Neck Muscle EMG-Force Relationship and Its Reliability During Isometric Contractions.

BACKGROUND: Susceptible to injury, the neck is subject to scientific investigations, frequently aiming to elucidate possible injury mechanisms via surface electromyography (EMG) by indirectly estimating cervical loads. Accurate estimation requires that the EMG-force relationship is known and that its measurement error is quantified. Hence, this study examined the relationship between EMG and isometric force amplitude of the anterior neck (AN), the upper posterior neck (UPN), and the lower posterior neck (LPN) and then assessed the relationships' test-retest reliability across force-percentiles within and between days. METHODS: EMG and force data were sampled from 18 participants conducting randomly ordered muscle contractions at 5-90% of maximal voluntary force during three trials over 2 days. EMG-force relationships were modeled with general linear mixed-effects regression. Overall fitted lines' between-trial discrepancies were evaluated. Finally, the reliability of participants' fitted regression lines was quantified by an intraclass correlation coefficient (ICC) and the standard error of measurement (SEM). RESULTS: A rectilinear model had the best fit for AN while positively oriented quadratic models had the best fit for UPN and LPN, with mean adjusted conditional coefficients of determination and root mean square errors of 0.97-0.98 and 4-5%, respectively. Overall EMG-force relationships displayed a maximum 6% between-trial discrepancy and over 20% of maximal force, and mean ICC was above 0.79 within day and 0.27-0.61 between days across areas. Corresponding SEM was below 12% both within and between days across areas, excluding UPN between days, for which SEM was higher. CONCLUSIONS: EMG-force relationships were elucidated for three neck areas, and provided models allow inferences to be drawn from EMG to force on a group level. Reliability of EMG-force relationship models was higher within than between days, but typically acceptable for all but the lowest contraction intensities, and enables adjustment for measurement imprecision in future studies.

Decelerations of Parachute Opening Shock in Skydivers.

INTRODUCTION: High prevalence of neck pain among skydivers is related to parachute opening shock (POS) exposure, but few investigations of POS deceleration have been made. Existing data incorporate equipment movements, limiting its representability of skydiver deceleration. This study aims to describe POS decelerations and compare human- with equipment-attached data. METHODS: Wearing two triaxial accelerometers placed on the skydiver (neck-sensor) and equipment (rig-sensor), 20 participants made 2 skydives each. Due to technical issues, data from 35 skydives made by 19 participants were collected. Missing data were replaced using data substitution techniques. Acceleration axes were defined as posterior to anterior (+ax), lateral right (+ay), and caudal to cranial (+az). Deceleration magnitude [amax (G)] and jerks (G · s-1) during POS were analyzed. RESULTS: Two distinct phases related to skydiver positioning and acceleration direction were observed: 1) the x-phase (characterized by -ax, rotating the skydiver); and 2) the z-phase (characterized by +az, skydiver vertically oriented). Compared to the rig-sensor, the neck-sensor yielded lower amax (3.16 G vs. 6.96 G) and jerk (56.3 G · s-1 vs. 149.0 G · s-1) during the x-phase, and lower jerk (27.7 G · s-1 vs. 54.5 G · s-1) during the z-phase. DISCUSSION: The identified phases during POS should be considered in future neck pain preventive strategies. Accelerometer data differed, suggesting human-placed accelerometry to be more valid for measuring human acceleration.Gladh K, Lo Martire R, Äng BO, Lindholm P, Nilsson J, Westman A. Decelerations of parachute opening shock in skydivers. Aerosp Med Hum Perform. 2017; 88(2):121-127.

Decelerations and muscle responses during parachute opening shock.

INTRODUCTION: Pain in the neck region among skydivers can be related to repeated parachute opening shocks (POS), but empirical data on ramair POS biomechanics is lacking in the literature. The aim of this study was to develop and evaluate a methodology for assessment of multidirectional accelerations and neck muscle activity during parachuting, and to describe preliminary data. METHODS: In an experimental design, four experienced skydivers made two consecutive skydives. Deceleration was recorded with two triaxial accelerometers, one placed on the skydiver's cervicothoracic junction (acc-neck) and the other on the harness (acc-rig). Surface electromyography (EMG) was sampled from four sites bilaterally: anterior-neck, posterior upper and lower neck, and upper shoulder muscles. EMG activity was normalized against a premeasured maximum voluntarily produced electrical signal (MVE). RESULTS: The measuring equipment did not interfere with the parachute jumps. High-quality signals were recorded. The median peak POS deceleration for the acc-neck vs. acc-rig indicated differences (4.0 G vs. 5.1 G), in addition to significant differences emerging for medians of average and maximum onset rates of deceleration (avg: 1.2 G x s(-1) vs. 19.5 G x s(-1); max: 23.0 G x s(-1) vs. 80.0 G x s(-1)). The median of overall muscleactivity was as high as 87% MVE, but no differences in peak activity were found between muscles. Temporal activations of the posterior upper neck occurred less than 50 ms after deceleration initiation, indicating feedforward control of the neck. CONCLUSION: All instruments recorded data of good quality without impeding the skydiving activity. Triaxial accelerometry on the neck vs. the harness yielded dissimilar results, underlining the importance of correct accelerometer placement. Muscle activity was high during POS and neck muscles showed anticipatory tendencies.