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Biomedical subjects

A Prochazka

Publications and source records attributed to A Prochazka.

At least 19 recordsLinked to original sources
PubMed1992

Attenuation of pathological tremors by functional electrical stimulation. I: Method.

In this study we explored the possibility of suppressing pathological tremors using closed-loop functional electrical stimulation (FES) to activate the tremorogenic muscles out-of-phase. A displacement signal monitored with a transducer was filtered so as to be "tuned" to the tremor frequency at the wrist or elbow. The filtered signal was used to amplitude-modulate the electrical stimulation. The design process was based on measurements of the open-loop frequency response characteristics of the forearm and hand to stimulation of the elbow and wrist flexors and extensors in a number of subjects. These data allowed us to identify closed-loop configurations, which attenuated 2-5 Hz tremors substantially, while only minimally attenuating functional movements in the 0-1 Hz range. There was a fairly delicate balance between efficacy and the risk of instability. However, designs were identified that offered enough tremor suppression and adequate immunity to muscle/load variations for the technique to be considered seriously for clinical application.

Electric Stimulation↗
PubMed1992

Attenuation of pathological tremors by functional electrical stimulation. II: Clinical evaluation.

In this study we evaluated a technique for tremor suppression with functional electrical stimulation (FES), the technical details of which were described in the previous paper. Three groups of patients were investigated: those with essential tremor, parkinsonian tremor, and cerebellar tremor associated with multiple sclerosis. In each group, tremor was attenuated by significant amounts (essential tremor: 73%; parkinsonian tremor: 62%; cerebellar tremor: 38%). These attenuations were in good accord with predictions based on the dynamic analyses and filter designs derived in the previous paper. With filters "tuned" to the lower mean tremor frequency encountered in the cerebellar patients, more attenuation was possible in this group as well. We identified some practical limitations in the clinical application of the technique in its present form. The most important was that in daily use, only one antagonist pair of muscles can realistically be controlled. At first sight, this restricts the usefulness of the system to patients with single-joint tremors. However, the concomitant use of mechanical orthoses may broaden the scope of application.

Adult↗
PubMed1992

Clinical experience with reinforced, anchored intramuscular electrodes for functional neuromuscular stimulation.

Implanted intramuscular electrodes must remain functional for many years if functional neuromuscular stimulation (FNS) is to become a standard treatment in paralysed individuals. In initial trials we found that 5 of 11 coiled single-wire FNS electrodes implanted in 3 patients failed within 8 months. Consequently, we turned to a reinforced electrode comprising 2 multi-stranded, insulated wires tandem-wound on a prolene core and terminated by a prolene anchor or tine (after Mortimer et al., 1986, 1987). The electrodes were implanted with a translumbar aortogram needle, the teflon sheath of which enabled us to stimulate through the tip to guide placement. We have monitored the electrical and functional properties of 8 reinforced electrodes implanted in 2 incomplete quadriplegic patients over 22 months. Four of the electrodes were used for at least 1 h daily to exercise muscles or to provide FNS in gait. Electrical impedances, thresholds and elicited limb motion remained constant in all 8 electrodes over the test period. Disadvantages of the reinforced electrodes are (1) difficulty of eventual removal, and (2) risk of pathogenic infiltration is increased by the 3-filament structure (fortunately dense tissue encapsulation seems to mitigate infection). We conclude that tandem-wound, prolene-reinforced FNS electrodes are much more robust than previous single-coil designs and may form the basis for FNS devices of the future.

Adult↗
PubMed1991

Properties of implanted electrodes for functional electrical stimulation.

Implanted wire electrodes are increasingly being used for the functional electrical stimulation of muscles in partially paralysed patients, yet many of their basic characteristics are poorly understood. In this study we investigated the selectivity, recruitment characteristics and range of control of several types of electrode in triceps surae and plantaris muscles of anaesthetized cats. We found that nerve cuffs are more efficient and selective (i.e., cause less stimulus spread to surrounding muscles) than intramuscular electrodes. Bipolar intramuscular stimulation was more efficient and selective than monopolar stimulation, but only if the nerve entry point was between the electrodes. Monopolar electrodes are efficient and selective if located close to the nerve entry point, but their performance declines with distance from it. Nonetheless, for a variety of reasons monopolar stimulation provides the best compromise in many current applications. Short duration pulses offer the best efficiency (least charge per pulse to elicit force) but high peak currents, increasing the risk of electrode corrosion and tissue damage. Electrode size has little effect on recruitment and should therefore be maximised because this minimises current density.

Action Potentials↗
PubMed1990

Human H-reflexes are smaller in difficult beam walking than in normal treadmill walking.

Hoffman (H) reflexes were elicited from the soleus (SOL) muscle while subjects walked on a treadmill and on a narrow beam (3.5 cm wide, raised 34 cm from the floor). The speed of walking on the treadmill was selected for each subject to match the background activation level of their SOL muscle during beam walking. The normal reciprocal activation pattern of the tibialis anterior and SOL muscles in treadmill walking was replaced by a pattern dominated by co-contraction on the beam. In addition, the step cycle duration was more variable and the time spent in the swing phase was reduced on the beam. The H-reflexes were highly modulated in both tasks, the amplitude being high in the stance phase and low in the swing phase. The H-reflex amplitude was on average 40% lower during beam walking than treadmill walking. The relationship between the H-reflex amplitude and the SOL EMG level was quantified by a regression line relating the two variables. The slope of this line was on average 41% lower in beam walking than treadmill walking. The lower H-reflex gain observed in this study and the high level of fusimotor drive observed in cats performing similar tasks suggest that the two mechanisms which control the excitability of this reflex pathway (i.e. fusimotor action and control of transmission at the muscle spindle to moto-neuron synapse) may be controlled independently.

Electromyography↗
PubMed1990

In-series compliance of gastrocnemius muscle in cat step cycle: do spindles signal origin-to-insertion length?

1. It has been claimed that stretch in the non-contractile (extramysial) portion of muscles is substantial, and may produce large discrepancies between the origin-to-insertion muscle length and the internal length variations 'seen' by muscle spindle endings. 2. In eight pentobarbitone-anaesthetized cats, we estimated stretch in the extramysial portion of medial gastrocnemius (MG) muscle with a method similar to the spindle null technique. 3. Length variations of MG previously monitored in a normal step cycle were reproduced with a computer-controlled length servo. The responses of test MG spindle endings were monitored in dorsal root filaments. Distributed stimulation of ventral root filaments, rate-modulated by the step-cycle EMG envelope, served to reproduce step-cycle forces. The filaments were selected so as to have no fusimotor action on the test spindle. 4. Spindle responses in active cycles were compared with those in passive cycles (stretch, but no distributed stimulation). In some cases concomitant tonic fusimotor stimulation was used to maintain spindle responsiveness throughout the cycle, both in active and passive trials. Generally, small discrepancies in spindle firing were seen. The passive trials were now repeated, with iterative adjustments of the length function, until the response matched the spindle firing profile in the active trial. The spindle 'saw' the same internal length change in the final passive trial as in the active trial. Any difference between the corresponding length profiles was attributed to extramysial displacement. 5. Extramysial displacement estimated in this was was maximal at short mean muscle lengths, reaching about 0.5 mm in a typical step cycle (force rising from 0 to 10 N). At longer mean muscle lengths where muscle force rose from say 2 to 12 N in the cycle, extramysial displacement was in the range 0.2-0.4 mm. 6. Except at very short lengths, the displacement was probably mainly tendinous. On this assumption, our results suggested that the stiffness of the MG tendinous compartment was force related, and about double that of cat soleus muscle at any given force. Calculations indicated that though the stretch was small, the MG tendon would store and release enough strain energy per cycle to contribute significantly to the E3 phase of the step cycle. The discrepancies in spindle firing were generally quite subtle, so we reject the claim that extramysial stretch poses a serious difficulty for inferences about fusimotion from chronic spindle afferent recordings.

Action Potentials↗
PubMed1989

Ensemble proprioceptive activity in the cat step cycle: towards a representative look-up chart.

Analysis of the control of movement in tasks such as stepping is severely restricted by the lack of quantitative data on the ensemble activity of afferents in the numerous muscles involved. We have started to build up a quantitative "look-up-chart" of the ensemble afferent and efferent profiles in the cat step cycle. To this end, we have developed software which allows us to digitize afferent firing, muscle length and electromyogram (EMG) activity, and to align segments for averaging by choosing one or more reference points in the step cycle. The ensemble firing of triceps surae Ia afferents showed lower than expected mean and peak rates, whereas triceps group II and Ib afferents were more active than predicted. There were small but significant transients in Ia firing at foot-off and touch-down which could not be explained in terms of origin-to-insertion length lone. They were most likely caused by propagated mechanical transients or tendon compliance effects giving rise to small differences between the origin-to-insertion length and the intramuscular length "seen" by spindles. Net ensemble Ia rates, based on previous estimates of spindle populations, probably exceed 25 kilo-impulses/second (ki.p.s.) in some muscles. Inputs as large as this are likely to contribute significantly to reflex control.

Animals↗
PubMed1989

Flexible fusimotor control of muscle spindle feedback during a variety of natural movements.

A refined version of an experimental iterative simulation method is described, which was used to infer, from chronic spindle afferent recordings, type and time course of static and dynamic fusimotor activation during a variety of voluntary movements. When used to estimate overall fusimotor drive (without distinction between static and dynamic action) the method provides unique solutions. However, when generating independent gamma s and gamma d activation profiles, the solutions no longer are strictly unique. Yet the boundary conditions imposed by the type specific characteristics of gamma-action nevertheless permit detection of powerful activation, especially of dynamic efferents. Extending the finding of selective dynamic fusimotor activation during unpredictably imposed and resisted stretches, evidence for powerful, often transient activation of dynamic efferents has now been obtained for three additional motor paradigms. First, initiation of walking was accompanied by mixed fusimotor action. Static drive was stepped up and then maintained, whereas dynamic drive declined after an initial abrupt peak. Second, corrective balancing on a narrow walk beam was characterized by largely maintained static background drive, whilst dynamic activation profiles often exhibited powerful surges or transients, when the animal crouched to regain balance. These preceded subsequent EMG bursts during the stretch phase of crouching by about 300 ms. Third, preparation for landing from rapid lowering featured prominent and possibly selective activation of dynamic fusimotor neurones, which peaked while the animal was in mid-air and declined upon landing, and which preceded the sharp onset of EMG after landing by several hundred milliseconds. In all cases the fusimotor activation profiles were unrelated to the parent muscle EMG and difficult to reconcile with the notion of alpha-gamma linkage or coactivation. These findings then clearly support the concept of flexible central control, particularly of dynamic gamma-motoneurones during certain motor tasks.

Animals↗
PubMed1989

Muscle afferent contribution to control of paw shakes in normal cats.

1. The discharge of various hindlimb muscle afferents was recorded during paw shakes in normal cats with the use of floating dorsal root electrodes. 2. Muscle spindle group Ia-afferents and tendon organ group Ib-afferents fired during muscle lengthening, reaching very high peak discharge rates and then silencing at or shortly after the onset of shortening. The timing of Ia firing was consistent with the predictions of a linear model as well as the responses of Ia endings subjected to identical length variations in separate anesthetized cats. 3. In the latter "reconstruction" experiments, waxing and waning dynamic fusimotor action straddling whole paw-shake sequences gave the most consistent matches with the data from the normal cats. The reproducibility of the inferred fusimotor action justifies the inclusion of paw shakes as a class of movement in which fusimotor set is high. 4. The peak ensemble Ia activity from single hindlimb muscles was estimated to be approximately 20 kiloimpulses/s (Kips). Ankle extensor and hamstrings length variations were nearly in phase in the first cycles of a paw-shake sequence. From published data on spindle populations in these muscles, this indicated that peak Ia input to the spinal cord exceeded 0.2 megaimpulses/s (Mips). 5. The phase relationship between origin-to-insertion muscle length and Ia firing during paw shakes was little affected by doubling or tripling the moment of inertia of the foot. We argue that this refutes the notion that in paw shakes phase reversals occur between muscle fibers and tendons in the muscles studied. 6. Inertial loading of the foot led to small but significant reductions in mean paw-shake frequency. This is consistent with an afferent contribution to the generation of these movements. 7. We conclude that in paw shakes in normal cats, the CNS "chooses" to sensitize Ia-afferents to muscle length variations by increasing dynamic fusimotor action. The resulting ensemble Ia input is very large and is likely to play a significant role in reflexly shaping the alpha-motoneuronal activity responsible for the paw shakes.

Action Potentials↗
PubMed1988

Instability in human forearm movements studied with feed-back-controlled muscle vibration.

1. Frequency-modulated vibration was applied to the elbow flexor and extensor tendons to produce reflex movements of the forearm in normal subjects. The modulating (command) signal caused equal and opposite deviations from the 40 Hz carrier frequency so that when flexor vibratory frequency increased, extensor frequency decreased, and vice versa. 2. It is argued that the movements resulted largely from the reflex action of muscle spindle primary afferents whose firing frequency had been 'taken over' and modulated by the vibration. 3. Bode plots relating forearm movements to command signal (modulating) frequency showed the transfer function of the Ia afferent-CNS-muscle-load system to have a low-pass filter characteristic. The phase lag of movement on command increased progressively with command signal frequency, exceeding 180 deg at 3-4 Hz. 4. The transduced forearm movements were fed back to provide the command signal to the vibrators (and thus indirectly to the spindle afferents) via a filter mimicking the dynamic responsiveness of muscle spindle primary endings. Our aim was to 'break into' the reflex arc, and re-route it so that we could artificially vary the gain without significantly altering the dynamics of the pathway. 5. Nearly all subjects developed forearm oscillations (tremor) when the gain exceeded a threshold value. Subjects varied widely in their threshold, though for a given subject the threshold remained fairly constant from day to day. The results suggest that reflexly active individuals may not have a large safety margin with respect to forearm instability. 6. The frequency range of the oscillations observed in seven subjects was 3-8 Hz. The frequencies depended upon the level of flexor-extensor co-contraction, and increased from 3 to 5 Hz at 10% co-contraction to 5-8 Hz at 100% co-contraction. An analysis of the mechanical impedance of the arm provided estimates of tremor frequencies consistent with these results. 7. These unexpectedly low tremor frequencies led us to propose that it may be erroneous to expect stretch reflexes to contribute to forearm tremor in the range 8-12 Hz (e.g. physiological and 'enhanced' physiological tremors). Rather, their contributions should be sought in the range 3-8 Hz (e.g. pathological tremors such as those of Parkinson's and cerebellar disease).

Action Potentials↗
PubMed1988

Instability in human forearm movements studied with feed-back-controlled electrical stimulation of muscles.

1. Amplitude-modulated electrical stimulation was applied to the elbow flexors and extensors to produce movements of the forearm in normal subjects. The parameters of the modulating (command) signal were set in isometric trials so as to produce equal and opposite background torques, and equal and supportive torque modulations. 2. Bode plots relating forearm movement to command signal (modulating) frequency showed the muscle-load to have a low-pass characteristic similar to that previously described in the cat, and a slightly larger bandwidth than described previously in man. 3. The transduced forearm signals were fed back to provide the command signal to the stimulators via a filter which mimicked the transfer function of muscle spindle primary endings. In effect this replaced the neural part of the reflex arc with an accessible model, but left the muscle-load effector intact. 4. All six subjects developed forearm oscillations (tremor) when the loop gain exceeded a threshold value. The mean tremor frequency at onset was 4.4 Hz, which was similar to that of the equivalent vibration-evoked tremor (previous paper, Prochazka & Trend, 1988). 5. With the linear spindle model, oscillations tended to grow rapidly in amplitude, and the stimuli became painful. The inclusion of a logarithmic limiting element resulted in stable oscillations, without significant alterations in frequency. This allowed us to study the effect on tremor of including analog delays in the loop, mimicking those associated with peripheral nerve transmission and central reflexes. In one subject, loop delays of 0, 20, 40 and 100 ms resulted in tremor at 4.0, 3.6, 3.0 and 2.1 Hz respectively, as quantified by spectral analysis. 6. By considering separately the phase contributions of the different elements of the reflex arc, including delays, it became clear that muscle-load properties were important in setting the upper limit of tremor frequencies which could conceivably be supported by reflexes. 7. The results support the conclusion of the related vibration study (Prochazka & Trend, 1988), that for moderate levels of background co-contraction, the contribution of stretch reflexes to tremor at the elbow should be sought in the 3-5 Hz range. Exaggerated long-latency reflexes would be expected to reduce these baseline frequencies by 1 or 2 Hz.

Adult↗
PubMed1987

An experimental simulation method for iterative and interactive reconstruction of unknown (fusimotor) inputs contributing to known (spindle afferent) responses.

A simulation technique, combining chronic recordings in freely moving alert cats with acute experiments on a nerve muscle preparation, has been designed to estimate fusimotor activity profiles underlying chronically recorded muscle spindle afferent responses to movements. Fusimotor stimulation patterns are iteratively generated and tested for their ability to simulate a target response during reproduction of the movement. The error between a simulated and the target response is incorporated into the current stimulation pattern, to generate the stimulation profile for the next cycle of iteration. The procedure is semi-automatic and offers a number of interactive features. For instance, the user has the choice to redraw manually critical segments of a given stimulation profile. The procedure converges rapidly, and solutions are unique, since target responses (first produced with known inputs) could be reconstructed by virtually identical, iterated, profiles. The method opens up the possibility of investigating complex transient adjustments of fusimotor drive, e.g. during adaptive motor performance.

Animals↗
PubMed1987

Relative displacements in muscle and tendon during human arm movements.

1. X-ray, cine and video recordings were made of the movement of radio-opaque markers injected into the musculo-tendinous junctions of biceps brachii muscle. 2. In strong isometric contractions, the distal tendon of the long head of biceps lengthened by about 2% of its estimated rest length. 3. During voluntary isotonic elbow flexion-extension movements at frequencies up to 5.5 Hz there was no detectable phase shift between intramuscular and joint displacements. 4. In the fastest alternating movements (5.5-6.7 Hz) small phase advances developed in the muscle. 5. We conclude that human tendons do stretch during muscle contraction, but not enough to cause intramuscular phase reversals in rapid unloaded movements. This in turn means that muscle spindles shorten and lengthen virtually in phase with joint movements under most conditions.

Arm↗
PubMed1986

Proprioception during voluntary movement.

In the last decade, a number of laboratories have accumulated data on the firing of single afferent fibres from muscle and skin during movement in awake cats, monkeys and human subjects. While there is general agreement on the firing behaviour of skin afferents and tendon organ (Ib) afferents during movement, there remains a significant divergence of opinion regarding the way in which the response of muscle spindle afferents (Ia and II) to length changes is modified by fusimotor action (e.g., alpha-gamma linkage versus "fusimotor set"). The controversies surrounding the fusimotor system have tended to overshadow the emergence of several important characteristics of proprioceptive behaviour, corroborated in separate laboratories. (i) Mean Ia firing rates during active movements are nearly always higher than at rest. Thus, activation of the fusimotor system is reserved for the control of, or preparation for, movement. In animals, there is now strong evidence that there is usually a tonic component of fusimotor action during rhythmical movements. (ii) During fast, unloaded movements (peak muscle speeds, 0.2 resting lengths/s or more), the firing of both Ia and II afferents usually increases during lengthening and decreases during shortening. Ib afferents fire during even the most rapid active shortening of their parent muscles. (iii) During powerful shortening contractions performed against significant loads, Ia firing is often appreciable, suggesting that there is at least some underlying alpha-gamma coactivation. (iv) During fast imposed muscle stretches, Ia afferents respond with segmented bursts of firing (threshold speed for segmentation, 0.5-1.0 resting length/s). Ib afferents show far less segmentation of discharge under similar circumstances.(ABSTRACT TRUNCATED AT 250 WORDS)

Afferent Pathways↗
PubMed1985

'Fusimotor set': new evidence for alpha-independent control of gamma-motoneurones during movement in the awake cat.

The discharge activity of single muscle spindle receptors was recorded in freely moving cats. Large changes in responsiveness to length variations was observed in different types of movement. In separate stimulations on anaesthetised cats, the activity of the gamma-fusimotor neurones responsible for these changes was reconstructed. The results suggested that fusimotor action on a given spindle afferent during the movements studied was not rigidly alpha-linked, but 'set' by the CNS to steady levels, and that it could switch from largely static (gamma s) to largely dynamic (gamma d) according to the motor tasks performed.

Animals↗
PubMed1985

Action of single dynamic fusimotor neurones on cat soleus Ia afferents during muscle shortening.

The ability of single dynamic fusimotor (gamma d) fibres to sustain the firing of muscle spindle primary (Ia) afferents during shortening was investigated in soleus muscles of anaesthetised cats. Of 11 gamma d fibres, 10 could maintain Ia firing during 10 mm/s shortening. Of the 7 tested at greater velocities, 5 could maintain Ia firing during shortening at velocities greater than 50 mm/s. This ability was, however, critically dependent upon the timing of the stimulation. In particular, it rapidly reduced with increasing duration of stimulation before the onset of shortening. Furthermore, if appreciable stretch occurred between the onset of gamma d stimulation and the onset of shortening, this could greatly reduce the ability of gamma d fibres to sustain Ia discharge. If gamma d neurones are on occasion phasically activated during voluntary shortening movements, their action could be an important determinant of Ia firing, even in the presence of weak gamma s action. Therefore in chronic recordings, observation of Ia firing during muscle shortening is not an adequate criterion for inferring gamma d activity.

Action Potentials↗
PubMed1985

Treatment of primary proliferative polycythaemia by venesection and low dose busulphan: retrospective study from one centre.

Sixty-five patients with primary proliferative polycythaemia (polycythaemia rubra vera) were followed during the period 1962-83 and analysed retrospectively. Primary control of PCV was by venesection only with low dose busulphan solely as required to keep the platelet count below 400 X 10(9)/l. Median survival was 11.1 years from diagnosis which is equal to or marginally better than with other reported regimens. Vascular causes of death were only a little higher than expected in a comparable normal population. Only deaths from acute leukaemia and myelofibrosis were significantly increased above the normal population incidence. There was no evidence to suggest that these transformations were busulphan induced. Analysis of the incidence of occlusive vascular lesions lends support to an earlier recommendation that the PCV level be maintained below 0.45. No support was found for the possible disadvantages of a predominantly venesection regimen, such as iron deficiency and reactive thrombocytosis. The case is put for this use of low dose busulphan. The data presented would warrant the future inclusion of this therapeutic regime as one limb of a controlled trial.

Adult↗