An individual's maximum rate of oxygen uptake (V[O.sub.2max]) has long been considered to be one of the key determinants of endurance performance since the individual's true V[O.sub.2max] sets the "upper limit on an individual's ability to take in and consume [O.sub.2]" (Bassett, 2002). In recent decades there have been countless attempts to design valid protocols for assessing V[O.sub.2max] in various populations, including athletes and sedentary and/or unhealthy individuals (Jamison et al., 2010; Marinov et al., 2003; Porszasz et al., 2003).
Numerous treadmill protocols have been developed to assess the aerobic power (V[O.sub.2max]) of runners. Many of these protocols differ with respect to speed, level and duration of the steps involved and inclination, leading more or less rapidly to physical exhaustion. This large number of different protocols has stimulated extensive discussion about which procedure is most optimal (Kang et al., 2001; Kuipers et al., 2003; Pollock et al., 1976) and how to calculate and define V[O.sub.2max] (McConnell, 1988; Poole et al., 2008; Roitman and Herridge, 2001).
Recent reports have emphasised that not only aerobic power, but also the central nervous system plays a major role in volitional exercise testing: the athlete's biological condition at the beginning of exercise including the emotional state (e.g. motivational self-belief) and the extent of mental and physical fatigue altogether contribute to the recruitment of an appropriate number of motor units thereby affecting performance (Noakes, 2008; Noakes, 2012). Thus, rather than externally imposed incremental protocols, an individually designed protocol might offer an alternative approach to determine an even higher V[O.sub.2max] value than during standardized test protocols, since an individual protocol allows the athlete to pace himself according to his present biological state. Well-trained runners are experienced in adjusting their speed in response to sensory feedback concerning fuel reserves, thermoregulation and hydration, as well as on the basis of personal characteristics (Noakes, 2008; Ross et al., 2010; Seiler and Sjursen, 2004).
Investigations of a self-paced test (five 2-min stages of cycling at power based on fixed increments by the rating of perceived exertion) by two different groups showed inconsistent results (Chidnok et al., 2013; Mauger and Sculthorpe, 2012). While Mauger and Sculthorpe (2012) observed greater V[O.sub.2max] values for self-paced when compared with standardized incremental tests, Chidnok and colleagues (2013) reported no difference in V[O.sub.2max].
In addition to the self-paced cycling protocol, Faulkner et al. (2014) and Hogg et al. (2014) examined a self-paced running protocol (within a predetermined and fixed range of perceived exertion) on a motorised treadmill. Although a self-paced test on a motorised treadmill is more challenging than completing a predetermined protocol, they found no significant differences in V[O.sub.2max] when compared to a standardized graded exercise test and demonstrated that a self-paced protocol, based on predefined stages of the RPE scale (11, 13, 15, 17, 20), can be an alternative to a predefined incremental test (Faulkner et al., 2014; Hogg et al., 2014).
To date, no study has examined the influence of a completely individually chosen protocol, not based on RPE, with freely chosen step duration to elicit exhaustion within a time range of 8-12 min, on a motorized treadmill. Therefore, it is hypothesized that a motorised treadmill protocol based entirely on individual adjustments (speed, inclination, step duration) will result in higher V[O.sub.2max] values than standardized testing procedures. Furthermore, the purpose was to assess the applicability of an individually designed protocol for treadmill experienced runners and the influence of different movement patterns leading to exhaustion (e.g. exhaustion by increasing treadmill inclination, speed, or a combination of speed and inclination).
Fourteen healthy, well trained male runners of national level, from local clubs participated (age: 26 [+ or -] 4 years, height: 1.84 [+ or -] 0.06 m; total body mass 78.5 [+ or -] 6.2 kg; total body fat: 11.4 [+ or -] 3.4%). All runners were instructed to follow their regular competition preparation strategy (i.e. fluid intake, nutrition, timing of going to bed) and to refrain from consuming alcohol or caffeine before each test. Prior to the study, all athletes were informed of the protocol and provided their written, informed consent to participate. All procedures were approved by the ethics committee of the German Sport University Cologne, Germany, and conducted in accordance with the Declaration of Helsinki.
The repeated measures design involved collecting data from five different exercise test protocols on five separate occasions one week apart at the same time of day. As illustrated in Figure 1, all participants first completed a 5 minute warm-up, followed by one of the 5 exercise test protocols (in randomized order), and thereafter 3 min of recovery.
Total body weight, lean mass and fat mass were determined using a four-electrode bio-impedance body scale (Tanita BC 418 MA, Tanita Corp., Tokyo, Japan). Thereafter, each participant performed the different exercise protocols to determine their V[O.sub.2max]: i) an incremental protocol ([INC.sub.S+I]) involving set increases in speed and inclination, 2 min at 8.64 km x [h.sup.-1], thereafter an increase in speed of 1.44 km x [h.sup.-1] every 30 s up to 14.4 km x [h.sup.-1], followed by a rise in the treadmill inclination from the initial 0[degrees] by 0.5[degrees] every 30 s; ii) incremental test (INQ) at constant speed (14.4 km x [h.sup.-1]) and with increasing inclination (2[degrees] every 2 min from the initial 0[degrees]); iii) incremental test ([INC.sub.S]) at a constant inclination of 0[degrees] and increasing speed (0.5 km x [h.sup.-1] every 30 s from the initial 12 km x [h.sup.-1]); iv) graded exercise protocol (GXP) at a constant inclination of 1[degrees] and increasing speed (1.44 km x [h.sup.-1] every 5 min from the initial 8.64 km x [h.sup.-1]) with 30-seconds of passive rest preceding each increment; and v) individual exercise protocol (INDXP) during which each participant adjusted the treadmill inclination and speed of running at will with the aim of reaching exhaustion within 8-12 min. The participants were asked to independently adjust their running speed and inclination without reducing the speed...