Hiking physiology and the "quasi-isometric" concept.

The literature indicates that the heart rate of a planing-dinghy sailor, in winds of 4 - 5 m . s(-1), is in the range seen in aerobic athletes, yet oxygen consumption (VO(2)) is roughly half that of the same individual cycling at that heart rate. Thus, although upper-body dynamic activity is a contributing factor, the dominant physiological demand must be the "quasi-isometric" stress on the lower-body anterior muscles - especially the quadriceps, which appears to impose 40 - 50% of the total oxygen demand in a typical hiking posture. Therefore, a non-trivial part of the sailor's fitness training should involve sustained quadriceps stress. Estimates of this stress on water vary widely in the literature, but about 25 - 30% maximal voluntary contraction (MVC) tallies with endurance times recorded both in the literature and in an outline of new work reported here. Muscle blood flow is restricted under such a load, but not occluded. Laser Doppler measurements of femoral blood flow on a leg-extension ergometer found similar values during 10 - 30% MVC, much less at 40%, and marked hyperaemia on relaxation from 20% MVC or more - implying metabolic debt. Adding low-amplitude alternating leg movements while holding the same overall load stationary, and therefore increasing only internal not external work, further elevates blood flow and VO(2) both during and after exercise. Femoral-vein lactate concentration is also higher after these movements. Speculations that unusually dynamic lower-body movements by elite sailors might assist hiking endurance are not supported by these findings. Nevertheless, afloat or ashore, capillary lactate concentrations hardly ever exceed 5 mmol . l(-1), even during the post-exercise surge - challenging assumptions that the quadriceps had been profoundly anaerobic while under load. On the contrary, it appears that aerobic metabolism contributes substantially, if not completely, to energy supply. A preliminary comparison of elite sailors with aerobic athletes suggests that isometric endurance at a given percentage MVC does not differ between the two groups, but the sailors have higher MVCs. In individuals not highly strength-trained, greater electromyogram activity immediately before capitulation than in an MVC performed while fresh indicates that physiological (not just volitional) limits have been reached. It is concluded that the literature and the outline of my recent work with colleagues support the view that the predominant physiological load during single-handed dinghy sailing is quasi-isometric in form and accounts for roughly half of the metabolic demand. Any more complete account of the physiology of hiking will require simultaneous on-water measurement of electromyographic, cardiovascular, and metabolic indicators in sailors extending from club to Gold Medal standard.