Quantifying the mechanical work of resting quadriceps muscle tone

Abstract

Our laboratory has shown that resting muscle, commonly thought to be mechanically inert, is actually mechanically active. We report a study of the mechanics of resting quadriceps muscle in adult surgical patients that determines how much metabolic activity can be attributed to quadriceps resting-muscle mechanical work. This was calculated by studying the motion of relaxed supine subjects’ instrumented legs dropped onto a pillow before and after anesthesia with muscle paralysis. By subtracting the acceleration of the dropping leg of the conscious subject before the quadriceps is paralysed from that found after paralysis, resting muscle tensile force and power of the quadriceps muscles can be calculated. Mechanomyography was also recorded, using an accelerometer. Paralysis produced an increase in acceleration in all cases (pre-paralysis 6.99 ± 1.51 m s⁻²; post-paralysis 7.65 ± 1.51 m s⁻²; P = 0.00007) and a decrease in mechanomyographic mean absolute amplitude (pre-paralysis 10.6 ± 3.7 mm s⁻²; post-paralysis 4.5 ± 2.6 mm s⁻²; P = 0.00003). Calculated force exerted by resting quadriceps was 22.6 ± 16.8 N; power 0.34 ± 0.17 W, corresponding to a daily caloric expenditure of 7.0 ± 3.6 kcal day⁻¹. This corresponds to approximately 205 kcal day⁻¹ for all skeletal muscle. Knowledge of the phenomenon of resting muscle mechanical activity may be of clinically importance in the study and treatment of obesity and of disorders of muscle tone.