Carl G. Kukulka

Postural adjustments preceding rapid arm movements in parkinsonian subjects

The electromyographic (EMG) responses in postural (thigh and trunk) and agonist (shoulder) muscles were examined in standing parkinsonian subjects and healthy controls prior to visual reaction time and self-paced rapid arm flexion movements. Recruitment of postural muscles typically preceded arm displacement in normals, but was less frequent, of shorter duration, and characterized by multiple EMG bursts which extended to the agonist in parkinsonians. Moreover, parallel delays in EMG recruitment times relative to the visual signal for both postural and agonist muscles were observed in akinetic patients. These abnormalities suggest that the basal ganglia may serve a preparatory motor function, by linking synergistic muscles through a common selection process.

Changes in human α-motoneuron excitability during sustained maximum isometric contractions

Abstract Experiments were conducted to evaluate the change in α-motoneuron excitability during sustained maximum isometric contractions of human triceps surae. A test H-reflex was used to assess motoneuron excitability 10 ms after a conditioning reflex was generated. The test reflex was compared to a reference H-reflex; both test and reference reflexes were of approximately equal amplitudes at the onset of the sustained maximum efforts. Both reflexes were assumed to be influenced by similar descending and peripheral inputs. In addition, the test reflex was influenced by the conditioning reflex. For the 4 subjects tested, the test reflex decreased in amplitude within the first 30–40 s of effort, while the reference reflex remained roughly constant or increased in amplitude. The decline of the test reflex relative to the reference was indicative of an inhibitory effect due to the conditioning reflex. In that the conditioning reflex was always generated 10 ms prior to the test reflex, the two factors most likely responsible for the inhibition would be recurrent inhibition and summation of motoneuron afterhyperpolarization. A combination of these two factors could also account for the associated slowing of motoneuron firing during sustained maximum efforts.

Electrical and mechanical changes in human soleus muscle during sustained maximum isometric contractions

Experiments were designed to evaluate changes in the electrical activation and force generating capabilities of human soleus muscle during sustained, maximum isometric contractions. Eighteen experiments were conducted on 7 healthy subjects. Surface EMG, and in select cases, intramuscular fine wire EMG recordings, were made to assess the electrical activation of soleus. Subjects performed maximum isometric plantarflexion contractions of 1-3 min during which time supramaximal electrical pulses were delivered to the tibial nerve at 5-s intervals to elicit maximum M waves. M wave areas were assessed for evidence of neuromuscular junction failure. The results revealed that, on average, maximum force declined to 80% of unfatigued maximum by 60 s of effort, 74% by 90 and 120 s, and 70% by 180 s. M waves were stable for efforts up to 3 min, thereby providing little evidence for neuromuscular junction failure. In 3 experiments, total spike counts from intramuscular recordings displayed a 50% reduction in firing by 30 s of effort, with little additional slowing for up to 3 min. Although all of the fatigue-induced electrical and mechanical alterations in muscle activation reported earlier for intrinsic hand and foot muscles were verified in these experiments on soleus, the magnitudes and time courses of these changes were quite different. All changes were consistent with a muscle designed to optimally resist fatigue.

An objective method for assessing graded electrically evoked afferent activity in humans

A problem arises in human sensorimotor studies when attempts are made to equate the intensity of electrical stimulation of a peripheral nerve with the amount of afferent activity generated. Results presented here reveal that sural recruitment curves exhibit large session to session differences both within and among subjects. These differences hinder the prediction of afferent activation based solely upon stimulus intensity. An alternative method based upon measurement of the evoked potentials is described for improving on these predictions. Sural nerve stimulation is used to demonstrate this method, but the method should be applicable wherever a peripheral nerve is accessible for electrical stimulation and evoked recordings.