Comparison of Noninvasively Determined Hydrogen Ion and Lactate Thresholds During Cycle Exercise

Abstract

The lactate threshold is a parameter utilized to assess physical fitness, for predicting aerobic performance, and to develop training protocols for endurance athletes. However, this measurement requires invasive blood sampling and produces only intermittent results. Interstitial fluid pH has been measured noninvasively and continuously using near infrared spectroscopy (NIRS) during ischemia, but additional techniques are required to apply this methodology during exercise. PURPOSE: To develop an exercise-based NIRS pH model and to compare H+ threshold from NIRS with lactate threshold from capillary blood sampling in subjects performing a graded exercise cycle protocol. METHODS: Intramuscular pH (pHm) was measured invasively in the flexor digitorum profundus during handgrip exercise in 8 subjects while NIR spectra were collected from a sensor placed on the skin over the muscle. Partial least squares regression was used to obtain a mathematical model relating the two measures. The model was subsequently applied to spectra collected from the vastus lateralis of 10 subjects performing a graded protocol in which the work rate was increased by 50 W every 3 minutes until volitional fatigue. Oxygen consumption (VO2) was measured continuously with a standard metabolic cart, and lactate concentration was measured at the end of each stage with capillary blood from a finger stick. Lactate and H+ thresholds were determined as a function of VO2 using bilinear regression. RESULTS: The root mean square error of prediction for the spectral pHm model was 0.035 pH units. Lactate threshold determined during cycle ergometry was strongly correlated with H+ threshold (R2=0.95) but occurred at higher VO2 (Δ=0.29 ± 0.23 L.min−1). CONCLUSIONS: NIRS can be utilized for noninvasive and continuous measurement of [H+] during incremental cycle exercise to determine a metabolic threshold associated with fitness and athletic performance. Supported by the National Space Biomedical Research Institute through NASA NCC 9-58.