Blunted presser and intramuscular metabolic responses to voluntary isometric exercise in multiple sclerosis

To test the hypothesis that a lower mean arterial pressure (MAP) response d uring voluntary isometric exercise in multiple sclerosis (MS) is related to a dampened muscle metabolic signal, 9 MS and 11 control subjects performed an isometric dorsiflexor contraction at 30% maximal voluntary contraction until target failure (endurance time). We made continuous and noninvasive m easurements of heart rate and MAP (Finapres) and of intramuscular pH and Pi (phosphorus magnetic resonance spectroscopy) in a subset of 6 MS and 10 co ntrol subjects. Endurance times and change in heart rate were similar in MS and control subjects. The decrease in pH and increase in Pi were less thro ughout exercise in MS compared with control subjects, as was the change in MAP response. Differences in muscle strength accounted for some of the diff erence in MAP response between groups. Cardiovascular responses during Vals alva and cold presser tests were similar in MS and control subjects, sugges ting that the blunted MAP response during exercise in MS was not due to a g eneralized dysautonomia. The dampened metabolic response in MS subjects was not explained by inadequate central muscle activation. These data suggest that the blunted presser response to exercise in MS subjects may be largely appropriate to a blunted muscle metabolic response and differences in cont racting muscle mass.