Repetitive head impacts are a complex public health problem. This protocol is a safe and reproducible method for validating whether clinical findings are due to head impacts or noise factor. From ball speed and impact placement to exercise effect and body temperature, this model gives researchers a comprehensive method for studying subconcussive head impacts.
This model allows researchers to validate clinical findings beyond acute profiles of brain-derived blood biomarkers and be used to investigate perturbations in ocular motor function, the vestibular system, and more. During play, the head makes contact with the ball in various ways. Therefore, it's important to visualize what is meant by soccer heading model as described in the literature.
Following baseline measurement collection pre-intervention, position a soccer ball launcher approximately 40 feet away from the subject. And confirm that the soccer balls to be used for the trial have been inflated to nine pounds per square inch. The face of the machine displays two identical dials that regulate the speed of the left and right wheels with an on/off switch in between.
Set both of the dials to a standardized speed of choice and place three inch blocks under the wheels of the ball launcher to allow the desired trajectory. When the last block has been placed, angle the launcher to 40 degrees between the ground and the midline of the rotating wheels. Next, fit the subject with a triaxial accelerometer embedded with a headband pocket positioned directly below the external occipital protuberance to monitor the linear and rotational head accelerations.
Before beginning the familiarization trials, explain to the subject that the ball launcher will volley the soccer ball and that contact with the ball simply needs to be simulated for the trials. Next, explain to the subject how contact should be made during the intervention trials. For example, heading subjects should only make forehead contact with the ball, avoiding potential impacts to the crown, parietal, and temporal bones.
Lastly, kicking subjects should make lower limb contact with the ball while it is in flight. For example, heading subjects will catch the ball in front of the forehead before head to ball contact is made. And Kicking subjects will trap the ball on the ground with the foot instead of volleying the ball back.
When the subject is ready, turn on the ball launcher and load a soccer ball onto the blue rails. Next, count down from three to one and push the ball into the rotating wheels. The subject should stop the ball as instructed.
Then, have the subject roll the ball back and repeat the trial two to four more times to be sure that the subject positioning is correct and that the interaction with the ball will be safe and controlled. Once the subject feels comfortable with how proper contact is to be made, you may begin the intervention trials. If needed, reiterate to the subject what proper contact entails as seen previously.
Next, instruct both heading and kicking subjects to volley the ball back to the target researcher approximately half the distance to the machine. Mimicking the trajectory of the ball during its flight towards the subject as closely as possible. Load the soccer ball onto the blue rails and push the ball into the rotating wheels after a three, two, one, countdown, making sure that an appropriate contact is made.
Then repeat the intervention nine more times with a 60 second rest between bouts. When the intervention has concluded, turn off the ball launcher and stop the triaxial recording before removing the headband. These representative data were derived from 34 subjects.
There were no significant differences in any demographic characteristics between the groups. The head impact kinematic data revealed that the heading group experienced a median linear head acceleration of 31.8 G force per head impact and a median rotational head impact of 3.56 kilo radians per second squared per head impact. In contrast, the control kicking group did not demonstrate detectable levels of head acceleration.
There is a gradual increase in plasma neurofilament light polypeptide expression every hour after 10 headers in the kicking subjects with no similar significant time effect for the kicking control group. Follow up analysis further revealed that a significant difference appeared at 24 hours post-heading compared to pre-heading and compared to the kicking control group 24 hours after the intervention. Familiarization of the protocol will ensure the participant's safety and uniformed impact.
Therefore, it is important to both describe and have each participant practice the heading technique until they are comfortable. This protocol is a powerful tool that can be used to validate neuroimaging, auditory responses, and protective gear assessment, as well as clinical trials on therapeutic factors.
