ADDUCTOR POLLICIS MUSCLE FATIGUE DURING ACUTE AND CHRONIC ALTITUDE EXPOSURE AND RETURN TO SEA-LEVEL

Large muscle exercise performance is impaired during acute exposure to normobaric or hypobaric hypoxia, but the effects of hypoxic condition s on fatigue of isolated smaller muscle groups per se are poorly defin ed. We studied how acute and chronic altitude (ALT) exposure and post- ALT return to sea level (SL) affects voluntary strength and fatigue of the adductor pollicis muscle. Eight healthy men (mean age 28 yr) were studied on five separate occasions: at SL, on days 1 (acute) and 13 ( chronic) at ALT (4,300 m), and on days 1 (post 1) and 3 or 4 (post 2) at SL after 20 days of residence at ALT. On each day, maximal voluntar y contractions (MVCs) of the adductor pollicis were obtained before an d at the end of each minute of submaximal intermittent contractions of the adductor pollicis (50% of MVC of rested muscle, 5 s of contractio n/5 s of rest) until exhaustion, defined as the inability to exert or maintain 50% of rested MVC. MVC of rested muscle did not differ among days. Time to exhaustion was shorter at acute ALT [5.1 +/- 0.5 (SE) mi n] than at SL (7.4 +/- 0.8 min, P < 0.05) and tended to be shorter tha n at chronic ALT (6.6 +/- 0.7 min, P > 0.05). Compared with acute and chronic ALT, time to exhaustion was prolonged during post 1 (9.0 +/- 1 .2 min, P < 0.05) but not post 2 (6.1 +/- 0.5 min, P > 0.05). We concl ude that 1) MVC of rested adductor pollicis muscle is not impaired dur ing or after ALT exposure, 2) compared with SL conditions, acute but n ot chronic ALT exposure leads to a more rapid decline in adductor poll icis MVC associated with submaximal contractions, and 3) time to exhau stion is prolonged for greater than or equal to 1 day after return fro m ALT.