Skeletal muscle properties and fatigue resistance in relation to smoking history

Abstract

Although smoking-related diseases, such as chronic obstructive pulmonary disease (COPD), are often accompanied by increased peripheral muscle fatigability, the extent to which this is a feature of the disease or a direct effect of smoking per se is not known. Skeletal muscle function was investigated in terms of maximal voluntary isometric torque, activation, contractile properties and fatigability, using electrically evoked contractions of the quadriceps muscle of 40 smokers [19 men and 21 women; mean (SD) cigarette pack years: 9.9 (10.7)] and age- and physical activity level matched non-smokers (22 men and 23 women). Maximal strength and isometric contractile speed did not differ significantly between smokers and non-smokers. Muscle fatigue (measured as torque decline during a series of repetitive contractions) was greater in smokers (P = 0.014), but did not correlate with cigarette pack years (r = 0.094, P = 0.615), cigarettes smoked per day (r = 10.092, P = 0.628), respiratory function (%FEV(1pred)) (r = -0.187, P = 0.416), or physical activity level (r = -0.029, P = 0.877). While muscle mass and contractile properties are similar in smokers and non-smokers, smokers do suffer from greater peripheral muscle fatigue. The observation that the cigarette smoking history did not correlate with fatigability suggests that the effect is either acute and/or reaches a ceiling, rather than being cumulative. An acute and reversible effect of smoking could be caused by carbon monoxide and/or other substances in smoke hampering oxygen delivery and mitochondrial function.

Fig. 1

Maximal torque capacity, maximal voluntary torque corrected for voluntary activation (MTC) of the quadriceps muscle expressed as a function of age in male (a) and female (b) smokers and non-smokers. The decrease in maximal strength over time (and smoke exposure) was similar between smokers and non-smokers (NS)

Fig. 2

Representative example of a fatigue test (a), with in b the first (continuous line) and last (dotted line) contraction enlarged

Fig. 3

Torque, expressed as percentage of initial value, during the fatigue protocol is plotted every 2 s during the fatigue protocol for male (a) and female (b) smokers and non-smokers. Women had a significantly higher fatigue resistance (P < 0.001). Both male and female smokers fatigued more than the non-smokers (P = 0.014)

Fig. 4

Maximal rates of relaxation (MRR), expressed as percentage of initial value, during the fatigue protocol are plotted every 2 s during the stimulation for male (a) and female (b) smokers and non-smokers. Significantly higher values were obtained in the females compared to the males (P < 0.001). MRR decreased to a significantly lower level in the smokers compared to the non-smokers (P = 0.008)

Fig. 5

Relationship between cigarette pack years and fatigue resistance of the quadriceps muscle for male (filled circle) and female (open square) smokers of the fatigue test. Fatigue index was calculated as torque at the end of the test as a percentage of the initial value. The correlation coefficient was 0.095 (P = 0.618) for fatigue index and similar results were obtained from other measures of smoking history (see text)