaller reduction in V O2max than in cardiovascular O 2 transport. Yet the latter appears to play the greatest role in limiting V O2max after bed rest (>70% of overall limitation), the remaining fraction being shared between peripheral O 2 diffusion and utilization.