PLASMA LACTATES AFTER 5KM TIME TRIALS REMAIN ELEVATED AND CORRELATE WITH % SLOW TWITCH FIBRES

Abstract

A 5 km cycling time trial (TT) demands high, sustained muscle power output and substantial oxidative and glycolytic energy delivery. The aims of this study were to determine the effects of fibre type and muscle oxidative capacity on TT performance and post-TT plasma lactate accumulation. 12 endurance trained cyclists with average speeds for a recent 105 km race of between 31.5 and 37.7 km/h were tested for laboratory exercise performance (peak sustained power output (PPO) and 2 × TT: TT1 and TT2), plasma lactate accumulation (before, during and after TT2) and skeletal muscle composition (% myosin heavy chain isoform content (MHC) and citrate synthase (CS) activity). The coefficient of variation for TT1 and TT2 was 0.6%. We therefore used the mean of TT1 and TT2 to determine TTm. PPO was 345.6 ± 32.7 W and average speed for TTm was 41.2 ± 1.4 km/h (average test duration: 7 min 16 s ± 15 s). %MHCI was 58.1 ± 10.9 and %MHCIIa was 41.5 ± 11 (one subject had 12% MHCIIx). CS activity was 83.9 ± 9.4 umol/g d.w./min. Plasma lactate concentration (LACT) was 13.9 ± 1.4 mmol/L at 2 min post-TT2. By 20 min post-TT2, LACT was still 9.6 ± 2.3 mmol/L. TTm correleated with PPO (r = 0.59, P < 0.05) but not 105 km performance or CS activity. Neither TTm, nor CS activity correlated with LACT at any time point, or LACT area under the curve (AUC) or plasma lactate disappearance rate from 2 to 20 min post-TT2. But %MHCI correlated with LACT at the end of TT2 (r = 0.76, P < 0.01) and with AUC from the start of TT2 to 20 min. of recovery (r = 0.57, P < 0.05). Since LACT remained high for 20 minutes following TT2 when lactate production itself had ceased, we concluded that lactate release from the muscle was prolonged due to saturation of the transport proteins, MCT1 and MCT4. Since muscle oxidative capacity was unrelated to plasma lactate disappearance rate during recovery, we conclude that prolonged high levels of muscle lactate release overshadowed any influence of plasma lactate disappearance and subsequent oxidation on plasma lactate concentration itself. In this study, higher muscle lactate release in vivo, as indicated by higher post-exercise plasma lactate concentrations, was related to a higher percentage of slow twitch fibers. Sarcolemmal vesicle lactate transport capacity in vitro has previoulsy been shown to correlate with % slow twitch fibers (Pilegaard et al. J. Appl. Physiol. 77:1858–1862, 1994). Sponsor: Medical Research Council of South Africa.