We assessed the effects of aerobic and/or resistance training on thermoregulatory responses in older men and analyzed the results in relation to the changes in peak oxygen consumption rate (VO(2 peak)) and blood volume (BV). Twenty-three older men [age, 64 +/- 1 (SE) yr; VO(2 peak), 32.7 +/- 1.1 ml. kg(-1). min(-1)] were divided into three training regimens for 18 wk: control (C; n = 7), aerobic training (AT; n = 8), and resistance training (RT; n = 8). Subjects in C were allowed to perform walking of ~10,000 steps/day, 6-7 days/wk. Subjects in AT exercised on a cycle ergometer at 50-80% VO(2 peak) for 60 min/day, 3 days/wk, in addition to the walking. Subjects in RT performed a resistance exercise, including knee extension and flexion at 60-80% of one repetition maximum, two to three sets of eight repetitions per day, 3 days/wk, in addition to the walking. After 18 wk of training, VO(2 peak) increased by 5.2 +/- 3.4% in C (P > 0.07), 20.0 +/- 2.5% in AT (P < 0.0001), and 9.7 +/- 5.1% in RT (P < 0.003), but BV remained unchanged in all trials. In addition, the esophageal temperature (T(es)) thresholds for forearm skin vasodilation and sweating, determined during 30-min exercise of 60% VO(2 peak) at 30 degrees C, decreased in AT (P < 0.02) and RT (P < 0.02) but not in C (P > 0.2). In contrast, the slopes of forearm skin vascular conductance/T(es) and sweat rate/T(es) remained unchanged in all trials, but both increased in subjects with increased BV irrespective of trials with significant correlations between the changes in the slopes and BV (P < 0.005 and P < 0.0005, respectively). Thus aerobic and/or resistance training in older men increased VO(2 peak) and lowered T(es) thresholds for forearm skin vasodilation and sweating but did not increase BV. Furthermore, the sensitivity of the increase in skin vasodilation and sweating at a given increase in T(es) was more associated with BV than with VO(2 peak).

Plasma volume (PV) expansion by endurance training and/or heat acclimatization is known to increase aerobic and thermoregulatory capacities in humans. Also, higher erythrocyte volume (EV) fractions in blood are known to improve these capacities. We tested the hypothesis that training in a hypobaric hypoxic and warm environment would increase peak aerobic power (VO(2)(peak)) and forearm skin vascular conductance (FVC) response to increased esophageal temperature (T(es)) more than training in either environment alone, by increasing both PV and EV. Twenty men were divided into four training regimens (n = 5 each): low-altitude cool (610-m altitude, 20 degrees C ambient temperature, 50% relative humidity), high-altitude cool (2,000 m, 20 degrees C), low-altitude warm (610 m, 30 degrees C), and high-altitude warm (HW; 2,000 m, 30 degrees C). They exercised on a cycle ergometer at 60% VO(2)(peak) for 1 h/day for 10 days in a climate chamber. After training, PV increased in all trials, but EV increased in only high-altitude trials (both P < 0.05). VO(2)(peak) increased in all trials (P < 0.05) but without any significant differences among trials. FVC response to increased T(es) was measured during exercise at 60% of the pretraining VO(2)(peak) at 610 m and 30 degrees C. After the training, T(es) threshold for increasing FVC decreased in warm trials (P < 0.05) but not in cool trials and was significantly lower in HW than in cool trials (P < 0.05). The slope of FVC increase/T(es) increase increased in all trials (P < 0.05) except for high-altitude cool (P > 0.4) and was significantly higher in HW than in cool trials (P < 0.05). Thus, against our hypothesis, the VO(2)(peak) for HW did not increase more than in other trials. Moreover, slope of FVC increase/T(es) increase in HW increased most, despite the similar increase in blood volume, suggesting that factors other than blood volume were involved in the highest FVC response in HW.

Effects of gravitational loading or unloading on the growth-associated increase in the cross-sectional area and length of fibers, as well as the total fiber number, in soleus muscle were studied in rats. Furthermore, the roles of satellite cells and myonuclei in growth of these properties were also investigated. The hindlimb unloading by tail suspension was performed in newborn rats from postnatal day 4 to month 3 with or without 3-mo reloading. The morphological properties were measured in whole muscle and/or single fibers sampled from tendon to tendon. Growth-associated increases of soleus weight and fiber cross-sectional area in the unloaded group were ∼68% and 69% less than the age-matched controls. However, the increases of number and length of fibers were not influenced by unloading. Growth-related increases of the number of quiescent satellite cells and myonuclei were inhibited by unloading. And the growth-related decrease of mitotically active satellite cells, seen even in controls (20%, P > 0.05), was also stimulated (80%). The increase of myonuclei during 3-mo unloading was only 40 times vs. 92 times in controls. Inhibited increase of myonuclear number was not related to apoptosis. The size of myonuclear domain in the unloaded group was less and that of single nuclei, which was decreased by growth, was larger than controls. However, all of these parameters, inhibited by unloading, were increased toward the control levels generally by reloading. It is suggested that the satellite cell-related stimulation in response to gravitational loading plays an essential role in the cross-sectional growth of soleus muscle fibers.

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These results suggest that increased Fmax improved walking efficiency by increasing energy utilization in the anterior-posterior/mediolateral directions while decreasing energy loss in the vertical direction.