THE PLANTAR REFLEX - RePub

present series). This has in turn led to the erroneous supposition that the
Babinski sign is mediated by the extensor hallucis brevis (Grimby, 1963 a).
The complex and overlapping results in patients with and without a
Babinski sign (Grimby, 1963 b, 1965 a, b) are easily explained by realizing
that these represent activity in an irrelevant muscle, evoked by inappropriate
stimuli.
In addition to the 'false-positive' controls, the present study also showed
failure of electrical stimuli to activate the extensor hallucis longus in five of
the 15 patients; in one of them this was observed on four separate
occasions in the course of a few months. In this case both spatial and
temporal summation of afferent impulses were necessary for activation of
the extensor hallucis longus, as neither prolonged electrical stimulation at
one site, nor simultaneous application of short pulse trains at several sites
on the lateral plantar border were effective. Only when combined with
subliminal mechanical stimulation a single pulse train could evoke a reflex
in the extensor hallucis longus.
Another anomalous feature of electrical stimuli when compared with
stroking the sole was the frequent co-activation of extensor hallucis longus
and flexor hallucis brevis; this occurred in all controls with electrically
induced reflexes in the extensor hallucis longus muscle, and in some of the
patients. Mechanical stimulation did not cause synchronous activity in the
two antagonist muscles in any of these subjects (18 in all). Co-activation of
extensor hallucis longus and flexor hallucis brevis after mechanical
stimulation was also rare in the entire group of patients and controls
(2/55), but it is found more often in equivocal cases (see next chapter).
For the cases on hand it is obvious that the organization of reciprocal
reflex patterns requires a stimulus of longer duration than a short train of
pulses. This stands in some contrast to the reciprocal activity which
Hagbarth (1960) found in various leg muscles of normal subjects after
brief electrical stimuli at different skin sites: physiological flexor muscles
were excited and their antagonists inhibited from most skin areas, except
from those covering extensor muscles. Intrinsic foot muscles were not
included in Hagbarth's study, but at any rate activation of the extensor
hallucis longus from the plantar surface is a frequent exception to his rule.
Dimitrijevic and Nathan (1968) applied electrical stimuli to legs of
paraplegic patients and recorded responses from all groups of leg muscles.
They ascribed this absence of reciprocal innervation to lack of supraspinal
control, but in the present study electrical activation of the extensor
hallucis longus without concomitant activity in the flexor hallucis brevis
occurred in patients only.
Generally speaking, I think that the results of this study justify caution
in the interpretation of any effect evoked by electrical skin stimuli lasting
only a few tens of milliseconds. Examples are:
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