Decelerations and muscle responses during parachute opening shock

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. 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). 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. 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.