Statistical Analysis

Repeated measures ANOVA (RMANOVA) was used to compare mean response variables between groups (i.e. EXP vs. CON) across different time points (i.e. PRE, POST-0h, POST-24h). Multivariate Wald test was computed and compared to the reference chi-squared distribution to test for the interactions between group and time. An unstructured covariance matrix was specified for underlying correlated measures across time points.

Statistical analyses were carried out in SAS (SAS Institute Inc., Cary, NC). For all tests, we used α = 0.05 as a threshold for statistical significance.

The outcome measures included the mean peak response to GVS for each participant for each balance variable computed (CoM-CoP Separation, Foot

Placement, Mediolateral Ankle Roll, Push Off, and Hip Adduction). For the SCAT5

21

we calculated the mean score of symptoms, symptoms severity, orientation, immediate memory, concentration, balance errors and delayed recall.

Statistical analyses were carried out in SPSS. For all tests, we used α = 0.05 as a threshold for statistical significance.

2.4 Results

Figures 3 to 11 show the balance responses to GVS in both groups across three time points. There were no significant group x time interaction effects for any of the balance mechanism response variables in the studied balance mechanisms.

The high variability observed in the balance mechanisms (table 1) was expected as observed with step width and step length in previous study (McLellan 2006).

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Table 1. Balance mechanisms means and standard deviations

Measurement Pre mean ±SD Post 0H mean ±SD Post 24h mean ±SD Control Heading Control Heading Control Heading Ankle eversion

Foot placement strategy: foot placement change, F=0.563, p=0.574, η2=0.030;

hip abduction change, F=0.038, p=0.963, η2=0.002; integrated gluteus medius EMG change, F= 0.537, p=0.589, η2=0.029.

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Figure 3. Foot placement strategy: foot placement, hip abduction and gluteus medius activity across 3 sessions (pre, post 0h and post 24h).

Ankle roll: integrated relative CoP change, F=0.311, p=0.734, η2=0.017; ankle inversion change, F=1.094, p=0.346, η2=0.057; integrated peroneus longus EMG change, F=0.305, p=0.739, η2=0.017.

Figure 4. Ankle roll strategy: CoM – CoP separation, ankle inversion and peroneus longus activity across 3 sessions (pre, post 0h and post 24h).

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Push off: step length change, F=0.909, p=0.412, η2=0.048; ankle plantar flexion change, F=0.610, p=0.549, η2=0.033; integrated medial gastrocnemius EMG change, F=0.547, p=0.583, η2=0.029.

Figure 5. Push off strategy: step length, ankle plantar flexion and medial

gastrocnemius activity across 3 sessions (pre, post 0h and post 24h).

There was no significant difference group x time interaction (F 2, 36 =1.022, p=0.370) to any of the variables collected with SCAT 5. The post priori power analysis resulted in a power of 0.830 and above for the main variables of interest demonstrating a sufficient sample size for all hypothesis testing.

2.5 Discussion

While the acute effects of concussion have been well characterized in the literature, the effects of RSHI on neurological function is poorly understood with some studies reporting functional impairments following RHI and others observing no deficits (Gysland 2012, Breedlove 2012; Lipton 2013; Talavage 2014; Montenigro 2017; Stewart 2018; Sollmann 2018, Caccese 2019). We aimed to quantify changes in

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neurological function through the assessment of vestibular processing and balance during walking following a controlled soccer heading paradigm. We hypothesized that individuals would have diminished balance responses to GVS during walking

immediately following the controlled soccer heading paradigm and that these balance responses would recover within 24 hours. However, our findings do not support our hypothesis and we did not observe any evidence of changes in balance mechanism response variables as a result of the RSHI paradigm. We observed substantial

variability in how individuals use these balance mechanisms when walking on a foam surface which was expected and previously described by MacLellan (2006). Although the observed balance mechanisms allow for dynamic control in response to balance perturbations, the variability within and across individuals makes it difficult to identify systematic changes during walking attributable to the RSHI.

Previous work identified diminished gain to GVS while standing with eyes closed on foam, suggesting that postural vestibular processing was disrupted following RSHI (Hwang 2017). However gains to GVS during standing are small, which makes changes in gains more difficult to identify and interpret. In healthy adults, vestibular information plays a greater role in tasks in which the relationship between the CoM and base of support is dynamic, such as during locomotor tasks (Bent 2005). For example, in response to GVS, healthy adults modulate their CoM-CoP separation about 2.5mm and their foot placement about 15mm in the direction of the perceived fall while walking along a firm walkway (Reimann 2017). There are three primary walking balance mechanisms investigated in our study which collectively are described as a stepping strategy to control subsequent foot placement. The foot placement is created by the variables foot placement, hip abduction and gluteus

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medius. The second one is the lateral ankle roll, which is the mechanism responsible for controlling the center of pressure under the stance foot. Lateral ankle roll strategy is the combination of the center of mass and center of pressure separation, the ankle eversion angle and peroneus longus activity. The last strategy is the push-off

mechanism that encompasses step length, ankle plantar/dorsiflexion and medial gastrocnemius activity (Reimann 2017). When comparing to Reimann (2017) our participants had a similar CoM-CoP separation response, but a much greater foot placement response, which may be a result of walking along a foam walkway instead of a firm walkway (MacLellan 2006). Although the vestibular contributions to maintaining balance during walking are larger than during standing, humans have several mechanisms available to maintain balance during walking (e.g. foot placement, mediolateral ankle roll, push off). These complementary mechanisms allow for

dynamic control in response to balance perturbations, yet make it difficult to identify changes in vestibular processing and balance during walking because of high

variability both within participants across trials and across participants. Therefore, balance responses to GVS are not sensitive enough to identify the subtle, transient changes in vestibular processing following RSHI.

The SCAT5 is broadly and successfully used for concussion assessment (Echemendia et al. 2017). Our studied population was not acutely concussed

(exclusion criteria was any concussion in the past 6 months) and it was expected that we wouldn’t find a difference between the three sessions. In addition, the participants were soccer practitioners used to perform soccer headings. Therefore it was not surprising that this cohort did not demonstrate any behavioral balance signs of impairment after the RSHI protocol that mirrors their sport participation.

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Limitations of this study included analyzing only the balance response to GVS.

Typical responses to GVS during walking are a combination of balance responses and deviation of the walking path, and previous work in healthy adults has suggested that the navigation response is at least partially decoupled from the balance response (Bent 2000; Bent 2005). The length of the foam walkway and the position of the force plates within the lab limited the space available for assessing the deviation of the walking path; however, incorporating measures of both balance response to GVS and deviation of the walking path should be considered in future research. This study was the first to use GVS to probe vestibular function during walking following head impact.

Therefore, we do not know how these balance mechanisms would change with greater exposure to RSHI [in this study participants only completed 10 controlled soccer headers], or with a more severe head impact, such as after a diagnosed concussion. In addition, we speculated that because all the participants were soccer athletes that are used to routinely performing soccer headings and at baseline may be different from non-soccer players. Significant functional deficits associated with RSHI should be placed in the context of frequency and magnitude of head impact and with respect to other clinical measures or biomarkers of head injury. Finally, human walking balance is a complex behavior and the fundamental properties of these balance mechanisms are still being investigated. Future research may determine the interdependence of balance mechanisms, which may provide additional insight in quantifying deficits in populations with diminished balance control, in light of the large variability within and across participants.

28 2.6 Conclusion

Although previous work demonstrated an effect to soccer headings in quiet stance, our results suggest that an acute bout of soccer headings does not indicate a balance deficit during walking. More research is necessary considering subconcussive head impact frequencies and different sports population.

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THE EFFECT OF SOCCER HEADING IN SENSORY REWEIGHTING IN STANDING BALANCE

3.1 Abstract

An important component of postural control is a complex, dynamic interaction of multiple sensory systems which allow humans to maintain balance despite changes in the environment or neurological state. Sensory reweighting is the process of

dynamic sensory regulation of balance control. The aim of this study is to investigate the effects of purposeful soccer heading on sensory reweighting during quiet stance in collegiate athletes. Thirty amateur adult soccer players were randomly assigned into two groups, soccer heading (EXP) and control (CON). Both groups underwent a clinical assessment (SCAT5) and a standing balance assessment. Subjects were tested across three sessions: baseline (PRE), immediately following soccer heading (POST-0h), and 24 hours following soccer heading (POST-24h). A standing balance

assessment, designed to simultaneously test all three sensory modalities -

somatosensory, vision, and the vestibular system was administered in all sessions.

Gains for leg and trunk angles relative to each modality were calculated and a

RMANOVA was used to compare means between groups across the three time points.

There were no changes in gain to vision, vibration, and GVS due to exposure to mild head impact. The results of this study suggest that although there may be a disruption in vestibular processing following RSHI, this disruption does not lead to measurable changes in quiet standing balance and sensory reweighting remains unaltered.

Chapter 3

30 3.2 Introduction

In soccer, purposeful heading is integral and frequent in both practice and in competition. Soccer headers can be characterized as repetitive head impacts (RHI) that do not result in acute clinical signs and symptoms of concussion (Bailes 2013).

Current thinking views sub-concussion as an under-recognized phenomenon that has the ability to cause significant current and future detrimental neurological effects, although studies reporting these effects are inconclusive (Tarnutzer et al 2016).

Previous research suggests that alterations in vestibular function may impair postural control during standing following an acute bout of soccer heading (Hwang et al., 2017). Deficits in postural control may lead to an increased risk of lower extremity injury following return to play (Howell 2015). Postural control impairments from sub-concussive head impacts may have similar consequences.

Postural control is achieved due to a complex dynamic interaction of multiple sensory systems (Horak 1996, 2006). This interaction between the somatosensory, visual, and vestibular systems allows humans to maintain balance despite changes in the environment. Dynamic sensory regulation is called sensory reweighting (Hwang 2014, Peterka 2002) and allows humans to balance in the presence of changing environmental or neurological conditions. Sensory reweighting can also be manipulated in a laboratory setting and a persistent adaptation process to sensory stimuli can be noticed after appropriate training (Hwang 2014; Allison 2006).

There are deficits in sensorimotor function following mild traumatic brain injury (mTBI) or concussion (Galea et al. 2018). Previous research has suggested that even repetitive subconcussive head impacts may lead to subtle balance disturbances during standing (Hwang 2017). Although previous studies have found no acute effect

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in postural balance post soccer headings with eyes open or closed and in a foam surface (Mangus 2004, Broglio 2004) other research demonstrated vestibular dysfunction following subconcussive impact. Hwang et al (2014) found diminished sway response to galvanic vestibular stimulation (GVS) while standing with eyes closed on foam after mild head impact. This disruption in postural vestibular

processing could be an underlying mechanism of balance problems after head impact.

To investigate how purposeful soccer heading disrupts sensory feedback and effects balance control in quiet stance, we used a soccer-heading model that controls head impact number, magnitude, and direction. The model consists of controlled soccer heading while assessing head impact kinematics and uses a sophisticated approach to characterize balance mechanisms disrupted post-heading. Ball speed and direction are controlled, and experienced soccer players simply stand and perform headers to control head impact location and direction (Hwang 2017, Caccese 2018).

The purpose of this study was to investigate the effects of purposeful soccer heading on sensory reweighting during quiet stance. We hypothesized that individuals would have diminished gains to GVS immediately following the controlled soccer heading paradigm that would be restored approximately twenty-four hours post heading. In addition, visual and proprioceptive processing would remain unaltered throughout the sessions.

3.3 Methods

3.3.1 Participants

Thirty amateur adult soccer players (15 males, 15 females, 21.8 ± 2.8 years, 69.9 ± 11.5 kg, 171.4 ± 8.2 cm) were randomly assigned into two groups, soccer

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heading (EXP) and control (CON) from the Newark, Delaware region volunteered for participation. All participants were active soccer players (i.e., collegiate, intramural, club) who were field players (i.e. not goalkeepers) and had at least 5 years of soccer heading experience. The exclusionary criteria were: any head, neck, face, or lower extremity injury in the six months prior to participation; pregnancy; history of balance problems or vestibular dysfunction; currently taking any medications affecting

balance; any neurological disorders; unstable cardiac or pulmonary disease;

goalkeepers. The University of Delaware institutional review board approved the study and participants provided written informed consent. Participants were instructed to abstain from performing soccer headings in between the sessions.

3.3.2 Experimental Design

The experiment used a repeated measures design across three time points (pre-heading, 0-hours post-(pre-heading, 24-hours post-heading) (Hwang et al., 2017). At each time point, participants completed a clinical assessment (SCAT) and a standing balance assessment, following the protocol described in the Standing Balance Assessment section. The pre-heading session (PRE) was a baseline measurement.

After approximately 24 hours, participants performed 10 headers following the protocol described in the Soccer Heading Paradigm section below. The same measurements were performed immediately following the heading (POST-0h) and then approximately 24 hours later (POST-24h).

3.3.3 Soccer Heading Paradigm

A controlled soccer heading paradigm was used as an in-vivo model of mild mechanical head impact (Higgins et al 2009). Soccer balls (size 5, 450 g, inflated to 8

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psi) were projected using a JUGS soccer machine (JUGS Sports, Tualatin, OR); the initial velocity was 11.2 m/s (25 mph), the angle of projection was 40 degrees, and the distance to the participant was approximately 12 m (40 ft) (Higgins 2009, Haran 2013, Caccese 2017, Caccese 2017, Caccese 2018). EXP participants performed 10 standing headers in 10 minutes (1 header per minute), while CON participants did not perform any soccer heading.

3.3.4 Clinical Assessment

In each session, subjects were administered the Standard Concussion Assessment Tool 5 (SCAT5), a standardized tool to aid evaluation of sign and

symptoms of concussion, which included the symptom checklist, cognitive screening (orientation, immediate memory, and concentration), balance examination (BESS), and delayed recall.

3.3.5 Standing Balance Assessment

Participants were instructed to stand upright looking straight ahead while their visual, somatosensory, and vestibular systems were perturbed as shown in Figure 6.

The visual feedback perturbation consisted of an oscillatory translation at 0.2 Hz of 500 3D pyramids randomly distributed projected on the surface of a dome that

surrounded the participant for 180 degrees of visual angle. Each pyramid had 30 cm of height and was projected about 10 meters from the subject base of support. The visual translation had two conditions: a low amplitude vision condition where the objects translated 20 centimeters and a high amplitude vision condition where the objects translated 80 centimeters. A pair of 20mm vibrators where strapped on each Achilles tendon, vibrating at an amplitude of 1 mm and frequency of 80 Hz based on a square

34

wave with equal on and off durations corresponding to a frequency of 0.28 Hz. To perturb the vestibular system, Galvanic Vestibular Stimulation (GVS) was

administered to evoke anterior-posterior sway. Binaural, bipolar GVS was delivered from two round electrodes with 3.2 cm diameter (Axelgaard Manufacturing Co., Fallbrook, CA, USA). A custom LabVIEW program (National Instruments Inc., Austin, TX, USA) generated an analog voltage, which was transformed into a square wave of 1 mA current using the neuroConn DC-Stimulator Plus (neuroCare Group, Munchen, Germany). Electrodes were placed bilaterally on both the mastoid processes and each scapula region approximately at the same height as the T2 spinous process.

The GVS stimulation was the same for both sides and consisted of ±1 mA as a sinusoidal wave at 0.36 Hz. (Hwang et al 2014, 2017).

The trials were randomized in four conditions of different combinations of sensory input. Condition one was a low vision, vibration, and GVS (LVG); condition two was a low vision and GVS (LG); condition three was a high vision, vibration, and GVS (HVG); and condition 4 was a high vision and GVS (HG). A total of twenty trials of 135 seconds were collected, five trials per condition. Throughout all trials, participants wore a harness to prevent falling, although no subjects lost balance during the experiment. Gain and phase of leg and trunk segments displacement related to each condition were calculated (refer to the data analysis section).

Twelve reflective markers were placed bilaterally on the temple (head), acromion (shoulder), great trochanter (hip), lateral femoral epicondyle (knee), lateral malleolus (ankle), and first metatarsal (foot). Kinematics were collected at 120 Hz using a thirteen-camera optical motion analysis system (Qualisys, Goteborg, Sweden).

35 Figure 6. Standing Assessment representation

3.3.6 Data Analysis

All the data collected was processed and analyzed in Matlab (MathWorks Inc.). The gain between each sensory input for leg and trunk segment displacements were calculated between groups and across days. The leg segment was defined by anteroposterior movement of the hip and ankle markers, and the trunk segment was defined by the anteroposterior movement of the shoulder and hip markers. Gain is the amplitude of the output (postural sway) divided by the amplitude of the input (sensory perturbation) at each driving frequency. To calculate gain we applied the frequency response function (FRF) analysis that is defined by the cross-spectral density divided by the power spectral density of the input. For example, if the gain to the GVS input

36

equals one, it means that the amplitude of the segment displacement (output) and the GVS perturbation (input) at the driving frequency are the same. Phase is a measure of the temporal relationship between the input and output; the output may lead the input (positive values) or lag behind it (negative values) (Hwang et al 2014).

3.3.7 Statistical Analysis

Repeated measures MANOVA (RMANOVA) was used to compare mean response variables between groups (i.e. EXP vs. CON) across different time points (i.e. PRE, POST-0h, POST-24h). Multivariate Wald test was computed and compared to the reference chi-squared distribution to test for the interactions between group and time. An unstructured covariance matrix was specified for underlying correlated measures across time points. Statistical analyses were carried out in SAS (SAS Institute Inc., Cary, NC). For all tests, we used α = 0.05 as a threshold for statistical significance.

3.4 Results

The clinical assessment (SCAT5) presented no significant differences in group x time (F2, 36=1.022, p=0.370) interactions.

3.4.1 Standing Balance Assessment – Leg AP Displacement

There were no changes in AP leg segment gain to vision (i.e. session X group effect; F=0.798, p=0.455, η2=0.028), AP leg segment gain to GVS (F=0.246, p=0.782, η2=0.009), or AP leg segment gain to vibration (F=0.662, p=0.520, η2=0.023) (Figure 1). In addition, there were no changes in sensory reweighting across any modality (i.e.

session X condition X group effect; vision, F=0.430, p=0.858, η2=0.015; GVS, F=0.763, p=0.600, η2=0.027; vibration, F=0.430, p=0.653, η2=0.015) (Figure 7).

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3.4.2 Standing Balance Assessment – Trunk AP Displacement

There were no changes in AP trunk segment gain to vision (i.e. session X group effect; F=0.490, p=0.615, η2=0.017), AP trunk segment gain to GVS (F=0.205, p=0.815, η2=0.007), or AP trunk segment gain to vibration (F=0.624, p=0.539, η2=0.022). In addition, there were no changes in sensory reweighting across any modality (i.e. session X condition X group effect; vision, F=0.395, p=0.881, η2=0.014; GVS, F=0.906, p=0.492, η2=0.031; vibration, F=0.761, p=0.472, η2=0.026) (Figure 7).

Soccer Heading Control

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Figure 7. Gains of the soccer heading and control group for GVS, vibration and vision

3.5 Discussion

We examined the effects of purposeful soccer heading on sensory reweighting during quiet stance in collegiate athletes before soccer heading (baseline), immediately

Soccer Heading Control

39

post heading, and twenty-four hours post heading. Although a previous study using a similar soccer heading protocol had shown a diminished gain to GVS immediately after soccer heading, our study results found no statistical difference in postural gain to any sensory modality (GVS, vibration, and visual input).

As expected, we did not observe any significant changes in the SCAT5. The SCAT5 has been shown to be sensitive to detect acute concussion (Echemendia et al.

2017); however, the RSHI experienced in this protocol was likely too small to alter balance or cognition in these conditioned soccer athletes. Although the participants

2017); however, the RSHI experienced in this protocol was likely too small to alter balance or cognition in these conditioned soccer athletes. Although the participants