The effects of local forearm heating and cooling on motor unit properties during submaximal contractions.

NEW FINDINGS: What is the central question of this study? How do temperature manipulations affect motor unit (MU) properties during submaximal contractions to the same relative percentage of maximal force? What is the main finding and its importance? MU recruitment patterns are affected by temperature manipulations at the forearm. However, the relationship between MU potential amplitude and recruitment threshold indicates no change to the order or recruitment. Additionally, the MU potential amplitude and firing rate relationship was affected by temperature, suggesting that smaller MUs are more affected by temperature changes than larger MUs. ABSTRACT: Temperature impacts muscle contractile properties, such that experiments with workloads based on thermoneutral values will produce different relative intensities if maximal force changes due to muscle temperature. We investigated how temperature affected motor unit (MU) properties with contractions performed at the same normalized percentage of maximal force. Twenty participants (10 females) completed evoked, maximal, and trapezoidal voluntary contractions during thermoneutral-, hot-, and cold-temperature conditions. Forearm temperature was established using 25 min of neutral (∼32°C), hot (∼44°C) or cold (∼13°C) water circulated through a tube-lined sleeve. Flexor carpi radialis MU properties were assessed with contractions at 30% and 60% MVC relative to each temperature using surface electromyography decomposition. Changes to contractile properties and electromechanical delay from the evoked twitch suggest that muscle contractility was changed from the thermal manipulations (effect size (d) ≥ 0.42, P < 0.05). Maximal force was not different between neutral and hot conditions (d = 0.16, P > 0.05) but decreased in the cold (d ≥ 0.34, P < 0.05). For both contraction intensities, MU potential (MUP) amplitude was larger and duration was longer in the cold compared to neutral and hot conditions (d ≥ 1.24, P < 0.05). Cumulative probability density for the number of MUs recruited revealed differences in MU recruitment patterns among temperature conditions. The relationship between MU recruitment threshold and firing rate or MUP amplitude was not different among temperature conditions (P > 0.05); however, the relationship between MUP amplitude and firing rate was (P < 0.05). Local temperature manipulations appear to affect MU recruitment patterns, which may act as compensatory mechanisms to the changes in muscle viscosity and contractile properties due to local temperature changes.

Dynamic Wrist Flexion and Extension Fatigue Induced via Submaximal Contractions Similarly Impairs Hand Tracking Accuracy in Young Adult Males and Females.

We evaluated the effects of muscle fatigue on hand-tracking performance in young adults. Differences were quantified between wrist flexion and extension fatigability, and between males and females. Participants were evaluated on their ability to trace a pattern using a 3-degrees-of-freedom robotic manipulandum before (baseline) and after (0, 1, 2, 4, 6, 8, and 10 mins) a submaximal-intensity fatigue protocol performed to exhaustion that isolated the wrist flexors or extensors on separate days. Tracking tasks were performed at all time points, while maximal voluntary contractions (MVCs) were performed at baseline, and 2, 6-, and 10-mins post-task termination. We evaluated movement smoothness (jerk ratio, JR), shape reproduction (figural error, FE), and target tracking accuracy (tracking error, TE). MVC force was significantly lower in females (p < 0.05), lower than baseline for all timepoints after task termination (p < 0.05), with no muscle group-dependent differences. JR did not return to baseline until 10-mins post-task termination (most affected), while FE returned at 4-mins post-task termination, and TE at 1-min post-task termination. Males tracked the target with significantly lower JR (p < 0.05), less TE (p < 0.05), and less FE (p < 0.05) than females. No muscle group-dependent changes in hand-tracking performance were observed. Based on this work, hand tracking accuracy is similarly impaired following repetitive submaximal dynamic wrist flexion or extension. The differences between male and female fatigability was independent of the changes in our tracking metrics.

A method for editing motor unit potential trains obtained by decomposition of surface electromyographic signals.

Rather than discarding motor unit potential trains (MUPTs) because they do not meet 100% validity criteria, we describe and evaluate a novel editing routine that preserves valid discharge times, based on decreasing shape variability (variance ratio, VR) within a MUPT. The error filtered estimation (EFE) algorithm is then applied to the remaining 'high confidence' discharge times to estimate inter-discharge interval (IDI) statistics. Decomposed surface EMG data from the flexor carpi radialis recorded from 20 participants during 60% MVC wrist flexion was used. There were two levels of denoising criteria (relaxed and strict) criteria for removing MUPs to decrease the VR and increase the signal-to-noise ratio (SNR) of a MUPT. In total, VR decreased 24.88% and SNR increased 6.0% (p's < 0.05). The MUP template peak-to-peak (P-P) amplitude and P-P duration were dependent on the level of denoising (p's < 0.05). The standard error of the estimate (SEE) of the mean IDI before and after editing using the relaxed criteria (3.2% versus 3.69%), was very similar (p > 0.05). The same was true for the SEE between denoising criteria, which increased only to 5.14% for the strict criteria (p > 0.05). Editing the MUPTs resulted in a significant decrease in MUP shape variability and in the measures extracted from the MUP templates, with trivial differences between the SEE of the mean IDI between the edited and unedited MUPTs.