Fine-wire recordings of flexor hallucis brevis motor units up to maximal voluntary contraction reveal a flexible, nonrigid mechanism for force control

Normative Extensor Hallucis Brevis Muscle Activity During Locomotion Following the Development of a Novel Ultrasound-Guided Fine-Wire Electromyography Protocol

Fine-wire electromyography (EMG) is a traditional laboratory technique to estimate muscle activity of the small foot muscles, however, recordings have not been reported from extensor hallucis brevis (EHB). As an extensor of the great toe, EHB is an important muscle when studying physiological changes associated with foot pathologies such as hallux valgus. The purpose of this study was to develop an ultrasound-guided fine-wire EMG protocol to record EHB muscle activity and report normative EMG profiles of healthy young adults during locomotion. Sixteen asymptomatic young adults completed 20 walking trials at a self-selected velocity. Ensemble averages were calculated from the time normalized linear envelopes and represented from 0% to 100% of the single stance phase of gait. EHB muscle bursts were observed between 0% and 20% of the stance phase of gait in all participants. A second burst of EMG was observed between 80% and 100% of stance in 50% of the participants. This study introduces a novel ultrasound-guided EMG protocol and normative data from EHB recordings suggest a synergistic role to anterior compartment musculature at contact. These results provide preliminary insights into understanding the functional role of EHB and may help elucidate the biomechanical factors exacerbating the progression of hallux pathologies.

Neuromechanical characterization of the abductor hallucis and its potential role in upright postural control

There is growing evidence to support a role for the abductor hallucis (AH) in standing balance control; however, functional properties of the muscle that may provide more insight into AH's specific contribution to upright posture have yet to be characterized. This study was conducted to quantify functional neuromechanical properties of the AH and correlate the measures with standing balance variables. We quantified strength and voluntary activation during maximal voluntary isometric contractions of the great toe abductor in nine (3 females and 6 males) healthy, young participants. During electrically evoked twitch and tetanic contractions, we measured great toe abduction peak force and constructed a force-frequency curve. We also evaluated peak abduction force, contraction time (CT), half-relaxation time (HRT), rate of force development (RFD), and relaxation rate (RR) from twitch contractions evoked using doublet stimuli. Strength, VA, CT, HRT, RFD, and RR were correlated to centre of pressure standard deviation (COP SD) and velocity (COP VEL) variables of the traditional COP trace and its rambling and trembling components during single-legged stance. AH twitch properties (e.g., CT: 169.8 ± 32.3 ms; HRT: 124.1 ± 29.2 ms) and force-frequency curve were similar to other slow contractile muscles. Contractile speed related negatively with COP VEL, suggesting AH may be appropriate for slow, prolonged tasks such as ongoing postural balance control. Correlation coefficient outcomes for all variables were similar between rambling and trembling components. Our results provide further evidence for the importance of AH neuromechanical function for standing balance control, at least during a challenging single-legged posture.

Flexor hallucis brevis motor unit behavior in response to moderate increases in rate of force development

Background

Studies on motor unit behaviour with varying rates of force development have focussed predominantly on comparisons between slow and ballistic (i.e., very fast) contractions. It remains unclear how motor units respond to less extreme changes in rates of force development. Here, we studied a small intrinsic foot muscle, flexor hallucis brevis (FHB) where the aim was to compare motor unit discharge rates and recruitment thresholds at two rates of force development. We specifically chose to investigate relatively slow to moderate rates of force development, not ballistic, as the chosen rates are more akin to those that presumably occur during daily activity.

Methods

We decomposed electromyographic signals to identify motor unit action potentials obtained from indwelling fine-wire electrodes in FHB, from ten male participants. Participants performed isometric ramp-and-hold contractions from relaxed to 50% of a maximal voluntary contraction. This was done for two rates of force development; one with the ramp performed over 5 s (slow condition) and one over 2.5 s (fast condition). Recruitment thresholds and discharge rates were calculated over the ascending limb of the ramp and compared between the two ramp conditions for matched motor units. A repeated measures nested linear mixed model was used to compare these parameters statistically. A linear repeated measures correlation was used to assess any relationship between changes in recruitment threshold and mean discharge rate between the two conditions.

Results

A significant increase in the initial discharge rate (i.e., at recruitment) in the fast (mean: 8.6 ±  2.4 Hz) compared to the slow (mean: 7.8 ± 2.3 Hz) condition (P = 0.027), with no changes in recruitment threshold (P = 0.588), mean discharge rate (P = 0.549) or final discharge rate (P = 0.763) was observed. However, we found substantial variability in motor unit responses within and between conditions. A small but significant negative correlation (R² = 0.33, P = 0.003) was found between the difference in recruitment threshold and the difference in mean discharge rate between the two conditions.

Conclusion

These findings suggest that as force increases for contractions with slower force development, increasing the initial discharge rate of recruited motor units produces the increase in rate of force development, without a change in their recruitment thresholds, mean or final discharge rate. However, an important finding was that for only moderate changes in rate of force development, as studied here, not all units respond similarly. This is different from what has been described in the literature for ballistic contractions in other muscle groups, where all motor units respond similarly to the increase in neural drive. Changing the discharge behaviour of a small group of motor units may be sufficient in developing force at the required rate rather than having the discharge behaviour of the entire motor unit pool change equally.

Effects of a Single Electrical Stimulation Session on Foot Force Production, Foot Dome Stability, and Dynamic Postural Control

CONTEXT

Mounting evidence suggests neuromuscular electrical stimulation (NMES) as a promising modality for enhancing lower limb muscle strength, yet the functional effects of a single electrical stimulation session for improving the function of the intrinsic foot muscles (IFM) has not been evaluated.

OBJECTIVE

To investigate the immediate effects of an NMES session compared with a sham stimulation session on foot force production, foot dome stability, and dynamic postural control in participants with static foot pronation.

DESIGN

Randomized controlled clinical trial.

SETTING

Laboratory.

PATIENTS OR OTHER PARTICIPANTS

A total of 46 participants (23 males, 23 females) with static foot pronation according to their Foot Posture Index (score ≥ 6) were randomly assigned to an NMES (n = 23) or control (n = 23) group.

INTERVENTION(S)

The NMES group received a single 15-minute NMES session on the dominant foot across the IFM. The control group received a 15-minute sham electrical stimulation session.

MAIN OUTCOME MEASURE(S)

All outcome measurements were assessed before and after the intervention and consisted of foot force production on a pressure platform, foot dome stability, and dynamic postural control. Statistical analysis was based on the responsiveness of the outcome measures and responder analysis using the minimum detectable change scores for each outcome measure.

RESULTS

In the NMES group, 78% of participants were classified as responders for at least 2 of the 3 outcomes, compared with only 22% in the control group. The relative risk of being a responder in the NMES group compared with the control group was 3.6 (95% CI = 1.6, 8.1]. Interestingly, we found that all participants who concomitantly responded to foot strength and navicular drop (n = 8) were also responders in dynamic postural control.

CONCLUSIONS

Compared with a sham stimulation session, a single NMES session was effective in immediately improving foot function and dynamic postural control in participants with static foot pronation. These findings support the role of NMES for improving IFM function in this population.

The Strategy of the Brain to Maintain the Force Production in Painful Contractions-A Motor Units Pool Reorganization

A common symptom in neuromuscular diseases is pain, which changes human movement in many ways. Using the decomposed electromyographic signal, we investigate the strategy of the brain in recruiting different pools of motor units (MUs) to produce torque during induced muscle pain in terms of firing rate (FR), recruitment threshold (RT) and action potential amplitude (MUAP_AMP). These properties were used to define two groups (G1/G2) based on a K-means clusterization method. A 2.0 mL intramuscular hypertonic (6%) or isotonic (0.9%) saline solution was injected to induce pain or act as a placebo during isometric and isokinetic knee extension contractions. While isometric torque decreases after pain induction with hypertonic solution, this does not occur in isokinetic torque. This occurs because the MUs re-organized after the injection of both solutions. This is supported by an increase in RT, in both G1 and G2 MUs. However, when inducing pain with the hypertonic solution, RT increase is exacerbated. In this condition, FR also decreases, while MUAP_AMP increases only for G1 MUs. Therefore, this study proposes that the strategy for maintaining force production during pain is to recruit MUs with higher RT and MUAP_AMP.

Distribution of motor unit properties across human muscles

The purpose of our review was to compare the distribution of motor unit properties across human muscles of different sizes and recruitment ranges. Although motor units can be distinguished based on several different attributes, we focused on four key parameters that have a significant influence on the force produced by muscle during voluntary contractions: the number of motor units, average innervation number, the distributions of contractile characteristics, and discharge rates within motor unit pools. Despite relatively few publications on this topic, current data indicate that the most influential factor in the distribution of these motor unit properties between muscles is innervation number. Nonetheless, despite a fivefold difference in innervation number between a hand muscle (first dorsal interosseus) and a lower leg muscle (tibialis anterior), the general organization of their motor unit pools, and the range of discharge rates appear to be relatively similar. These observations provide foundational knowledge for studies on the control of movement and the changes that occur with aging and neurological disorders.

One more time about motor (and non-motor) synergies

We revisit the concept of synergy based on the recently translated classical book by Nikolai Bernstein (On the construction of movements, Medgiz, Moscow 1947; Latash, Bernstein's Construction of Movements, Routledge, Abingdon 2020b) and progress in understanding the physics and neurophysiology of biological action. Two aspects of synergies are described: organizing elements into stable groups (modes) and ensuring dynamical stability of salient performance variables. The ability of the central nervous system to attenuate synergies in preparation for a quick action-anticipatory synergy adjustments-is emphasized. Recent studies have demonstrated synergies at the level of hypothetical control variables associated with spatial referent coordinates for effectors. Overall, the concept of synergies fits naturally the hierarchical scheme of control with referent coordinates with an important role played by back-coupling loops within the central nervous system and from peripheral sensory endings. Further, we review studies showing non-trivial changes in synergies with development, aging, fatigue, practice, and a variety of neurological disorders. Two aspects of impaired synergic control-impaired stability and impaired agility-are introduced. The recent generalization of the concept of synergies for non-motor domains, including perception, is discussed. We end the review with a list of unresolved and troubling issues.