No abstract

Afassachti~etts~The ctirreiit literattire on iiiale-female diffiieiices in respoiise to tlteriiial stress has beeii revien.ed. hlorphologically, ti-oiiieii average 20% siiialler body iiiass, 14% iiiore body fat, 33% less leaii body iiiass, biit oiil-v 18% less siirface area thaii iiieii. Woiiieii have greater body irisiilatioii wlieri vasocoiistricted (except liaiids aiid feet) atid a larger peripheral lieat siiik, biit at the cost of ( I ) greater body fat btirdeii. (2) less iiiriscle iiiass arid streiigth, aiid (3) siiialler circtilatiiig blood voliiiiies i\*hich reqiiires greater pliysiological straiii to balairce lieat prodiictioii arid loss. Uiider lieat stress, it'oiiieii geiierally sIi011' ( I ) relqtively wore periplieral blood pooliiig. (2) greater lieart rate iiicreases, (3) iiiore freqiieiit circtilatoy eiiibarassiiieiit, (4) lower iiiaxiiiial sweat rates, (5) higher skiii teiiiperattires rcvWi greater body heat storage, arid (6) poorer iiiniiiteiiaiice of circiilatiiig blood voliiiiie ntith iiiore iiiipact frorii dehydratioii. Proportioiiately feitver it'oriieii tliaii iiieii caii be sticcessfiilly heat accliiiiated. I i i the cold, woiiieii geiierally have ( I ) less capability for'iiiaxiiiiiiiii heat prodiictioit by either erercise or sliiveriiig, (2) a iiiore extensively vasocoiistricted peripliety, (3) lower foot, haiid, aiid iiieaii skiii teiiiperatiires, (4) greater siirface lieat losses, especially froiii the geometrically thiiiiter extreiiiities. (5) iiicreased rates of extreiiiity. biit not core, cooliiig. aiid (6) relatitpely greater risk of cold iiijtiry.

The force output of the ankle dorsiflexors was studied during a 40-day simulated ascent of Mt. Everest in a hypobaric chamber; both electrically activated and maximal voluntary contractions (MVCs) were employed. The purpose of this study was to establish whether, under conditions of progressive chronic hypoxia, there was a decrease in muscle force output and/or increased fatigability. We also attempted to identify the main site of any failure, i.e., central nervous system, neuromuscular junction, or muscle fiber. Muscle twitch torque (Pt), tetanic torque (Po), MVC torque, and evoked muscle compound action potential (M wave) were monitored during 205-s exercise periods in five subjects at three simulated altitudes (760, 335, and 282 Torr). All three types of torque measurement were well preserved at the three altitudes. In some subjects, the responses to stimuli interpolated during repeated MVCs provided evidence of "central" fatigue at altitude. In addition, the rate of fatigue during 20-Hz electrical stimulation was greater (P less than 0.01) at altitude and there was increased fatigability of the twitch (P less than 0.025); however, the M wave amplitude was maintained. We conclude that central motor drive becomes more precarious at altitude and is associated with increased muscle fatigue at low excitation frequencies; the latter is the result, in part, of chronic hypoxia and occurs in the muscle fiber interior because no impairment in neuromuscular transmission could be demonstrated.