Effects of body orientation on maximum voluntary arm torques

Quantified clinical measures linked to ambulation speed in hemiparesis

Background: In spastic paresis, the respective contributions to active function of antagonist hypoextensibility, spasticity, and impaired descending command remain unknown. Objectives: We explored correlations between ambulation speed and coefficients of shortening, spasticity and, weakness for three lower limb extensors.Methods: This retrospective study identified 140 subjects with chronic hemiparesis (>6 months since injury) assessed during a single visit with barefoot 10-meter ambulation at comfortable and fast speed, and measurements of passive range of motion (X_V1), angle of catch at fast stretch (X_V3) and active range of motion (X_A) against the resistance of gastrocnemius, rectus femoris, and gluteus maximus. Coefficients of shortening (C_SH=[X_N-X_V1]/X_N; X_N, normal expected amplitude based on anatomical values), spasticity (C_SP=[X_V1-X_V3]/X_V1), and weakness (C_WK=[X_V1-X_A]/X_V1) were derived. For each muscle, multivariable analysis explored C_SH, C_SP, and C_WK as potential predictors of ambulation speed.Results: Ambulation speed was 0.62±0.28m/s (mean±SD, comfortable) and 0.84±0.38m/s (fast) and was correlated with C_SH and C_WK against gastrocnemius (C_SH, comfortable, ns; fast, β=-0.20, p=.03; C_WK, comfortable, β=-0.21, p=.010; fast, β=-0.21, p =.012), rectus femoris (C_SH, comfortable, β=-0.41, p=6E^-7; fast, β=-0.43, p=5E^-7; C_WK, comfortable, β=-0.36, p=5E^-5; fast, β=-0.33, p=.0003) and gluteus maximus (C_SH, comfortable, β=-0.19, p=.02; fast, β=-0.26, p=.002; C_WK, comfortable, β=-0.26, p=.002; fast, β=-0.22, p=.010). Ambulation speed was not correlated with C_SP.Conclusions: In chronic hemiparesis, ambulation speed correlates with coefficients of shortening and of weakness in lower limb extensors, but not with their spasticity level. This may encourage therapists to focus treatment primarily on muscle shortening by stretching programs and on impaired descending command by active training.

The neurophysiology of deforming spastic paresis: A revised taxonomy

This paper revisits the taxonomy of the neurophysiological consequences of a persistent impairment of motor command execution in the classic environment of sensorimotor restriction and muscle hypo-mobilization in short position. Around each joint, the syndrome involves 2 disorders, muscular and neurologic. The muscular disorder is promoted by muscle hypo-mobilization in short position in the context of paresis, in the hours and days after paresis onset: this genetically mediated, evolving myopathy, is called spastic myopathy. The clinician may suspect it by feeling extensibility loss in a resting muscle, although long after the actual onset of the disease. The neurologic disorder, promoted by sensorimotor restriction in the context of paresis and by the muscle disorder itself, comprises 4 main components, mostly affecting antagonists to desired movements: the first is spastic dystonia, an unwanted, involuntary muscle activation at rest, in the absence of stretch or voluntary effort; spastic dystonia superimposes on spastic myopathy to cause visible, gradually increasing body deformities; the second is spastic cocontraction, an unwanted, involuntary antagonist muscle activation during voluntary effort directed to the agonist, aggravated by antagonist stretch; it is primarily due to misdirection of the supraspinal descending drive and contributes to reducing movement amplitude; and the third is spasticity, one form of hyperreflexia, defined by an enhancement of the velocity-dependent responses to phasic stretch, detected and measured at rest (another form of hyperreflexia is "nociceptive spasms", following flexor reflex afferent stimulation, particularly after spinal cord lesions). The 3 main forms of overactivity, spastic dystonia, spastic cocontraction and spasticity, share the same motor neuron hyperexcitability as a contributing factor, all being predominant in the muscles that are more affected by spastic myopathy. The fourth component of the neurologic disorder affects the agonist: it is stretch-sensitive paresis, which is a decreased access of the central command to the agonist, aggravated by antagonist stretch. Improved understanding of the pathophysiology of deforming spastic paresis should help clinicians select meaningful assessments and refined treatments, including the utmost need to preserve muscle tissue integrity as soon as paresis sets in.

On Denny-Brown's 'spastic dystonia' - What is it and what causes it?

In this review, we will work around two simple definitions of two different entities, which most often co-exist in patients with lesions to central motor pathways: Spasticity is "Enhanced excitability of velocity-dependent responses to phasic stretch at rest", which will not be the subject of this review, while Spastic dystonia is tonic, chronic, involuntary muscle contraction in the absence of any stretch or any voluntary command (Gracies, 2005). Spastic dystonia is a much less well understood entity that will be the subject this review. Denny-Brown (1966) observed involuntary sustained muscle activity in monkeys with lesions restricted to the motor cortices . He further observed that such involuntary muscle activity persisted following abolition of sensory input to the spinal cord and concluded that a central mechanism rather than exaggerated stretch reflex activity had to be involved. He coined the term spastic dystonia to describe this involuntary tonic activity in the context of otherwise exaggerated stretch reflexes. Sustained involuntary muscle activity in the absence of any stretch or any voluntary command contributes to burdensome and disabling body deformities in patients with spastic paresis. Yet, little has been done since Denny-Brown's studies to determine the pathophysiology of this non- stretch or effort related sustained involuntary muscle activity following motor lesions and there is a clear need for research studies in order to improve current therapy. The purpose of the present review is to discuss some of the possible mechanisms that may be involved in the hope that this may guide future research. We discuss the existence of persistent inward currents in spinal motoneurones and present the evidence that the channels involved may be upregulated following central motor lesions. We also discuss a possible contribution from alterations in synaptic inputs from surviving or abnormally branched sensory and descending fibres leading to over-activity and lack of motor coordination. We finally discuss evidence of alterations in motor cortical representational maps and basal ganglia lesions.

Neck rotation modulates flexion synergy torques, indicating an ipsilateral reticulospinal source for impairment in stroke

The effect of reticular formation excitability on maximum voluntary torque (MVT) generation and associated muscle activation at the shoulder and elbow was investigated through natural elicitation (active head rotation) of the asymmetric tonic neck reflex (ATNR) in 26 individuals with stroke and 9 age-range-matched controls. Isometric MVT generation at the shoulder and elbow was quantified with the head rotated (face pointing) contralateral and ipsilateral to the paretic (stroke) and dominant (control) arm. Given the dominance of abnormal torque coupling of elbow flexion with shoulder abduction (flexion synergy) in stroke and well-developed animal models demonstrating a linkage between reticular formation and ipsilateral elbow flexors and shoulder abductors, we hypothesized that constituent torques of flexion synergy, specifically elbow flexion and shoulder abduction, would increase with contralateral head rotation. The findings of this investigation support this hypothesis. Increases in MVT for three of four flexion synergy constituents (elbow flexion, shoulder abduction, and shoulder external rotation) were observed during contralateral head rotation only in individuals with stroke. Electromyographic data of the associated muscle coactivations were nonsignificant but are presented for consideration in light of a likely underpowered statistical design for this specific variable. This study not only provides evidence for the reemergence of ATNR following stroke but also indicates a common neuroanatomical link, namely, an increased reliance on ipsilateral reticulospinal pathways, as the likely mechanism underlying the expression of both ATNR and flexion synergy that results in the loss of independent joint control.