Participation of ipsilateral cortical descending influences in bimanual wrist movements in humans

Estimating descending activation patterns from EMG in fast and slow movements using a model of the stretch reflex

Due to spinal reflex loops, descending activation from the brain is not the only source of muscle activation that ultimately generates movement. This study directly estimates descending activation patterns from measured patterns of muscle activation (electromyographic; EMG) during human arm movements. A simple model of the spinal stretch reflex is calibrated in a postural unloading task and then used to estimate descending activation patterns from muscle EMG patterns and kinematics during voluntary arm motion performed at different speeds. We observed three key features of the estimated descending activation patterns: 1) Within about the first 15% of movement duration, descending and muscle activations are temporally aligned. Thereafter, they diverge and develop qualitatively different temporal profiles. 2) The time course of descending activation is monotonic for slow movements, nonmonotonic for fast movements. 3) Varying model parameters such as the spinal reflex gain or the level of cocontraction do not qualitatively change the temporal pattern of estimated descending activation. Our findings highlight the substantial contribution of spinal reflex loops to movement generation, while at the same time providing evidence that the brain must generate qualitatively different descending activation patterns for movements that vary in their mechanical dynamics.NEW & NOTEWORTHY We propose a new method that directly estimates descending activation from measured electromyographic (EMG) signals and arm kinematics by inverting a model of the spinal stretch reflex, without the need for muscle models or an arm dynamics model. This approach identifies key features of the time structure of descending activation as movement speed is varied, while also revealing the significant contribution of the spinal stretch reflex to movement generation.

Postural adjustments to self-triggered perturbations under conditions of changes in body orientation

We studied anticipatory and compensatory postural adjustments (APAs and CPAs) associated with self-triggered postural perturbations in conditions with changes in the initial body orientation. In particular, we were testing hypotheses on adjustments in the reciprocal and coactivation commands, role of proximal vs. distal muscles, and correlations between changes in indices of APAs and CPAs. Healthy young participants stood on a board with full support or reduced support area and held a standard load in the extended arms. They released the load in a self-paced manned with a standard small-amplitude arm movement. Electromyograms of 12 muscles were recorded and used to compute reciprocal and coactivation indices between three muscle pairs on both sides of the body. The subject's body was oriented toward one of three targets: straight ahead, 60° to the left, and 60° to the right. Body orientation has stronger effects on proximal muscle pairs compared to distal muscles. It led to more consistent changes in the reciprocal command compared to the coactivation command. Indices of APAs and CPAs showed positive correlations across conditions. We conclude that the earlier suggested hierarchical relations between the reciprocal and coactivation command could be task-specific. Predominance of negative or positive correlations between APA and CPA indices could also be task-specific.

Repetitive Peripheral Magnetic Stimulation to Improve Ankle Function and Gait in Cerebral Palsy at Adulthood: An open-label Case Study

Repetitive peripheral magnetic stimulation (rPMS) is noninvasive and painless. It drives plasticity of the primary motor cortex (M1) in children with cerebral palsy (CP) and this improves the ankle function and gait. Our study explored whether rPMS of muscles could influence motricity in an adult CP case. A 30-year-old woman with mixed CP participated in four sessions (S1 to S4, one per week) where rPMS was applied bilaterally on leg and trunk muscles (tibialis anterior-TA, hamstrings, transverse abdominis, paraspinal multifidus). Clinical scores and M1 excitability (probed by transcranial magnetic stimulation) were tested at pre-rPMS at S1 (baseline) and S4, then 40 days later (follow-up). The active ankle dorsiflexion was significantly increased and the plantar flexors resistance to stretch reduced as compared to baseline. The improvement of the ankle function was carried-over to the quality of locomotor patterns. Changes persisted until follow-up and were paralleled by drastic changes of M1 excitability. These original findings of rPMS influence on M1 plasticity and motricity are promising for the functional improvement of adult people living with CP and should be replicated in larger-sampled studies.

Physiology of Tremor Reduction by Putting the Hands Together in Essential Tremor

BACKGROUND

Essential tremor is a common movement disorder, characterized by 4-12 Hz tremor of the hands and arms that can affect many activities of daily living. It has been reported by patients that when performing tasks bimanually their tremor is reduced, but why this happens is unknown.

OBJECTIVES

We measured patients' tremors in different conditions when performed with 1 hand and 2 hands to observe if bimanual task performance changes the characteristics of the tremor.

METHODS

A total of 10 patients with essential tremor participated in the study. Electromyographic electrodes were attached bilaterally to the wrist flexor and extensor muscles, and accelerometers were attached to the dorsum of the hands. For each condition, holding a cup, wingbeat, and extending both arms up, data were collected with a single hand and bimanually with the hands touching.

RESULTS

When the hands were touching, there was a significant decrease in both accelerometric and electromyographic power at the tremor frequency. In addition, there was a decrease in coherence between accelerometer and electromyography on the same side. There was no change in the tremor frequency.

CONCLUSIONS

Tremor amplitude does decrease when the hands are together. Together, the characteristics underlying the decrease in tremor amplitude may indicate a decrease in power of the central oscillator driving the tremor, which we speculate is attributed to the differences in unimanual and bimanual motor control. However, given the small sample size, we note that future hypothesis-driven studies with an a priori power analysis will be required to further explore this phenomenon.

Effect of Object Texture and Weight on Ipsilateral Corticospinal Influences During Bimanual Holding in Humans

We tested the hypothesis that the ipsilateral corticospinal system, like the contralateral corticospinal system, controls the threshold muscle length at which wrist muscles and the stretch reflex begin to act during holding tasks. Transcranial magnetic stimulation was applied over the right primary motor cortex in 21 healthy subjects holding a smooth or coarse block between the hands. Regardless of the lifting force, motor evoked potentials in right wrist flexors were larger for the smooth block. This result was explained based on experimental evidence that motor actions are controlled by shifting spatial stretch reflex thresholds. Thus, the ipsilateral corticospinal system is involved in threshold position control by modulating facilitatory influences of hand skin afferents on motoneurons of wrist muscles during bimanual object manipulation.

Motor evoked potential latency and duration from tibialis anterior in individuals with chronic stroke

Ipsilateral motor pathways from the contralesional hemisphere to the paretic limbs may be upregulated to compensate for impaired function after stroke. Onset latency and duration of motor evoked potentials (MEPs) evoked by transcranial magnetic stimulation (TMS) provide insight into compensatory pathways but have been understudied in the lower limb. This study assessed MEP onset latency and duration in the lower limb after stroke, and compared ipsilateral and contralateral MEPs in the paretic and non-paretic limb. We hypothesized that: (1) onset latency would be longer for ipsilateral than contralateral MEPs and longer for the paretic than the non-paretic limb, and (2) duration would be shorter for ipsilateral than contralateral MEPs and longer for the paretic than the non-paretic limb. Data were collected as a part of a pre-test of a randomized controlled trial. TMS was applied to the ipsilateral and contralateral hemisphere of the paretic and non-paretic limb. MEP onset latency and duration were calculated from the tibialis anterior. Thirty-five participants with chronic stroke were included in the final analysis. Onset latency was longer in the paretic than the non-paretic limb (~ 6.0 ms) and longer after ipsilateral than contralateral stimulation (~ 1.8 ms). Duration was longer in the paretic than the non-paretic limb (~ 9.2 ms) and longer after contralateral than ipsilateral stimulation (~ 5.2 ms). Ipsilateral MEPs may be elicited through ipsilateral pathways with fewer fibers with a higher activation threshold and/or greater spinal branching. MEPs from the paretic limb may reflect slower central motor conduction, peripheral changes, or changes in motor pathway.