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.

The volunteers for this study were 83 women, aged 19-65 yr, drawn from several different occupations. Three minutes after exerting maximal handgrip strength (MVC) each subject held a tension of 40 percent MVC to fatigue. Blood pressures and heart rates were measured before, during, and after the endurance of contraction. Age was associated with a reduction of strength of the women, whereas their endurance at 40 percent at 40 per cent MVC increased. There was evidence that menopause enhanced those age effects for strength and endurance. At rest, age was associated with a decreased heart rate. As expected, the heart rates of all the women increased during the endurance contraction. But that increase was greater for the younger than for the older women, thereby exaggerating the difference due to age already seen at rest. Systolic blood pressure at rest was higher with age and, in a similar manner, that relationship was also exaggerated throughout the isometric contraction. Diastolic blood pressure, however, was not changed with age at rest, and although the diastolic pressure increased during the isometric exercise, the rate of increase was unaffected by age. The results obtained are compared with those from a similarly large number of men examined in identical circumstances.

No abstract

Use of leg weights for physical conditioning was evaluated in 8 middle-aged male Ss; four Ss of similar age served as a control group. Pre- and post-training evaluation consisted of heart rate and oxygen uptake responses to five submaximal work loads which involved either level walking or cycling. Differentiated ratings of perceived exertion elicited for each work load were: a local muscular rating; a central or cardio-pulmonary rating; and an over-all or general rating. Submaximal heart rate decreased 6 to 9 beats/min. from pretraining values for all work load after training. The differentiated ratings for training generally reflected a reduced strain on the cardiovascular system and also improved functioning of the working muscles with training. However, when one set of sensations dominated the exertional perception the others appear to have been perceptually de-emphasized. Local muscular factors seemed to dominate the exertional perception for cycling, but central factors appeared to play a more important role for treadmill walking, at least within the range of velocities investigated.

A new approach has been devised to assess the "static component" of dynamic exercise. This technique involves the measurement of the isometric endurance of muscles which have just taken part in rhythmic exercise and depends on the repeatability of trained subjects in isometric effort. The premise is that isometric endurance will be inversely related to the static component of the preceeding dynamic exercise. The subjects worked on a bicycle ergometer at known fractions of their maximal aerobic capacity (max Vo2). The rate of pedalling was varied from 30 to 90 rpm, so that for a given % max Vo2, the belt tension varied inversely with the speed of cycling. At any one speed of cycling, isometric endurance decreased as the belt tension increased. Following exercise at 30 rpm, the isometric endurance was 25 to 50% lower than that found at the most advantageous speed of cycling for our subuects; at these faster rates of cycling two subjects showed least static component following exercise at 90 rpm while the remaining subject performed best after cycling at 50 rpm.

No abstract