A detailed electromyographic investigation of the actions of the biceps brachii (both heads), the brachialis, and the brachioradialis was carried out in twenty normal young adults with a Stanley Cox multi-channel electromyograph equipped with audio and cathode-ray units and a thirty-five-millimeter linagraph camera. Records on film were made simultaneously from four bipolar concentric-needle electrodes in each subject during a large variety of movements and positions of the upper limb with and without a load of two pounds.
For each muscle and for each Inovelnent or Posture a general trend could be determined. However, what proved most striking was the wide range in the degrees of activity in the series and in the sequence of appearance and disappearance of activity during various movements. An appreciable number of muscles differed from the average or general trend for any movement.
The muscles were completely inactive when the forearm was in the relaxed or resting position.
The conclusions presented indicate the general trend of the behavior of these muscles in the majority of the subjects.
The short and long heads of the biceps have similar, although not identical, actions except that the long head of the biceps is generally more active in most of the movements.
The biceps is a flexor, both isometric and isotonic, of the supine forearm under all conditions and is a flexor of the semiprone forearm when a load is lifted. However, it plays little or no part in the flexion of the prone forearm even when a load of two pounds is lifted.
The biceps (both heads) is not a supinator of the extended forearm unless supination is firmly resisted ; ordinary supination is performed chiefly by the supinator.
The biceps takes part in flexion of the shoulder joint, showing a slight activity in both heads, but it is not active in abduction unless the forearm is supinated.
The brachialis is active during flexion of the forearm under all conditions and is generally active during most movements and postures. It is a flexor par excellence and is the workhorse of the elbow joint.
The brachioradialis comes into action either during quick flexion of the forearm or when weight is lifted during slow flexion, especially of the semiprone and prone forearm. It is a reserve which is called upon when quick or powerful movements are required.
The brachioradialis acts neither in supination of the extended prone forearm nor in pronation of the extended supine forearm unless these movements are strongly resisted.
All three muscles act simultaneously, with the greatest activity when weight is lifted during flexion of the semiprone forearm.
The antagonistic activity of the muscles during slow extension shows variable patterns, with the brachialis generally always active. During quick extension, the short, sharp burst of activity (found in all the muscles, but most pronounced in the biceps) probably is the result of a protective stretch reflex.