Other studies confirmed these findings in race walkers (Hagberg & Coyle, 1983), in road racers competing at distances from 5 to 21 km (Kumagai et al, 1982; LaFontaine et al, 1981; S.R. Powers et al, 1983; Tanaka et al, 1983; C. Williams & Nute, 1983), and in marathoners (Lehman et al, 1983; Noakes et al, 1990a; Sjodin & Jacobs, 1981; Sjodin & Svedenhag, 1985; Tanaka & Matsuura, 1984). In addition, Sjodin and colleagues (1981) showed that a runners lactate tumpoint was related to the percentage ST fibers and the capillary density in the runners leg muscles and to the runners training volume. Thus, the greater the percentage of ST fibers, the greater the training volume; also, the more muscle capillaries the athlete had, the greater the speed at which the runners lactate tumpoint occurred and therefore the faster the runners marathon pace. Training-induced changes in road racing performance at distances from 5 to 10 km also correlate best with changes in the lactate tumpoint (Tanaka et al, 1984).
Recently, researchers have proposed a practical method to predict the lactate tumpoint on the basis of heart rate changes during exercise. Italian researchers (Conconi et al, 1982) recorded the heart rates of a group of athletes during track running. During the experiment, the runners ran on a 400-m track; they started running at 12 to 14 km/hr and increased their running paces each lap by about 0.5 km/hr until they were exhausted. Exhaustion usually occurred within 8 to 12 laps, with the runners having reached speeds that ranged from 18 to 25 km/hr. Their heart rates were measured electronically at the end of each lap. A few days later the athletes underwent a conventional treadmill test to determine their lactate tumpoints. The results of the experiment showed that at low to moderate running speeds, there was a linear increase in heart rate measured on the track, and this increase corresponded to a gradual rise in blood lactate levels measured during the treadmill test. However, at and above the running speeds that corresponded to the lactate tumpoint in the treadmill test, heart rates measured on the track no longer rose linearly but reached plateaus, giving a heart rate inflection or breakpoint (see Exercises 3.14).
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The authors suggested that determination of the speed at which the heart rate breakpoint occurred would allow prediction of the lactate tumpoint. The authors also showed that this test could be used to measure training improvement, signal established illness, and predict overtraining and imminent illness. The authors found that correct training caused the heart rate breakpoint to occur at a higher running speed, whereas overtraining or illness caused the heart rate breakpoint to occur at a lower running speed (see Exercises 10.3). The method has also been used in cyclists, cross-country skiers, rowers, and walkers (Droghetti et al, 1985) and appears to be particularly reliable in children (Gaisl & Wiesspeiner, 1988).
A more recent study (Ribeiro et al, 1985) suggested, however, that only about 50% of subjects show true heart rate breakpoints; the authors also found that in those in whom a breakpoint does develop, this point occurs at work loads well beyond the true lactate tumpoint, shown diagrammatically in Exercises 3.14. If this is indeed the case, it means that Conconis method overestimates the true lactate tumpoint. It also means that runners who use Conconis method to determine
Postulated relationship between the lactate tumpoint and the heart rate inflection point exercise intensities that boost them to or beyond their lactate tumpoints would exercise at intensities that are too high and would thereby increase the risk of overtraining (see post 10). This issue is currently unresolved. Some studies have failed to confirm the existence of a heart rate breakpoint (Dennis et al, 1991). Similarly, Kuipers et al. (1988) found that the heart rate breakpoint, when present in cyclists, did not coincide with the lactate tumpoint, and in runners they could not identify a heart rate breakpoint; thus they also question the validity of the heart rate breakpoint.