ORIGINAL ARTICLE Jimmy C. Smith Æ Daniel P. Stephens Æ Emily L. Hall Allen W. Jackson Æ Conrad P. Earnest Effect of oral creatine ingestion on parameters of the work rate-time relationship and time to exhaustion in high-intensity cycling Accepted: 1 September 1997 Abstract The relationship between work rate ( _ W ) and time to exhaustion (t) during intense exercise is com- monly described by either a hyperbolic function (NLin), t  W 0 = _ W  _ W cp ), or by its linear equivalent (LinW) W lim  W 0  _ W cp (t). The parameter _ W cp (critical power) has been described as an inherent characteristic of the aerobic energy system, while W ¢ has been shown to be a ralid estimate of anaerobic work capacity. Recent studies have demonstrated that oral supplementation of creatine monohydrate (CrH 2 O) increases total muscle creatine stores, and have linked these increases to im- proved performances in intense intermittent exercise. This study was conducted to determine the eect of CrH 2 O supplementation on estimates of W ¢ and _ W cp derived from the NLin and LinW equations, and to determine the eect of CrH 2 O on t in exhaustive con- stant power exercise of dierent intensities. Fifteen ac- tive but untrained university students completed three phases of testing on a cycle ergometer: (1) familiariza- tion, three learning trials, (2) baseline determination of W ¢ and _ W cp , four bouts performed at a _ W selected to elicit fatigue in 90–600 s, and (3) experimental determi- nation of W ¢ and _ W cp , four bouts performed at the same _ W as baseline, but performed after 5 days of ingesting either a placebo (4 · 6 g of glucose/day) or CrH 2 O (4 · 5 g of CrH 2 O and 1 g glucose/day). Testing was administered in a double-blind manner. Analyses of covariance revealed a significant eect for CrH 2 O on both estimates of W ¢ (NLin, P  0:04; LinW, P < 0:01), but not on estimates of _ W cp (NLin, P  0:37; LinW; P  0:30). Within groups, t was significantly dierent for only CrH 2 O at the two highest _ W s(P  0:04). It is concluded that oral ingestion of CrH 2 O increases esti- mates of W ¢ due to an improved t at the shorter, more intense exercise bouts. Key words Critical power Æ Creatine Æ Exercise Æ Anaerobic capacity Introduction The relationship between work rate ( _ W ) and time to exhaustion (t) during intense exercise can be described by a hyperbolic function (Monod and Scherrer 1965; Poole et al. 1988; Gaesser et al. 1995; Hill et al. 1995). This relationship is often expressed in the form, t  W 0 = _ W  _ W cp , where t represents time to exhaus- tion, _ W cp the work rate asymptote, and W ¢ the degree of curvature in the relationship (see Fig. 1A). Although it is appropriate to express t as the dependent variable, being determined by the magnitude of _ W (Gaesser et al. 1995), this relationship has often been ‘‘linearized’’ into a mathematically equivalent form, W lim  W 0  _ W cp t, where W lim is the total amount of work performed during a given exercise bout and is the product of _ W and t, (i.e., W lim  _ W  t). In this linear relationship, the slope is _ W cp and W’ is the Y-intercept (see Fig. 1B). While _ W cp has been characterized as the power output that can be sustained ‘‘for a very long time without fa- tigue’’ (Monod and Scherrer 1965), and as representing ‘‘an inherent characteristic of the aerobic energy supply system’’ (Gaesser and Wilson 1988), it has been sug- gested that W ¢ is anaerobic in nature and represents the energetic reserve of the muscle (Monod and Scherrer 1965). Evidence of the anaerobic character of this pa- rameter has been provided by Moritani et al. (1981) who Eur J Appl Physiol (1998) 77: 360–365 Ó Springer-Verlag 1998 J.C. Smith (&) Æ E.L. Hall Southwestern University, Department of Kinesiology, Georgetown, TX 78627, USA D.P. Stephens University of Texas Health Science Center-San Antonio, Department of Physiology, San Antonio, TX 78284, USA A.W. Jackson The University of North Texas, Department of KHPR, Denton, TX 76203, USA C.P. Earnest Department of Kinesiology, Texas Women’s University, Denton, TX 76202, USA