Charles T. Leonard

Intra- and interrater reliabilities of the myotonometer when assessing the spastic condition of children with cerebral palsy

The purposes of this study were to assess intra- and interrater reliabilities by novice users of the Myotonometer® (Neurogenic Technologies, Inc., Missoula, MT), a portable electronic device that quantifies muscle tone (stiffness) and paresis, in assessing children with cerebral palsy. Two raters used the Myotonometer to assess the biceps brachii and medial gastrocnemius muscles of 10 children with spastic-type cerebral palsy. Muscles were measured in a relaxed state and during a voluntary isometric contraction. Intraclass correlation coefficients and repeatability coefficients were calculated for each muscle and for each condition (relaxed and contracted). Intrarater reliabilities ranged from 0.82 to 0.99 (biceps brachii muscles) and 0.88 to 0.99 (medial gastrocnemius muscles). Interrater reliabilities ranged from 0.74 to 0.99 (biceps brachii muscles) and 0.84 to 0.99 (medial gastrocnemius muscles). Repeatability coefficients indicated a 98% level of agreement between raters across all conditions. Novice users of the Myotonometer, with few exceptions, had high to very high intra- and interrater reliabilities for measurements of the biceps brachii and medial gastrocnemius muscles of children with spastic-type cerebral palsy. (J Child Neurol 2004;19:894—901).

Neural modulation of muscle contractile properties during fatigue: afferent feedback dependence

H reflex amplitudes, an indirect measure of the excitability of the alpha motoneuron pool, were recorded from 10 males during fatigue induced by submaximal, isotonic, voluntary contractions of the soleus muscle. H reflex changes were correlated with electromyographic changes (mean power frequency (MPF); root mean square (rms EMG)), under ischemic and non-ischemic conditions. The purpose of the ischemia was to block transmission of Ia and possibly Ib afferents to assess whether changes in sensory feedback had any effect on alpha motoneuron and EMG activity during fatigue. Significant interactions were found between ischemic and non-ischemic conditions. After an initial decrease (1.21 +/- 0.56 mV to 0.54 +/- 0.39 mV), H reflex amplitudes increased during non-ischemic trials (0.54 +/- 0.39 mV to 1.13 +/- 0.84 mV). Under ischemic conditions H reflex amplitudes decreased (2.11 +/- 1.10 mV to 0.70 +/- 0.74 mV; P < 0.003). During non-ischemic conditions, MPF decreased across 5 consecutive trials (157.7 +/- 17.9 Hz to 124.7 +/- 17.2 Hz), as compared to an increase under ischemic conditions (132.8 +/- 21.2 Hz to 197.1 +/- 53.6 Hz; P < 0.001). Root mean square amplitude decreased during the non-ischemic trials (31.07 +/- 14.62 mV to 25.98 +/- 8.26 mV). A greater decrease was noted during the ischemic trials (34.00 +/- 23.61 mV to 4.95 +/- 3.77 mV; P < 0.001). Data suggest that the CNS modulates muscle contraction in order to preserve force output and neuromuscular transmission during fatigue. This modulation appears dependent on Ia and/or Ib afferent feedback.

MYOTATIC REFLEX RESPONSES OF NON-DISABLED CHILDREN AND CHILDREN WITH SPASTIC CEREBRAL PALSY

Surface electromyography (EMG) was used to examine lower‐extremity myotatic reflex responses following patellar or Achilles tendon taps to normally developing, non‐disabled infants and to individuals with cerebral palsy (CP). Reflex irradiation was present in non‐disabled infants and infants with CP under two years of age. The only significant differences in myotatic reflex responses between the two groups at this age was the higher amplitude of the directly stimulated muscle of children with CP. After two years the amplitude did not differ between groups. Reflex irradiation, however, was greatly reduced in the non‐disabled children but not in the children with cerebral palsy. These findings and those of non‐human animal studies indicate the possible neural mechanisms that underlie reflex irradiation of individuals with CP. The potential clinical relevance of these findings is discussed.

CORRELATION BETWEEN IMPAIRMENT AND MOTOR PERFORMANCE DURING REACHING TASKS IN SUBJECTS WITH SPASTIC HEMIPARESIS

OBJECTIVE The main purposes of this study were to examine, in subjects with chronic hemiparesis following a stroke: (i) the correlations between tests of muscle tone, stiffness, spasticity, paresis and co-contraction, and (ii) the correlations of these tests and measurements of impairment to upper extremity motor performance. DESIGN Prospective, cross-sectional, correlation matrix using sample of convenience. SUBJECTS Thirteen subjects with chronic hemiparesis secondary to a cerebrovascular accident (stroke) were tested. METHODS Subjects were assessed using the Fugl-Meyer Upper Extremity Motor Assessment, modified Ashworth scale, deep tendon reflexes, and muscle characteristics that included quantification of muscle stiffness, paresis and co-contraction during a voluntary reaching task and during passive movements. Surface electromyographic and myotonometric muscle stiffness data were obtained during movement trials. RESULTS Biceps and triceps brachii muscle paresis and excess biceps brachii co-contraction during voluntary reaching had the highest correlations to decreased motor performance. Muscle tone measurements did not have significant correlations to upper extremity performance. CONCLUSION Paresis of elbow flexors and extensors and excess co-contraction of the biceps brachii during voluntary reaching appear to be most predictive of upper extremity motor performance. Results are discussed in relation to the specific challenges these findings pose for spastic paresis clinical management.

Long-latency contributions to reciprocal inhibition during various levels of muscle contraction

Reciprocal inhibition is a functional term and refers to the proportional decrease in antagonist motoneuron activity that accompanies an agonist contraction. A condition-test (C-T) H-reflex paradigm (conditioned stimulus applied to the common peroneal nerve; test reflex elicited by posterior tibial nerve stimulation) was used during: (1) rest, (2) a tonic isometric tibialis anterior (TA) contraction at 10% of its maximal voluntary contraction (MVC) and, (3) a TA contraction at 25% MVC. The purpose of the study was to assess whether or not long-latency contributions to reciprocal inhibition of soleus H-reflexes changed with increasing levels of TA contraction. C-T intervals ranged from 5 to 150 ms. Subjects (n=14) had long-latency inhibition at rest (x = -35.0 +/- 18.7%). This inhibition was enhanced during 10% (x = -46.1 +/- 17.9%; p = 0.17) and 25% MVCs (x = -56.3 +/- 14.0%; p < 0.01). Findings indicate that long-latency contributions to reciprocal inhibition of the soleus motoneuron pool are enhanced with increasing force of TA muscle contraction up to 25% MVC. These results indicate that long-latency contributions to reciprocal inhibition of soleus H-reflexes are not static but rather are task-specific and change in relation to levels of TA muscle activity.

Construct validity of myotonometric measurements of muscle compliance as a measure of strength

Myotonometric measurement of muscle compliance represents new technology that quantifies muscle tone. Compliance change during muscle contraction might provide an indirect measure of strength. The purpose of this study was to determine relationships among myotonometric measurements of muscle compliance, surface electromyographic (sEMG) measurements of muscle activation and joint force production during voluntary isometric knee extensions. The level of relationship will contribute to the construct validity of use of muscle compliance as an indirect strength measurement. Thirteen male subjects, mean age 25 +/- 1.5 years, participated. Simultaneous recordings of myotonometric, sEMG of the rectus femoris and isometric knee extension force measurements were taken at rest, during maximal voluntary contraction (MVC), and during 33% and 66% MVC contractions. Relationships among the three measurement procedures were calculated using correlation and regression analyses. Myotonometric measures of muscle compliance, sEMG and force measurements were highly correlated. Myotonometric measurements were best represented by a curvilinear (quadratic) relationship to sEMG (r = 0.82, p < 0.001) and joint force (r = 0.83, p < 0.001). The present experiments establish the construct validity of myotonometric measurements of muscle compliance as an indirect means of quantifying muscle strength and activation levels. This method, therefore, offers a possible alternative for cases in which direct measurement of joint force or sEMG is difficult or inappropriate.

Short- and long-latency contributions to reciprocal inhibition during various levels of muscle contraction of individuals with cerebral palsy

Deficits in reciprocal inhibition likely contribute to excessive antagonist muscle cocontraction during voluntary movements of individuals with cerebral palsy. This study examined neural contributions to reciprocal inhibition of the soleus motoneurons of individuals with spastic, diplegic cerebral palsy and nondisabled individuals during various levels of voluntary tibialis anterior contraction. A condition-test H-reflex paradigm examined short- and long-latency contributions to reciprocal inhibition of soleus neural pools during changing levels of voluntary tibialis anterior contraction. Electrically induced short- and long-latency inhibition was similar between healthy, neurologically intact control subjects and subjects with cerebral palsy during rest. With increasing levels of tibialis anterior contraction, control subjects experienced increasing levels of soleus motoneuron inhibition, especially of long-latency inhibitory responses. In contrast, there was no evidence of modulation of short- or long-latency inhibition with increasing levels of tibialis anterior contraction among subjects with cerebral palsy. Deficits in long-latency (presynaptic) inhibition appear to contribute prominently to voluntary movement impairment of individuals with cerebral palsy. (J Child Neurol 2006;21:240—246).

Human myotatic reflex development of the lower extremities

Lower extremity (LE) myotatic reflexes were tested by percussion (taps) to the patellar and Achilles tendons. Surface electromyographic recordings were obtained from 5 LE muscles during tendon taps. Results indicated that LE myotatic reflexes underwent considerable change during early human development. The changes were non-linear and highly variable. Reflex irradiation (the presence of reflex responses in muscles other than the one being directly stimulated by a tendon tap) was present in the newborn but to a lesser extent than was in evidence later on during the first year of life. The percentage of time reflex irradiation was detected in heteronymous muscle groups appeared to achieve maximal levels during the first year and then progressively decline. The decline in reflex irradiation was most dramatic between the first and second years of life. Irradiated responses were still recorded from 2 year-old children but with less frequency than in children less than 1 year of age. With the exception of responses in muscles that were direct antagonists to the stimulated muscle, irradiation was not observed in children 3-5 years of age.

The effect of enslaving on perception of finger forces

The primary purpose was to examine the effect of enslaving on finger force perception during isometric finger force production using an ipsilateral force-matching paradigm. Fourteen subjects were instructed to produce varying levels of reference forces [10, 20, 30, and 40% maximal voluntary contraction (MVC)] force using one finger (index, I or little, L) and to reproduce these forces using the same finger (homo-finger tasks, I/I and L/L) or a different finger (hetero-finger tasks, I/L and L/I). Forces of all fingers were recorded. During homo-finger tasks, no differences were found in force magnitude or relative level of force (expressed as a proportion of MVC). The index finger matching force magnitudes were greater than the little finger reference force magnitudes, with significantly lower levels of relative force during L/I tasks; while the little finger matching forces underestimated the index finger reference forces with significantly higher levels of relative force during I/L tasks. The difference in the matching and reference forces by the instructed finger(s), i.e., matching error, was larger in hetero-finger tasks than in homo-finger tasks, particularly at high reference force levels (30, 40% MVC). When forces of all fingers were considered, enslaving (uninstructed finger forces) significantly minimized matching errors of the total force during both I/L and L/I hetero-finger tasks, especially at high reference force levels. Our results show that there is a tendency to match the absolute magnitude of the total force during ipsilateral finger force-matching tasks. This tendency is likely related to enslaving effects. Our results provide evidence that all (instructed and uninstructed) finger forces are sensed, thus resulting in perception of the absolute magnitude of total finger force.

H-reflex modulations during voluntary and automatic movements following upper motor neuron damage.

OBJECTIVES It is not known whether similar mechanisms account for the impairments of voluntary movement and automatic postural responses of individuals with spasticity secondary to damage to the sensorimotor cortex and its projections (i.e. upper motor neuron syndrome (UMN)). METHODS The present study examined changes in soleus H-reflexes preceding and during voluntary tibialis anterior (TA) muscle contraction of standing subjects and during balance platform induced postural perturbations that elicited similar TA muscle contractions. Twenty-two subjects (12 non-disabled; 4 with spastic-type cerebral palsy; 6 with adult-onset cerebral vascular accident) participated in the study. Data were analyzed using ANOVAs and Tukey HSD post-hoc comparison tests to assess the timing and magnitude of soleus H-reflex amplitude changes relative to the onset of TA muscle activation. RESULTS Results indicated that, regardless of the level of TA activation, soleus H-reflexes of subjects with UMN involvement did not demonstrate inhibition either. during voluntary movements or during automatic postural perturbations. CONCLUSIONS These findings indicate that postural reflexes, as well as volitional movements, are impaired following UMN damage and that deficits in neural pathways subserving reciprocal inhibition contribute to the impairments.