Bursts of activity in the surface electromyogram (EMG) during a sustained contraction have been interpreted as corresponding to the transient recruitment of motor units, but this association has never been confirmed. The current study compared the timing of trains of action potentials discharged by single motor units during a sustained contraction with the bursts of activity detected in the surface EMG signal. The 20 motor units from 6 subjects [recruitment threshold, 35.3 +/- 11.3% maximal voluntary contraction (MVC) force] that were detected with fine wire electrodes discharged 2-9 trains of action potentials (7.2 +/- 5.6 s in duration) when recruited during a contraction that was sustained at a force below its recruitment threshold (target force, 25.4 +/- 10.6% MVC force). High-pass filtering the bipolar surface EMG signal improved its correlation with the single motor unit signal. An algorithm applied to the surface EMG was able to detect 75% of the trains of motor unit action potentials. The results indicate that bursts of activity in the surface EMG during a constant-force contraction correspond to the transient recruitment of higher-threshold motor units in healthy individuals, and these results could assist in the diagnosis and design of treatment in individuals who demonstrate deficits in motor unit activation.

Observations on the relationship of administered dosage per unit of body surface area and blood concentration achieved when different amounts of sulfadiazine and acetylsalicylic acid were given patients varying widely in size indicate that a direct proportionality exists for these two drugs regardless of patient size. On the basis of this finding body surface area was used as the common denominator in the calculation of dosages of a large variety of drugs for a period of more than one year on a service dealing with patients ranging from small prematures to young adults. The clinical results suggest that this method of dosage calculation offers the advantages of simplification and greater accuracy over calculations based on a unit of body mass or on the patient's age.