Time-dependent effects of pre-conditioning activation on muscle fiber conduction velocity and twitch torque

Firing behavior of single motor units of the tibialis anterior in human walking as non-invasively revealed by HDsEMG decomposition

Objective.Investigation of the firing behavior of motor units (MUs) provides essential neuromuscular control information because MUs are the smallest organizational component of the neuromuscular system. The MUs activated during human infants' leg movements and rodent locomotion, mainly controlled by the spinal central pattern generator (CPG), show highly synchronous firing. In addition to spinal CPGs, the cerebral cortex is involved in neuromuscular control during walking in human adults. Based on the difference in the neural control mechanisms of locomotion between rodent, human infants and adults, MU firing behavior during adult walking probably has some different features from the other populations. However, so far, the firing activity of MUs in human adult walking has been largely unknown due to technical issues.Approach.Recent technical advances allow noninvasive investigation of MU firing by high-density surface electromyogram (HDsEMG) decomposition. We investigated the MU firing behavior of the tibialis anterior (TA) muscle during walking at a slow speed by HDsEMG decomposition.Main results.We found recruitment threshold modulation of MU between walking and steady isometric contractions. Doublet firings, and gait phase-specific firings were also observed during walking. We also found high MU synchronization during walking over a wide range of frequencies, probably including cortical and spinal CPG-related components. The amount of MU synchronization was modulated between the gait phases and motor tasks. These results suggest that the central nervous system flexibly controls MU firing to generate appropriate force of TA during human walking.Significance.This study revealed the MU behavior during walking at a slow speed and demonstrated the feasibility of noninvasive investigation of MUs during dynamic locomotor tasks, which will open new frontiers for the study of neuromuscular systems in the fields of neuroscience and biomedical engineering.

Estimation of contractile parameters of successive twitches in unfused tetanic contractions of single motor units - A proof-of-concept study using ultrafast ultrasound imaging in vivo

During a voluntary contraction, motor units (MUs) fire a train of action potentials, causing summation of the twitch forces, resulting in fused or unfused tetanus. Twitches have been important in studying whole-muscle contractile properties and differentiation between MU types. However, there are still knowledge gaps concerning the voluntary force generation mechanisms. Current methods rely on the spike-triggered averaging technique, which cannot track changes in successive twitches' properties in response to individual neural firings. This study proposes a method that estimates successive twitches contractile parameters of single MUs during low force voluntary isometric contractions in human biceps brachii. We used a previously developed ultrafast ultrasound imaging method to estimate unfused tetanic activity signals of single MUs. A twitch decomposition model was used to decompose unfused tetanic activity signals into individual twitches. This study found that the contractile parameters varied within and across MUs. There was an association between the inter-spike interval and the contraction time (r = 0.49,p < 0.001) and the half-relaxation time (r = 0.58,p < 0.001), respectively. The method shows the proof-of-concept to study MU contractile properties of individual twitches in vivo, which can provide further insights into the force generation mechanisms of voluntary contractions and response to individual neural discharges.

Ultrasound-based detection of glucocorticoid-induced impairments of muscle mass and structure in Cushing's disease

PURPOSE

To investigate the glucocorticoid-induced impairments of muscle mass and structure in patients presenting different stages of steroid myopathy progression.

METHODS

Thirty-three patients (28 women) affected by active (N = 20) and remitted (N = 13) Cushing's disease were recruited and the following variables were assessed: walking speed, handgrip strength, total body and appendicular muscle mass by bioelectrical impedance analysis (BIA), thickness and echo intensity of lower limb muscles by ultrasonography.

RESULTS

The two groups of patients showed comparable values of both handgrip strength [median (interquartile range) values: active disease: 27.4 (7.5) kg vs. remitted disease: 26.4 (9.4) kg; P = 0.58] and walking speed [active disease: 1.0 (0.2) m/s vs. remitted disease: 1.1 (0.3) m/s; P = 0.43]. Also, the thickness of the four muscles and all BIA-derived sarcopenic indices were comparable (P > 0.05 for all comparisons) between the two groups. On the contrary, the echo intensity of vastus lateralis, tibialis anterior (lower portion), and medial gastrocnemius was significantly (P < 0.05 for all comparisons) higher in patients with active disease compared to patients with remitted disease. Finally, significant negative correlations were found in the whole group of patients between muscle echo intensity and muscle function assessments.

CONCLUSIONS

We provided preliminary evidence that the ultrasound-derived measurements of muscle thickness and echo intensity can be useful to detect and track the changes of muscle mass and structure in patients with steroid myopathy and we suggest that the combined assessment of muscle mass, strength, and performance should be systematically applied in the routine examination of steroid myopathy patients.

Physiological consequences of doublet discharges on motoneuronal firing and motor unit force

The double discharges are observed at the onset of contractions of mammalian motor units (MUs), especially during their recruitment to strong or fast movements. Doublets lead to MU force increase and improve ability of muscles to maintain high force during prolonged contractions. In this review we discuss an ability to produce doublets by fast and slow motoneurons (MNs), their influence on the course of action potential afterhyperpolarization (AHP) as well as its role in modulation of the initial stage of the firing pattern of MNs. In conclusion, a generation of doublets is an important strategy of motor control, responsible for fitting the motoneuronal firing rate to the optimal for MUs at the start of their contraction, necessary for increment of muscle force.

Motor unit

Movement is accomplished by the controlled activation of motor unit populations. Our understanding of motor unit physiology has been derived from experimental work on the properties of single motor units and from computational studies that have integrated the experimental observations into the function of motor unit populations. The article provides brief descriptions of motor unit anatomy and muscle unit properties, with more substantial reviews of motoneuron properties, motor unit recruitment and rate modulation when humans perform voluntary contractions, and the function of an entire motor unit pool. The article emphasizes the advances in knowledge on the cellular and molecular mechanisms underlying the neuromodulation of motoneuron activity and attempts to explain the discharge characteristics of human motor units in terms of these principles. A major finding from this work has been the critical role of descending pathways from the brainstem in modulating the properties and activity of spinal motoneurons. Progress has been substantial, but significant gaps in knowledge remain.

History dependence of human muscle-fiber conduction velocity during voluntary isometric contractions

The conduction velocity (CV) of a muscle fiber is affected by the fiber's discharge history going back ∼1 s. We investigated this dependence by measuring CV fluctuations during voluntary isometric contractions of the human brachioradialis muscle. We recorded electromyogram (EMG) signals simultaneously from multiple intramuscular electrodes, identified potentials belonging to the same motor unit using EMG decomposition, and estimated the CV of each discharge from the interpotential interval. In 12 of 14 subjects, CV increased by ∼10% during the first second after recruitment and then fluctuated by about ±2% in a way that mirrored the fluctuations in the instantaneous firing rate. The CV profile could be precisely described in terms of the discharge history by a simple mathematical model. In the other two subjects, and one subject retested after cooling the arm, the CV fluctuations were inversely correlated with instantaneous firing rate. In all subjects, CV was additionally affected by very short interdischarge intervals (<25 ms): it was increased in doublets at recruitment, but decreased in doublets during continuous firing and after short interdischarge intervals in doubly innervated fibers. CV also exhibited a slow trend of about -0.05%/s that did not depend on the immediate discharge history. We suggest that measurements of CV fluctuations during voluntary contractions, or during stimulation protocols that involve longer and more complex stimulation patterns than are currently being used, may provide a sensitive approach for estimating the dynamic characteristics of ion channels in the human muscle-fiber membrane.

Sympathetic activation by the cold pressor test does not increase the muscle force generation capacity

A positive inotropic action by the sympathetic nervous system on skeletal muscles has been observed and investigated in animal and in vitro studies. This action provided a theoretical basis for the putative ergogenic action of catecholamines and adrenergic agonists, although there is no clear evidence of this effect in humans. The aim of this study was to investigate the occurrence of inotropic effects associated to physiological sympathetic activation in healthy subjects. The muscle force capacity was investigated in the tibialis anterior (n = 9 subjects) and in the soleus (n = 9) muscles electrically stimulated with single pulses and double pulses with variable interspike interval (4-1,000 ms) and short pulse trains (frequency: 5-14 Hz) before, during, and after sympathetic activation by the cold pressor test (CPT). CPT significantly decreased by 10.4 ± 7.2 and 10.6 ± 4.4% the force produced by single and double pulse stimulation, respectively, and produced smaller decreases in the force obtained by train stimulation in the tibialis anterior, while no significant changes were observed in either type of contraction in the soleus muscle. CPT failed to induce any increase in the force capacity of the investigated muscles. The prevalent decrease in force evidenced in this study supports the concept that the weakening sympathetic action on type I fiber, already shown to occur in humans, prevails over the putative potentiating action.