An analysis of double discharges in the human electromyogram

Different characteristics of double discharges (doublets) of motor units in m. biceps brachii and tibialis anterior were examined. Doublets may occur physiologically at the beginning and termination of muscular effort and at the minimum rhythmic firing frequency, but doublets arising within a regular sequence of motor unit potentials can be regarded as abnormal. In neuromuscular diseases about 3% of motor units showed this firing pattern, but it was never found in intact limb muscles. Most of the abnormal doublets have the following characteristics: the interval between the first and the second component is 9 msec or less (longer intervals up to 22 msec occur very rarely); both components have nearly the same form; the amplitude of the second component is slightly smaller than that of the first; a stepwise or continuous jitter between doublet components is common; doublets recur randomly during a sequence of motor unit potentials; the interval between the doublet and the next MUP is usually about 50% longer than the mean MUP interval. A number of doublets differ, however, in one way or another from the above. The second component of the doublet may differ completely in amplitude and even in form from the first component; or they may fuse. It is thus hardly possible to define the lower limit of the interval between doublet components. Sometimes the post-doublet interval increase is absent. When applying all the criteria there is seldom any uncertainty about the identification of doublets.