(C-87) Using wearable sensors to characterize neck muscle activation patterns during soccer heading at different kinematics levels

Introduction:: There is concern that repetitive sub-concussive impacts sustained from soccer heading may cause long-term neurological damage, as immediate and measurable alterations in brain function have been detected after a single heading session (1). It has been suggested that greater neck muscle strength and neck muscle activation may decrease the risk of brain injury. Tensing the neck muscles before and during head impacts may decrease head kinematics, which is deemed one of the most important indicators of brain injury risk (2). In support of this hypothesis, one study has found that neck strength is negatively correlated with head acceleration in soccer players (3), and that pre-tensing the neck muscles decreases head accelerations during non-injurious impacts (4). However, studies employing computer based musculoskeletal models have found that large superficial neck muscles do not significantly contribute to head stabilization during short, fast impacts (5). Despite this, few studies have investigated neck muscle activation and timing during real-world soccer heading.

In this study, we developed a multi-modality wearable sensor system to record synchronized measurements of head impact kinematics and electromyography (EMG) of the sternocleidomastoid muscles during soccer heading. Our objective is to characterize neck muscle activation patterns at different head impact kinematics levels, with the long-term goal of identifying muscle activation strategies that may contribute to improved head stabilization during impact.


Materials and Methods:: Wearable multi-modality sensor system: The participant was fitted with a mouthguard, to which a PCB 356A32 accelerometer and DTS ARS3 Pro gyroscope were secured via custom bite bar. The Mentalab ExplorePlus amplifier was used to collect left and right sternocleidomastoid EMG. Electrodes were placed 1/3 of the distance from the mastoid process to the sternal notch (4). The IMU and EMG data were synchronized using a custom trigger circuit that delivered a simultaneous, debounced signal with sub-millisecond resolution to the recording systems. We examined EMG data within a 2-second window around the impact kinematics peak for each header.

Heading protocol: A ball delivery system was used to deliver balls at low, medium, or high (approx. 7, 10 and 13 m/s), initial speeds. One participant performed three standing frontal headers at each speed. Prior to data collection the participant pushed maximally against a dynamometer to obtain their maximum voluntary isometric contraction (MVIC). The protocol was approved by the UBC Research Ethics Board (H21-00958).

Data analysis: Kinematic data were sampled at 2500Hz and filtered using a 4th order lowpass Butterworth filter at 300 Hz and 256 Hz respectively (6). We calculated resultant peak linear acceleration (PLA), peak rotational velocity (PRV) and peak rotational acceleration (PRA) for each impact. EMG data were collected at 2000Hz, bandpass-filtered using an 8th order Butterworth filter at 20-350 Hz, rectified, and low-pass filtered at 10Hz to obtain the envelope (7). The EMG data was normalized to MVIC.


Results, Conclusions, and Discussions:: Results: All kinematic parameters (PLA, PRV and PRA) increased with increasing impact speed. The magnitude of peak muscle activation also increased with increasing impact speed and was found to be significantly correlated with PLA (p< 0.05). The onset of muscle activation occurred approximately 500ms before peak impact kinematics for most headers (Figure 1). The time between the peak muscle activation and head impact decreased with increasing impact speed and muscle activation (Table 1, Fig 1).

Discussion: At low impact speeds peak EMG occurred before the impact ( > 40 ms), indicating that the most significant role of muscle activation was to pre-tense the neck and move the head into the intended heading position. However, at the highest impact speed peak EMG occurred close (< 30 ms) to the time of head impact, potentially indicating that neck muscle contraction increases to stabilize the head. During the highest impact conditions, peak EMG for both the right and left sternocleidomastoid muscles were above the MVIC (%). This may indicate that the participant is able to further activate their neck muscles beyond the voluntary contraction level. However, the impact levels we tested represent low-severity impacts based on field data (8), and it is likely that there exists a limit to muscle activation increase, beyond which neck tensing may no longer be an effective head stabilization strategy.

Conclusion: We have collected a novel set of synchronized head kinematics and neck muscle activation data during real-world soccer headers using a custom-developed sensor system. We show that neck muscle activation is highly correlated with impact speed during mild soccer heading, and that neck muscle tensing may play a role in head stabilization at the tested impact levels. A limitation of the study was the small sample size and low range of impact speeds. In the future, we intend to test additional participants with a larger range of ball speeds. We would like to investigate if there is a threshold speed above which neck activation can no longer increase, to determine heading conditions that may pose a greater risk for brain injury.


Acknowledgements (Optional): :

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