Unchanged muscle fiber conduction velocity relates to mild acidosis during exhaustive bicycling

Abstract

Muscle fiber conduction velocity (MFCV) has often been shown to decrease during standardized fatiguing isometric contractions. However, several studies have indicated that the MFCV may remain constant during fatiguing dynamic exercise. It was investigated if these observations can be related to the absence of a large decrease in pH and if MFCV can be considered as a good indicator of acidosis, also during dynamic bicycle exercise. High-density surface electromyography (HDsEMG) was combined with read-outs of muscle energetics recorded by in vivo (31)P magnetic resonance spectroscopy (MRS). Measurements were performed during serial exhausting bouts of bicycle exercise at three different workloads. The HDsEMG recordings revealed a small and incoherent variation of MFCV during all high-intensity exercise bouts. (31)P MRS spectra revealed a moderate decrease in pH at the end of exercise (~0.3 units down to 6.8) and a rapid ancillary drop to pH 6.5 during recovery 30 s post-exercise. This additional degree of acidification caused a significant decrease in MFCV during cycling immediately after the rest period. From the data a significant correlation between MFCV and [H(+)] ([H(+)] = 10(-pH)) was calculated (p < 0.001, Pearson's R = -0.87). Our results confirmed the previous observations of MFCV remaining constant during fatiguing dynamic exercise. A constant MFCV is in line with a low degree of acidification, considering the presence of a correlation between pH and MFCV after further increasing acidification.

Fig. 1

Image of the MR-compatible bicycle ergometer. Parts indicated in the image are: 1 wooden flywheel, 2 mechanical brake, 3 pedals and 4 nylon transmission belt

Fig. 2

Schematic overview of the exercise protocol at workload intensities of 35-, 50- and 65-N braking force. Duration of the first exercise bout lasted until exhaustion or until the pedaling rate of 80 rpm could no longer be met. The order of the three workloads was randomly chosen

Fig. 3

Mean muscle fiber conduction velocity (MFCV) for individual subjects (a) and pooled per workload (mean ± SD, n = 6) (b)

Fig. 4

Normalized muscle fiber conduction velocity (MFCV) during exercise at with a braking load of 35 N (a), 50 N (b) and 65 N (c). Data points represent mean ± SD (n = 6). MFCV data were normalized with respect to the mean MFCV

Fig. 5

Normalized muscle fiber conduction velocity (MFCV, a), pH (b) and inorganic phosphate concentration ([Pi], c) during the full intermittent exercise protocol (3 exercise bouts separated by 2 rest periods) at a workload of 65-N braking force. Data points represent mean ± SD (n = 6). The data points recorded during exercise are indicated by a closed circle, the data points recorded during rest are indicated by an open circle

Fig. 6

Relation between normalized MFCV and muscle [H⁺] ([H⁺] = 10^−pH) calculated from three exercise bouts at 65-N braking load (Fig. 5a, b solid black dots). The dashed line indicates pH = 7.0. The solid line indicates the calculated linear regression model (MFCV = 1.068 – 554,227*[H⁺], Pearson’s R = −0.87; p < 0.001). Error bars indicate SD