Scaling of lactate threshold by peak oxygen uptake and by fat-free mass super(0.67)

The lactate threshold (LT) represents the onset of metabolic acidosis during cardiopulmonary exercise testing (CPET). It is measured as a [unconverted image]O sub(2) in the units of ml min super(-1). In order to make comparisons among subjects, LT is often scaled or normalized by [unconverted image]O sub(2) peak resulting in the LT/[unconverted image]O sub(2) peak ratio. Ratio variables have underlying assumptions. One assumption is that the relationship between the numerator and denominator is linear with a zero y-intercept. If the relationship has a positive y-intercept, then the ratio will decrease with increasing values of the scaling variable thereby penalizing subjects with larger values of the scaling variable. Our purpose was to examine the validity of scaling LT by [unconverted image]O sub(2) peak and by fat-free mass raised to 0.67 power (FFM super(0.67)) as dimensional analysis predicts that LT is proportional to FFM super(0.67). Cycle ergometer CPET was administered to 204 healthy, sedentary subjects (103 males) to the limit of tolerance. Lactate threshold was estimated noninvasively using the V-slope technique. Fat-free mass was assessed by skinfolds. The relationship of LT versus [unconverted image]O sub(2) peak was linear with a positive y-intercept for both sexes. Consequently, the LT/[unconverted image]O sub(2) peak ratio decreased as [unconverted image]O sub(2) peak increased for both sexes. The relationship of LT versus FFM super(0.67)was linear with a zero y-intercept for both sexes. Consequently, the plot of the LT/FFM super(0.67) ratio versus FFM resulted in a straight line with a slope of zero for both sexes. The results of this study support the conclusion that FFM super(0.67), but not [unconverted image]O sub(2) peak, is a valid scaling variable for LT.