In Vivo Protocol of Controlled Subconcussive Head Impacts for the Validation of Field Study Data

Summary

The subconcussive soccer heading model is a safe and concise methodological approach to isolate and measure the effects of subconcussive head impacts.

Abstract

Subconcussive hits pose a threat to neuronal health as they have shown to induce neuronal structural damage and functional impairment without causing outward symptomology and appear to be a key contributor to an irreversible neurodegenerative disease, chronic traumatic encephalopathy (CTE). In addition, athletes can incur more than 1,000 of these hits per season. The subconcussive soccer heading model (SSHM) is a relevant, reproducible, and leading method of isolating and examining the effects of these subconcussive head impacts. By controlling variables such as ball traveling speed, the frequency of impacts, interval, ball placement to the head, as well as by measuring head impact magnitude, the SSHM provides the scientific community with a superior avenue of investigating the acute subconcussive effects on neuronal health. In this paper, we demonstrate the utility of SSHM in studying a time-course expression of neurofilament-light polypeptide (NF-L) in plasma in a repeated measures fashion. NF-L is an axonal injury marker that has previously been shown to be elevated in boxers and football players following subconcussive head trauma. Thirty-four adult aged soccer players were recruited and randomly assigned to either a soccer heading (n = 18) or kicking (n = 16) group. The heading group executed 10 headers with soccer balls projected at a velocity of 25 mph over 10 min. The kicking group followed the same protocol with 10 kicks. Plasma samples were obtained before and at 0 h, 2 h, and 24 h after heading/kicking and assessed for NF-L expressions. The heading group showed a gradual increase in plasma NF-L expression and peaked at 24 h after the heading protocol, whereas the kicking group remained consistent across the time points. These results confirmed the NF-L data from clinical field studies, encouraging the use of SSHM to validate clinical subconcussion data.

Introduction

Long-term, repetitive exposure to subconcussive head impacts has been proposed as one of the key contributors for developing the neurodegenerative disease CTE^1,2,3,4,5. Each year, approximately 2.5 million high school and college athletes engage in contact sports that frequently induce these subconcussive insults through rapid acceleration-deceleration of the body and head^6,7. Specifically, contact sport athletes may experience several 100 up to a 1,000 of such impacts per season^6,8,9. Additionally, other populations, such as military men and women, have registered more than 300,000 head injuries since 2001, which has manifested into the recent diagnosis of CTE within a retired military veteran¹⁰. This diagnosis parallels with 110 postmortem CTE brains of American football players and four postmortem soccer players to present a burgeoning public health issue^11,12. In light of the staggering prevalence, head impact research must shift its gaze to incorporate sound, precise methods of analyzing the acute debt subconcussive hits are inducing in a variety of arenas.

The SSHM presented here is one that satisfies the current methodological need to safely induce common mechanical stresses placed upon neural tissue during contact sport activities. The implementation of this model allows investigators to meticulously manage ball traveling speed, the frequency of impacts, interval, ball placement to the head, as well as measurements of head impact magnitude^13,14. While these factors are virtually impossible to control in the field setting, the SSHM provides an outlet for researchers to isolate the effects of subconcussive head impacts. Furthermore, through the elimination of confounding variables seen during play (e.g., effects from vigorous exercise, body damage, body temperature change, and hydration/perspiration), the SSHM provides a superior method of validating clinical observations.

The SSHM has direct parallels to head impacts seen specifically within the realm of sport. As such, the literature has already begun to show its utility and corroborate the findings of cumulative head impact burden from other investigators. For example, we have demonstrated that the burden of subconcussive head impacts significantly drive neuro-ophthalmologic dysfunction amongst soccer athletes^13,15. In addition, as few as 10 subconcussive impacts have been shown to immediately perturb vestibular function which can be normalized after 24 h of resting¹⁶. In this methodological report, we describe the application of SSHM to safely study the effects of subconcussive head impacts and introduce one of our findings that repetitive subconcussive head impacts gradually increase the concentration of a neuron-derived blood biomarker, namely NF-L¹⁴. This finding not only substantiates previous results of NF-L presence due to repetitive subconcussive blows to the head^17,18 but also validates that the SSHM can reproduce such findings in a controlled clinical manner.

Protocol

The authors verify that the Indiana University Institutional Review Board approved the study (protocol # 1610743422) and written informed consent was obtained.

NOTE: The SSHM was integrated into a repeated measures design that is intended to investigate changes among dependent variables within subjects at 0 h, 2 h, and 24 h post-intervention compared to individual pre-intervention values. This study design allows researchers to track changes for a 24 h period, which is the typical timeframe between athletic practices. In the present study, soccer players were randomly assigned to either a soccer heading (n = 18) or a soccer kicking group (n = 16).

1. Setup

1. Following baseline measurement collection (pre-intervention), begin the SSHM with the positioning of a soccer ball launcher approximately 40 ft away from the subject, as well as ensuring the soccer ball is inflated to 9 psi.
2. The face of the machine displays two identical dials, which regulate the speed of the left and right wheels, and an on/off switch in between them. Set both these dials to a standardized speed of choice.
   NOTE: For this demonstration purpose, the ball traveling speed has been set to 30 mph. This speed was chosen as it simulates a soccer throw-in from the sideline to midfield. Soccer players frequently perform this maneuver during practice and games.
3. Place 3 inch blocks underneath the wheels of the ball launcher to allow for the desired trajectory (no blocks needed for kicking subjects). Once this is complete, the ball launcher is then angled to 40°, which is measured as the angle between the ground and the midline of the rotating wheels.
   NOTE: The angle measurement is taken with a goniometer, and the machine can be adjusted after loosening a knob located along the blue rails where the soccer ball is loaded.
4. Once the soccer machine is properly set, fit the subjects with a triaxial accelerometer embedded within a head-band pocket and positioned directly below the external occipital protuberance (inion) to monitor linear and rotational head accelerations.
5. Start up the corresponding software for the accelerometer, and enter the subject’s information accordingly. At this point, the subject is ready to begin the familiarization trials of the intervention (heading, kicking).

2. Familiarization trials

1. Position the subject approximately 40 ft in front of the ball launcher.
2. Be sure to explain to the subject that the ball launcher will volley the soccer ball to them and that they simply need to simulate intervention contact with the ball (i.e., heading subjects will catch the ball with their hands in front of their forehead before head-to-ball contact is made, kicking subjects will “trap” the ball on the ground with their foot instead of volleying the ball back, and standing subjects will remain static and not make contact with the ball).
3. When the subject understands and feels ready, have the researcher turn the ball launcher on, load the soccer ball onto the blue rails, and finally push the ball into the rotating wheels after a 3-2-1 countdown.
4. After stopping the ball as described previously (step 2.2), have the subject roll the ball back.
5. Repeat steps 2.3 and 2.4 two to four additional times (no rest time required between) to ensure that the subject positioning is correct and interaction with the ball will be safe and controlled. This concludes the familiarization trials.

3. Intervention

1. Verbally confirm that the subject is ready. Once confirmed, give instructions to the heading subjects to only make forehead contact with the ball; tell the subjects to avoid potential impacts to the crown, parietal, and temporal lobes. Instruct the kicking subjects to kick the ball only while it is in flight as ball contact with the ground will attenuate the subsequent impact to the foot.
2. Instruct both heading and kicking subjects to volley the ball to a target (additional researcher) approximately half the distance between them and the machine. As best they can, ask subjects to do this in a manner that will mimic the arched trajectory the ball took during its flight toward the subject.
3. Activate the triaxial accelerometer and begin the recording.
4. Load the soccer ball onto the blue rails, push the ball into the rotating wheels after a 3-2-1 countdown, and make sure appropriate contact is made.
5. Repeat step 3.4 nine more times with a 60 s rest in between bouts. If the subject forgoes interaction with the ball (due to inopportune placement or suspected contact with the body in an area that must be avoided), then volley the ball to the subject again promptly, without any rest period.
6. In between each head contact with the ball, verify that the triaxial accelerometer registered an impact (using triaxial software; kicking subjects should not register G-force).
7. Once the intervention is concluded, turn off the ball launcher and stop the triaxial recording (important, as the movement required to remove the headband could record another “impact”). Once the recording has stopped, remove the headband.

Results

The results represented here were interpreted from a previous article¹⁴, in which the SSHM was utilized as previously described. In this particular study, we aimed to show how the SSHM could induce changes in plasma levels of NF-L, which is an axonal injury marker that is hypothesized to filter out of the cranium and into the peripheral blood following head impacts.

SSHM and head kinematics

Discussion

While contact sports like American football might appear to be driving the need for a concise research model to study subconcussive impacts, other sports like soccer may account for the dominant share of subconcussive exposure around the globe as approximately 265 million people participate in what is perhaps the world’s most popular sport¹⁹. However, while the majority of the suspected long-term neurodegenerative effects of subconcussion have been autopsied in American football players, the...

Materials

Name	Company	Catalog Number	Comments
JUGS Soccer Machine	JUGS Sports	http://jugssports.com/products/soccer-machine.html
SIM-G Triaxial Accelerometer	Triax Technologies	https://www.triaxtec.com/workersafety/wp-content/uploads/2017/08/SIM-G-User-Manual_V4-2-01.pdf