Optimized maximum voluntary exertion protocol for normalizing shoulder muscle activity

Evaluating the Relationship Between Surface and Intramuscular-Based Electromyography Signals: Implications of Subcutaneous Fat Thickness

Intramuscular (iEMG) and surface electromyographic (sEMG) signals have been compared previously using predictive regression equations, finite element modeling, and correlation and cross-correlation analyses. Although subcutaneous fat thickness (SCFT) has been identified as a primary source of sEMG signal amplitude attenuation and low-pass filter equivalence, few studies have explored the potential effect of SCFT on sEMG and iEMG signal characteristics. The purpose of this study was to investigate the relationship between normalized submaximal iEMG and sEMG signal amplitudes collected from 4 muscles (rectus femoris, vastus lateralis, infraspinatus, and erector spinae) and determine whether SCFT explains more variance in this relationship. The effect of sex was also explored. Linear regression models demonstrated that the relationship between sEMG and iEMG was highly variable across the muscles examined (adjusted coefficient of determination [Adj R2] = .02-.74). SCFT improved the model fit for vastus lateralis, although this relationship only emerged with the inclusion of sex as a covariate. Thus, this research suggests that SCFT is not a prominent factor affecting the linearity between sEMG and iEMG. Researchers should investigate other parameters that may affect the linearity between sEMG and iEMG signals.

Is the YES/NO classification accurate in screening scapular dyskinesis in asymptomatic individuals? - A novel validation study utilizing surface electromyography as a surrogate measure in identifying movement asymmetries

BACKGROUND

Scapular dyskinesis is a known risk factor for shoulder pain, making it important to screen for prevention. Physical therapists screen scapular dyskinesis by visually comparing asymmetries in scapular movement during overhead reach using the Scapular Dyskinesis Test Yes/No classification (Y/N). Although scapular kinematics has been used to quantify scapular dyskinesis, current measurement techniques are inaccurate. Optimal scapular muscle activity is crucial for normal shoulder function and is measured using surface electromyography (sEMG). Research suggests that impaired scapular muscles can lead to scapular dyskinesis. Despite kinematics being a poor reference standard, there is currently no validated method to identify movement asymmetries using muscle activity as an alternative. We utilized sEMG to establish Y/N's validity. We hypothesized that Y/N is a valid tool using sEMG as a viable surrogate measure for identifying scapular dyskinesis.

METHODS

We employed a known-groups (symmetrical vs. asymmetrical shoulders) validity design following the Standards for Reporting Diagnostic Accuracy Studies. Seventy-two asymptomatic subjects were evaluated using Y/N as the index test and sEMG as the reference standard. We created a criterion to assign the sEMG as the reference standard to establish the known groups. We calculated the sensitivity (Sn), specificity (Sp), positive and negative predictive values (PPV, NPV), likelihood ratios (LR+, LR-), and diagnostic odds ratio (DOR) using a 2 × 2 table analysis.

RESULTS

The diagnostic accuracy values were Sn = 0.56 (0.37-0.74), Sp = 0.36 (0.08-0.65), PPV = 0.68 (0.49-0.88), NPV = 0.25 (0.04-0.46), LR+ = 0.87 (0.50-1.53), and LR- = 1.22 (0.50-2.97).

CONCLUSION

The Y/N's diagnostic accuracy was poor against the sEMG, suggesting clinicians should rely less on Y/N to screen scapular dyskinesis in the asymptomatic population. Our study demonstrated that sEMG might be a suitable alternative as a reference standard in validating methods designed to screen movement asymmetries.

Variability in musculoskeletal fatigue responses associated with repeated exposure to an occupational overhead drilling task completed on successive days

Emerging research suggests that muscular and kinematic responses to overhead work display a high degree of variability in fatigue-related muscular and kinematics changes, both between and within individuals when evaluated across separate days. This study examined whether electromyographic (EMG), kinematic, and kinetic responses to an overhead drilling task performed until volitional fatigue were comparable to those of a repeated identical exposure of the task completed 1 week later. Surface EMG and intramuscular EMG, sampled from 7 shoulder muscles, and right upper limb kinematics and kinetics were analyzed from 15 male and 14 female participants. No significant day-to-day changes in EMG mean power frequency (MPF) were observed, though serratus anterior displayed significantly less fatigue-related increase in EMG root-mean-squared (RMS) signal amplitude on day 2. Unfatigued upper kinematics on day 2 featured an increase in thoracohumeral elevation, elbow flexion, and decrease in wrist ulnar deviation compared to unfatigued state on day 1. Fatigue-related changes in shoulder joint flexion moment that were present on day 1 were reduced on day 2, suggesting that a more efficient overhead work strategy was learned and preserved across successive days. Day-to-day changes in upper limb joint angle variability, quantified by median absolute deviation (MdAD), were joint dependent. Despite yielding a variable fatigue-related kinetic strategy on both days, kinematic and kinetic fatigue-related changes on a second day of completing an overhead drilling task suggested a potential kinematic learning effect.

Unlatching school bus seat belt buckles: Considerations for young passengers

Automobile seat belts reduce the risk of injuries and fatalities resulting from a crash. As seat belts become more prevalent on large school buses, characterizing the capabilities of children to operate the unlatching mechanism of a seat belt is crucial to ensure the post-crash safety of young passengers. This study evaluated the strength capabilities of children and their abilities to unlatch a school bus seat belt when a school bus is in both the upright and rolled-over orientations. Push force exertions on a seat belt buckle push button were measured and compared to the seat belt assembly release force requirements specified in Federal Motor Vehicle Safety Standard (FMVSS) No. 209. Results of the study suggested that children do not have the strength to exert the maximum force of 133 N to release a seat belt assembly as specified in FMVSS No. 209; however, most children could unlatch a typical school bus seat belt assembly in the upright and rolled-over orientations.

Ergonomic arm support prototype device for smartphone users reduces neck and shoulder musculoskeletal loading and fatigue

Smartphone use is a risk factor for both neck and shoulder musculoskeletal disorders. The objective of this study was to evaluate an ergonomic arm support prototype device, which may help improve posture while using a smartphone, by determining its effect on muscle activity, muscle fatigue, and neck and shoulder discomfort. Twenty-four healthy young adult smartphone users performed 20 min of smartphone game playing under two different conditions, smartphone use with support prototype device (i.e. intervention condition) and without (i.e. control condition), while neck and shoulder posture were controlled at 0° neck flexion and 30° shoulder flexion. Activity and fatigue of four muscles were measured using surface electromyography (sEMG), these were: anterior deltoid (AD), cervical erector spinae (CES), upper trapezius (UT) and lower trapezius (LT). The intervention condition showed significantly decreased activity of all muscles. Fatigue of all muscles, except LT, significantly increased over time compared to the start point in the control condition. There was no significant difference in muscle fatigue between each time point in the intervention condition. In conclusion, the ergonomic arm support prototype device can be used as ergonomic intervention to reduce neck and shoulder muscle loading and fatigue.

Using EMG Amplitude and Frequency to Calculate a Multimuscle Fatigue Score and Evaluate Global Shoulder Fatigue

OBJECTIVE

The authors developed a function to quantify fatigue in multiple shoulder muscles by generating a single score using relative changes in EMG amplitude and frequency over time.

BACKGROUND

Evaluating both frequency and amplitude components of the electromyographic signal provides a more complete evaluation of muscle fatigue than either variable alone; however, little effort has been made to combine time and frequency domains for the evaluation of myoelectric fatigue.

METHOD

Surface EMG was measured from 14 shoulder muscles while participants performed simulated, repetitive work tasks until exhaustion. Each 60-s work cycle consisted of four tasks (dynamic push, dynamic pull, static drill, static force target matching task) scaled to participants' anthropometrics and strength. The function was generated to calculate a multimuscle fatigue score (MMFS) based on changes in EMG frequency, amplitude, and the number of muscles showing signs of myoelectric fatigue (increase in EMG amplitude; decrease in EMG frequency).

RESULTS

The function was evaluated through changes in MMFS over time: first (31.8 ± 14.6), middle (47.6 ± 25.3), last (58.6 ± 35.5) reference exertions ( p < .05). The evaluation of the relationships between MMFS and changes in strength ( r = -0.510) and MMFS and perceived fatigue (RPF) ( r = 0.298) showed significant relationships over time ( p < .05). MMFS scores increased over time ( p < .05) with significant relationships between MMFS and strength changes and RPF ( p < .05).

CONCLUSION AND APPLICATION

The MMFS allows for comparisons between workplace tasks, which can aid in workplace design to mitigate the development of fatigue.

Characterizing trunk muscle activations during simulated low-speed rear impact collisions

Objective: The purpose of this study was to evaluate the activation profiles of muscles surrounding the lumbar spine during unanticipated and braced simulated rear-end collisions. Methods: Twenty-two low-speed sled tests were performed on 11 human volunteers ( △ V = 4 km/h). Each volunteer was exposed to one unanticipated impact and one braced impact. Accelerometers were mounted on the test sled and participants' low back. Six bilateral channels of surface electromyography (EMG) were collected from the trunk during impact trials. Peak lumbar accelerations, peak muscle activation delay, muscle onset time, and peak EMG magnitudes, normalized to maximum voluntary contractions (MVCs), were examined across test conditions. Results: Though not statistically significant, bracing for impact tended to reduce peak lumbar acceleration in the initial rearward impact phase of the occupant's motion by approximately 15%. The only trunk muscles with peak activations exceeding 10% MVC during the unanticipated impact were the thoracic erector spinae. Time of peak muscle activation was slightly longer for the unanticipated condition (unanticipated = 296 ms; braced = 241 ms). Conclusions: Results from this investigation demonstrate that during an unanticipated low-speed rear-end collision, the peak activation of muscles in the lumbar spine are low in magnitude. As such, muscle activation likely has minimal contribution to the internal joint loads that are experienced in the lumbar intervertebral joints during low-speed rear impact collisions. These findings justify the use of simplified joint models in estimating the joint loads in the lumbar spine during low-speed rear impact collisions and support the application of cadaveric and anthropomorphic test device (ATD) testing in understanding the resultant joint loads in the lumbar spine associated with rear-end collisions.

Muscular and kinematic adaptations to fatiguing repetitive upper extremity work

Repetitive work is common in the modern workplace and the effects are often studied using fatigue protocols; however, there is evidence that the manner in which fatigue develops impacts the kinematic and muscular response to reduced physical capacity. The purpose of this study was to simultaneously evaluate muscular and kinematic adaptations during fatiguing, repetitive work until exhaustion. We measured muscle activity in 13 muscles in the shoulder and trunk, and captured full body kinematics while participants completed simulated, repetitive work tasks. Every 12 min, reference data were collected to quantify fatigue. This sequence continued until they reached the termination criteria. Participants displayed significant signs of muscle fatigue, loss of strength and increased perceived fatigue (p < 0.05). Analysis revealed a significant effect of time on posture and muscle activity that was both task and time dependent, and variable both between and within individuals. Participants were able to compensate for reduced physical capacity and maintain task performance through coordinated compensation strategies.