External load training does not alter balance performance in well-trained women

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1 Sports Biomechanics ISSN: (Print) (Online) Journal homepage: External load training does not alter balance performance in well-trained women Jeffrey D. Simpson, Brandon L. Miller, Eric K. O Neal, Harish Chander & Adam C. Knight To cite this article: Jeffrey D. Simpson, Brandon L. Miller, Eric K. O Neal, Harish Chander & Adam C. Knight (2017): External load training does not alter balance performance in well-trained women, Sports Biomechanics To link to this article: Published online: 21 Jul Submit your article to this journal View related articles View Crossmark data Full Terms & Conditions of access and use can be found at Download by: [Mississippi State University Libraries] Date: 21 July 2017, At: 07:48

2 Sports Biomechanics, External load training does not alter balance performance in well-trained women Jeffrey D. Simpson a, Brandon L. Miller a, Eric K. O Neal b, Harish Chander a and Adam C. Knight a a Department of Kinesiology, Mississippi State University, Mississippi State, MS, USA; b Department of Health, Physical Education and Recreation, University of North Alabama, Florence, AL, USA ABSTRACT This study investigated the influence of external load training (ELT) on static and dynamic balance. Nineteen females stratified into two groups (ELT = 9, control = 10) completed three testing sessions over 6 weeks. The ELT group wore weighted vests (WV) of ~8% body mass for 32 h/week during daily living and three training sessions/week for 3 weeks. Following completion of ELT, a 3 week detraining (DET) phase was completed. Bilateral and unilateral static balance were assessed with eyes open and closed. Dynamic balance was assessed using the star excursion balance test (SEBT). Static and dynamic balance variables were analysed using a 2 (group) x 3 (time) between participants repeated measures ANOVA (p < 0.05). Results revealed significant reductions in average centre of pressure (COP) velocity in the control group on the non-dominant limb with eyes closed, and significantly greater reach distances in the ELT group on the SEBT for the posteromedial and medial directions on the dominant limb (p < 0.05). These findings suggest the ELT group did not significantly improve their balance in comparison to the control group. However, future research should further examine this unique, supplemental training method and the impact on balance performance. ARTICLE HISTORY Received 26 September 2016 Accepted 6 June 2017 KEYWORDS Star excursion balance test; unilateral balance; weighted vest Introduction Balance is considered to be a critical component of athletic performance and injury prevention (Chander & Dabbs, 2016; Plisky, Rauh, Kaminski, & Underwood, 2006). Acute non-contact musculoskeletal injuries to the lower extremity account for approximately 40% of injuries sustained during sports related activities (Hrysomallis, McLaughlin, & Goodman, 2007; McGuine & Keene, 2006; Plisky et al., 2006). Research indicates that female athletes are 4 6 times more likely to sustain musculoskeletal and ligament injuries to the lower extremity with marked evidence attributing such injuries to impaired balance performance (Filipa, Byrnes, Paterno, Myer, & Hewett, 2010; Holden, Boreham, Doherty, Wang, & Delahunt, 2014; Paterno, Myer, Ford, & Hewett, 2004). Therefore, balance assessments are warranted to identify possible balance deficits in female athletes (Bressel, Yonker, CONTACT Jeffrey D. Simpson jds1313@msstate.edu 2017 Informa UK Limited, trading as Taylor & Francis Group

3 2 J. D. SIMPSON ET AL. Kras, & Heath, 2007; Chander & Dabbs, 2016; Chander et al., 2014; Holden et al., 2014; Knight, Holmes, Chander, Kimble, & Stewart, 2016), which will further assist in developing effective training programs designed to enhance balance and reduce potential risks of lower extremity injury (Filipa et al., 2010; Paterno et al., 2004; Plisky et al., 2006). Multiple investigations have examined the influence of various training modalities on improving static and dynamic balance, with reports that improved balance enhances performance and reduces the potential risk of injury (Bruhn, Kullmann, & Gollhofer, 2004; Filipa et al., 2010; Holm et al., 2004; Paterno et al., 2004; Rasool & George, 2007). Filipa et al. (2010) reported that high school female soccer players participating in 8 weeks of lower extremity strength and core stability training significantly improved reach distance on a modified star excursion balance test (SEBT), whilst no changes were reported in the control group who received no additional training outside of normal activities. Holm and colleagues (2004) concluded that 7-weeks of injury prevention training focused on preventing anterior cruciate ligament (ACL) injuries resulted in a 15.8% improvement in dynamic balance using a stability platform pre- to post-intervention, whilst failing to elicit any changes in single leg static balance in a group of elite female handball players. Additionally, Paterno et al. (2004) reported that a group of high school female athletes demonstrated significant improvements in anterior/posterior single leg stability following 6 weeks of ACL injury prevention training. Additional studies that examined alternative training programs focusing on plyometric (Myer, Ford, Brent, & Hewett, 2006) and resistance training (Bruhn et al., 2004) have reported significant improvements in single leg balance performance. External load training (ELT), which consists of wearing a weighted vest (WV) during daily living activities and training, is an alternative training method commonly used by athletes seeking improvements in high intensity task performance. A seminal investigation conducted by Bosco and colleagues (1984) reported that 3-weeks of ELT significantly improved explosive jumping characteristics in a treatment group of internationally competitive athletes, whilst no improvements were reported in the control group. The authors (Bosco et al., 1984) postulated that the observed changes in explosive jumping characteristics were a result of the greater muscular forces required to overcome the additional load carried during waking hours to maintain an upright posture. Thus, the WV provided the main stimulus to elicit adaptations such as improved recruitment rates of fast-twitch motor units and increased strength in lower extremity extensor muscles (Bosco et al., 1984). The findings by Bosco and colleagues (1984) were further supported in two subsequent studies using ELT in elite level athletes, which both reported significant improvements in jumping performance following 3 weeks of ELT (Bosco, 1985; Bosco, Rusko, & Hirvonen, 1986). Additional studies examining this training method have shown similar improvements in performance (Lowe et al., 2016; Sands et al., 1996; Scudamore et al., 2016). However, a study conducted by Barr and colleagues concluded that 8 days of ELT was not sufficient to elicit significant improvements in performance (Barr, Gabbett, Newton, & Sheppard, 2015). Whilst previous studies have reported improvements in athletic performance following long term ELT, which were likely due to an increase in muscular strength and motor unit recruitment (Bosco, 1985; Bosco et al., 1984, 1986; Lowe et al., 2016; Sands et al., 1996; Scudamore et al., 2016), it is possible that improvements may be seen in both static and dynamic balance if muscular strength improves following ELT, particularly in dynamic balance (Filipa et al., 2010; Plisky et al., 2006).

4 SPORTS BIOMECHANICS 3 Improvements in static and dynamic balance can potentially enhance performance and reduce the potential risk of injury (Filipa et al., 2010; Paterno et al., 2004; Plisky et al., 2006). Previous studies indicate ELT interventions can result in significant improvements in lower extremity strength and athletic performance (Bosco, 1985; Bosco et al., 1984, 1986; Sands et al., 1996; Scudamore et al., 2016). However, the scope of previous studies have not included examination of the potential impact of ELT on static and dynamic balance. Thus, the aim of this study was to determine the impact of 3-week ELT on static and dynamic balance in well-trained females that regularly participated in resistance and high intensity training. Previous research indicates that ELT can result in increased lower extremity strength, which could positively impact balance performance. Therefore, the authors hypothesised that wearing a WV during activities of daily living and training for three weeks would result in significant improvements in measures of static and dynamic balance. Moreover, we also hypothesised that 3 weeks removal of the WV would mitigate any observed improvements in static and dynamic balance performance. Methods Participants Initially, 21 participants were recruited for participation in the study, but 2 participants sustained injuries unrelated to study procedures and did not complete all round of testing. Therefore, 19 healthy collegiate aged female participants (age: 21.0 ± 2.0 years; height: ± 3.4 cm; mass: 64.2 ± 5.9 kg) that were free from any musculoskeletal injury and were physically active, participating in resistance and/or high intensity interval training 4 days per week for the previous 6 months, were recruited and volunteered to participate in the study. The study was approved by the Mississippi State University Institutional Review Board and all participants read and signed the informed consent to participate in this study. Sample size estimation was determined a priori based on previous static and dynamic balance data from our laboratory, using G-power software with a desired power of 0.8 and effect size of 0.50, using an alpha level of Height was measured using a stadiometer (Webb City, MO), mass was recorded using a physician s scale (Tanita Corporation, Tokyo, Japan) and body fat percentage was estimated using Lange skinfold calipers (Cambridge, MD, USA) with the 3-site method (tricep, superilliac, thigh) (Pollack, Schmidt, & Jackson, 1980). Experimental procedures Following collection of required documents and anthropometric assessments, all participants completed a familiarisation trial. During the familiarisation trial, the testing protocol was explained and each participant was allowed to practice each balance task (described below) as many times as desired to reduce any potential learning effects. During the familiarisation trial participants were stratified into ELT (n = 9) and control groups (CON) (n = 10) based off their performance on the single countermovement jump and 25-m sprint, which are commonly used field tests to assess athletic performance (Vescovi & Mcguigan, 2008). After completion of the familiarisation trial, participants returned to the lab less than a week later to complete their baseline trial (session one). Following completion of the baseline trial, participants then commenced the ELT phase of the investigation that lasted three weeks.

5 4 J. D. SIMPSON ET AL. Participants in the ELT group were provided with a WV (Ironwear Fitness, Pittsburgh, PA, USA) loaded with ~8% individual body mass (5.3 ± 0.3 kg) that was evenly distributed on the anterior and posterior portion of the upper torso. The ELT group was instructed to wear the WV 4 days per week, 8 h per day during daily activities and 3 training sessions each week. The load of the WV used in this study was selected based on seminal work conducted by Bosco et al. (1986) using a small group of female athletes. During the study participants in the CON group were instructed to continue routine training, but were restricted from wearing a WV. After completion of the ELT phase, participants returned to the lab for a post-treatment session (session two). Following completion of the second testing session, participants then completed the detraining (DET) phase of the investigation, which lasted three weeks. During the DET phase, the ELT group was instructed to continue normal training regimens but was restricted from wearing WVs. After completion of the DET phase, both groups returned to the lab for their final round of testing (session three). Activity logs were provided to monitor participants training frequency, intensity, and modalities in both groups for the duration of the study. Balance assessments The first set of tasks completed was bilateral and unilateral static balance assessments. Bilateral (BL) and unilateral static balance were assessed using a portable AMTI (Watertown, MA, USA) AccuGait force platform sampling at 100 Hz. During the bilateral assessment, participants were instructed to stand in the centre of the force platform on both feet shoulder width apart with their eyes facing forward and hands placed at their side. Unilateral assessments were completed on the dominant (DL) and non-dominant limb (NDL) and participants were instructed to stand with their testing foot in the centre of the force platform, with the contralateral hip and knee flexed, eyes facing forward and hands placed at their side. A total of three trials lasting 20s with eyes open (EO) and three trials with eyes closed (EC) were completed in the bilateral stance. For the unilateral stance condition, three trials on the DL and NDL were completed lasting 20s with the eyes open and lasting 10s with the eyes closed. If participants were unable to maintain their balance for the duration of the trial it was marked as a failed attempt and the trial was repeated. The duration of the EC condition in the unilateral stance was selected based off previous work by Knight et al. (2016) assessing static balance in athletic populations. The order of bilateral and unilateral assessments with EO and EC were completely randomised using Excel (Microsoft Corporation, Redmond, WA, USA). Data from the force platform was collected on a laptop computer and analysed using AMTI s BioAnalysis software. Centre of pressure (COP) variables were recorded from the force platform and the average displacement of the COP in medial/lateral (ML) and the anterior/posterior (AP) directions (Equations 1 & 2), average COP velocity (cm/s) (equation 3), and 95% ellipse area (cm 2 ) were calculated using the following equations: Avg. ML Displacement = Avg. AP Displacement = N i=1 x i AVG x N N i=1 y i AVG y N (1) (2)

6 SPORTS BIOMECHANICS 5 Figure 1. Participant completing the star excursion balance test (SEBT). where L Avg.COP Velocity = N Δt n (xi ) 2 ( ) 2. (L) = x i 1 + yi y i 1 i=2 (3) Average COP displacements in the ML and AP directions were calculated by taking the sum of the absolute value for the initial x (x i ) and y (y i ) subtracted by the average x (AVG x ) and y (AVG y ) and dividing by the number of successive data points over the duration of the balance trial (N), respectively. For average COP velocity, the total length of the COP path (L) was divided by the number of successive data points over the duration of the balance trial (N) multiplied by the change in time (Δt), where (L) was calculated by taking the square root of the sum of squares in the ML and AP directions. After completion of the static balance assessment, participants completed the SEBT to assess dynamic balance. This test was completed on both the DL and NDL as the stance limb

7 6 J. D. SIMPSON ET AL. Table 1. Descriptive characteristics of participants (mean ± SD). Variable ELT (n = 9) CON (n = 10) p-value Age (y) 21 ± 2 22 ± Height (cm) ± ± Body mass (kg) 66.0 ± ± Body fat (%) 18.0 ± ± and consisted of reaching with the contralateral limb in eight different directions (Anterior, Anteromedial, Medial, Posteromedial, Posterior, Posterolateral, Lateral and Anterolateral) extending out from the centre of the base of support (Figure 1). Participants were instructed to keep both hands on their hips, reach as far as possible with the contralateral limb, touch the marked line with their toes, and return back to the stance limb. Failed attempts were marked if the participant could not return to the stance limb without losing their balance, if they removed their stance foot from the ground, or if their hands were removed from the hips. Three successful trials were completed in each direction on both legs and trials were completely randomised using Excel. The average reach distance in each direction was recorded in inches by one of two researchers and converted to centimeters for analysis. In order to account for variability in footwear and sock characteristics all balance assessments were completed in a barefoot condition. Statistical analysis Data are represented as mean ± SD for descriptive variables and were calculated using SPSS software version 24 (IBM, Armonk, NY, USA). An independent sample t-test was computed to analyse anthropometric variables, training log data for ELT and DET phases, and baseline measures between experimental and control groups. To compare changes in performance a series of 2 (group) 3 (time) repeated measures ANOVAs were computed to compare dependent variables of static and dynamic balance. Bonferroni post hoc tests were computed when necessary. In addition, change in performance on the SEBT for the two phases of the investigation are represented as absolute and Δ percent and were calculated using the formula: Δ% = (treatment baseline / baseline). All results were considered significant when p < Results Descriptive characteristics for both the experimental and control groups are presented in Table 1. Results from the independent sample t-test revealed no significant differences for age, height, body mass or body fat percentage between the ELT and CON groups (p > 0.05). Additionally, training frequency, intensity and modalities were not significantly different between groups for the duration of the study (p > 0.05). Participants in the ELT group exhibited significantly greater 95% ellipse area on the dominant limb with EO and reach distances on the SEBT in posteromedial and medial directions on the DL and anteromedial direction on the NDL at baseline (p < 0.05). All other baseline measures of static and dynamic balance revealed no significant differences between groups (p > 0.05). This data can be found in Tables 2 6.

8 SPORTS BIOMECHANICS 7 Table 2. Average static balance variables on both legs (mean ± SD). p-value Variable Baseline Post-ELT Post-DET Time Group Avg. M/L COP displacement (cm) eyes open ELT ± ± ± CON ± ± ± Avg. A/P COP displacement (cm) eyes open ELT ± ± ± CON ± ± ± Avg. Sway Velocity (cm/s) eyes open ELT ± ± ± # CON ± ± ± # 95% Ellipse Area (cm 2 ) eyes open ELT ± ± ± CON ± ± ± Avg. M/L COP displacement (cm) eyes closed ELT ± ± ± CON ± ± ± Avg. A/P COP displacement (cm) eyes closed ELT ± ± ± CON ± ± ± Avg. Sway Velocity (cm/s) eyes closed ELT ± ± ± # CON ± ± ± # 95% Ellipse Area (cm 2 ) eyes closed ELT ± ± ± CON ± ± ± # Indicates significant difference between baseline and post-det (p < 0.05). Indicates significant difference between post-elt and post-det (p < 0.05). Results of the static balance assessments revealed a significant group by time interaction (p = 0.028) for average COP velocity in the NDLEC condition. Significant main effects for time were identified for BLEO average COP velocity (p = 0.010), BLEC average COP velocity (p = 0.004), and NDLEC 95% ellipse area (p = 0.046). Post hoc analysis showed significant reductions in average COP velocity from baseline to post-det, and post-elt to post-det in the BLEO and BLEC conditions, respectively (Table 2). In addition, a significant reduction in 95% ellipse area from baseline to post-elt was also identified in the NDLEC condition (Table 4). There was also a significant main effect for group (p = 0.011) for the anterior/posterior displacement of the COP in the DLEO condition between the ELT group (0.699 ± 0.212) and the CON group (0.592 ± 0.097). No other significant interactions or main effects for measures of static balance were identified. Data from static balance assessments can be found in tables 2 4. Analysis of reach distance on the SEBT revealed no significant time by group interactions for any of the eight directions for the DL and NDL (p > 0.05). Significant main effects for time were identified for the posterior (p = 0.041), posteromedial (p = 0.002), and medial (p = 0.001) directions on the DL and in the posterior (p = 0.042) direction on the NDL. Reach distance significantly improved from baseline to post-elt and from baseline to post- DET on the DL in the posteromedial and medial directions (Table 5) and from baseline to post-elt in the posterior direction on the NDL (Table 6). There was also a significant main effect for group in the posteromedial (p = 0.021) and medial (p = 0.033) directions on the DL between the ELT and CON groups. The ELT group had a greater mean reach distance of 75.9 ± 7.4 cm and 68.6 ± 9.3 cm in the posteromedial and medial directions on

9 8 J. D. SIMPSON ET AL. Table 3. Average static balance variables on the dominant leg (mean ± SD). p-value Variable Baseline Post-ELT Post-DET Time Group Avg. M/L COP displacement (cm) eyes open ELT ± ± ± CON ± ± ± Avg. A/P COP displacement (cm) eyes open ELT ± ± ± CON ± ± ± Avg. Sway Velocity (cm/s) eyes open ELT ± ± ± CON ± ± ± % Ellipse Area (cm 2 ) eyes open ELT ± * ± ± CON ± ± ± Avg. M/L COP displacement (cm) eyes closed ELT ± ± ± CON ± ± ± Avg. A/P COP displacement (cm) eyes closed ELT ± ± ± CON ± ± ± Avg. Sway Velocity (cm/s) eyes closed ELT ± ± ± CON ± ± ± % Ellipse Area (cm 2 ) eyes closed ELT ± ± ± CON ± ± ± * Indicates significantly different than CON (p < 0.05). the DL compared to the CON group that had a mean of 67.8 ± 7.4 cm and 59.3 ± 9.3 cm, respectively. No other significant interactions or main effects for reach distance on the SEBT were identified. Reach distance and calculated change (absolute and Δ%) in performance on the SEBT in each direction on the DL and NDL can be found in Tables 5 and 6. Discussion and implications The ability of an athlete to maintain their centre of gravity within their base of support in both static and dynamic conditions is a critical aspect of athletic performance. Previous literature provides evidence that implementing ELT for 3 weeks can improve high intensity performance tasks improvements (Bosco, 1985; Bosco et al., 1984, 1986; Lowe et al., 2016; Sands et al., 1996; Scudamore et al., 2016). However, the effects of ELT on static and dynamic balance in athletic populations are unknown. Whilst we hypothesised ELT would improve balance, there was also a possibility that wearing a WV during training and daily living may negatively impact balance once the WV was removed. The primary purpose of this study was to examine the potential impact of ELT on static and dynamic balance performance in women who participate in high intensity and resistance training. To the knowledge of the authors, this was the first study to examine the impact of ELT on static and dynamic balance performance in collegiate aged women participating in resistance and high intensity training. The main findings of the current study are ELT did not result in significant decrements in balance at any time point in comparison to CON, and for some variables showed improvements.

10 SPORTS BIOMECHANICS 9 Table 4. Average static balance variables on the non-dominant leg (mean ± SD). p-value Variable Baseline Post-ELT Post-DET Time Group Avg. M/L COP displacement (cm) eyes open ELT ± ± ± CON ± ± ± Avg. A/P COP displacement (cm) eyes open ELT ± ± ± CON ± ± ± Avg. Sway Velocity (cm/s) eyes open ELT ± ± ± CON ± ± ± % Ellipse Area (cm 2 ) eyes open ELT ± ± ± CON ± ± ± Avg. M/L COP displacement (cm) eyes closed ELT ± ± ± CON ± ± ± Avg. A/P COP displacement (cm) eyes closed ELT ± ± ± CON ± ± ± Avg. Sway Velocity (cm/s) eyes closed^ ELT ± ± ± CON ± ± ± % Ellipse Area (cm 2 ) eyes closed ELT ± ± $ ± CON ± ± $ ± $ Indicates significant difference between baseline and post-elt (p < 0.05). ^Indicates significant time x group interaction (p < 0.05). Static balance was assessed and revealed significant reductions in average COP velocity with EO and EC for both groups in bilateral stance from baseline to post-det (Table 2). Sway area in the NDLEC condition was reduced from baseline to post-elt in both groups, whilst failing to improve post-det (Table 4). Whilst participants in both the ELT and CON groups showed similar improvements in static balance performance, our hypothesis that ELT would enhance static balance performance was not confirmed. Bruhn and colleagues (2004) examined single leg postural stability using a Posturomed swinging platform following a 4-week intervention consisting of either strength training focused on high intensity resistance exercises, sensorimotor training focused on joint stability and rehabilitation exercises, or no additional training. The authors reported significant reductions in platform displacement for the strength training group compared to sensorimotor and control groups post-intervention (Bruhn et al., 2004). Additionally, a study by Myer et al. (2006) examined single leg balance after 6 weeks of plyometric or balance training amongst high school female athletes using a 50 cm hop forward and land on the same leg whilst maintaining balance for 10s. Significant reductions in medial/lateral COP deviations on the DL for both training groups were observed in this study. Balance assessments were performed three times over the course of the study which might have generated a learning effect resulting in the lack of between group differences. Although training was not controlled by the investigators in the current study, training volume and type between groups was not statistically different. Whilst studies examining static balance following interventions that did not specifically target balance training have shown improvements in balance post-intervention (Bruhn et al., 2004; Myer et al., 2006), the lack of between group differences in static balance indicate

11 10 J. D. SIMPSON ET AL. Table 5. Average reach distance (cm) for the dominant stance leg on the SEBT (mean ± SD). Δ BL to ELT Δ ELT to DET p-value Direction Baseline Post-ELT Post-DET Absolute % Absolute % Time Group Anterior ELT 70.7 ± ± ± ± ± ± ± CON 68.5 ± ± ± ± ± ± ± 6.1 Anterolateral ELT 75.4 ± ± ± ± ± ± ± CON 73.4 ± ± ± ± ± ± ± 3.4 Lateral ELT 77.9 ± ± ± ± ± ± ± CON 74.8 ± ± ± ± ± ± ± 3.1 Posterolateral ELT 79.0 ± ± ± ± ± ± ± CON 76.0 ± ± ± ± ± ± ± 4.2 Posterior ELT 78.4 ± ± ± ± ± ± ± CON 73.0 ± ± ± ± ± ± ± 4.2 Posteromedial ELT 73.0 ± 7.9 * 76.7 ± 8.8 $ 78.0 ± 8.8 # 3.6 ± ± ± ± CON 64.9 ± ± 8.3 $ 71.4 ± 9.0 # 3.7 ± ± ± ± 7.8 Medial ELT 66.0 ± 8.6 * 69.9 ± 8.1 $ 69.9 ± 10.6 # 3.9 ± ± ± ± CON 55.0 ± ± 12.0 $ 62.7 ± 10.2 # 5.0 ± ± ± ± 7.51 Anteromedial ELT 62.2 ± ± ± ± ± ± ± CON 57.9 ± ± ± ± ± ± ± 7.7 $ Indicates significant difference between baseline and post-elt (p < 0.05). # Indicates significant difference between baseline and post-det (p < 0.05). * Indicates significantly different than CON (p < 0.05).

12 SPORTS BIOMECHANICS 11 Table 6. Average reach distance (cm) for the non-dominant stance leg on the SEBT (mean ± SD). Δ BL to ELT Δ ELT to DET p-value Direction Baseline Post-ELT Post-DET Absolute % Absolute % Time Group Anterior ELT 70.4 ± ± ± ± ± ± ± CON 67.1 ± ± ± ± ± ± ± 3.2 Anterolateral ELT 75.1 ± ± ± ± ± ± ± CON 71.6 ± ± ± ± ± ± ± 3.8 Lateral ELT 77.6 ± ± ± ± ± ± ± CON 74.4 ± ± ± ± ± ± ± 4.1 Posterolateral ELT 80.6 ± ± ± ± ± ± ± CON 76.1 ± ± ± ± ± ± ± 6.3 Posterior ELT 79.1 ± ± 9.1 $ 83.2 ± ± ± ± ± CON 72.4 ± ± 9.1 $ 78.4 ± ± ± ± ± 7.0 Posteromedial ELT 73.0 ± ± ± ± ± ± ± CON 69.3 ± ± ± ± ± ± ± 8.5 Medial ELT 67.1 ± ± ± ± ± ± ± CON 61.0 ± ± ± ± ± ± ± 7.9 Anteromedial ELT 63.3 ± 4.2 * 62.5 ± ± ± ± ± ± CON 58.3 ± ± ± ± ± ± ± 4.5 $ Indicates significant difference between baseline and post-elt (p < 0.05). * Indicates significantly different than CON (p < 0.05).

13 12 J. D. SIMPSON ET AL. ELT during daily living whilst incorporating WVs into resistance and plyometric training may not elicit further improvements in static balance in comparison to routine strength and plyometric training. An additional study by Paterno and colleagues (2004) using female athletes investigated the changes in single leg stability before and after 6 weeks of combination training including balance, resistance and plyometric exercises. The authors measured single leg stability of high school basketball, soccer and volleyball athletes using a Biodex Stability System and reported significant improvements in single leg total stability and anterior/posterior stability for both legs (Paterno et al., 2004). Transient reductions in sway area in the current study for both groups were evident for the NDLEC post-elt, which indicates ELT is unlikely to increase sway area. This provides positive implications as greater sway area during single leg balance, particularly when performing sports related tasks such as jumping, landing, changing direction, stopping and/or rapid accelerations that challenge an athlete s ability to maintain their centre of gravity within their base of support, has previously been attributed to an increased susceptibility of injury to the lower extremity (McGuine, Greene, Best, & Leverson, 2000). In regards to dynamic balance, Filipa et al. (2010) concluded that 8 weeks of lower extremity strength and core stability training improved SEBT scores by 8.2 and 6.5% on the right and left legs in high school female soccer players. Additionally, significant improvements in reach distance were reported for both legs in the posterolateral direction, and in the posteromedial direction on the left leg. Similarly, Rasool and George (2007) reported 2 weeks of single leg stability training was sufficient to elicit increased reach distance in each direction on the SEBT in a group of healthy trained men. However, an additional 2 weeks of training resulted in further improvements in reach distance, with distinct improvements in the posterior (25.8%) and anterolateral (35.5%) directions. In the current study, the ELT group showed a 5.2% and 6.3% improvement in the posteromedial and medial direction, whilst the CON group showed a 6.5% and 9.3% increase from baseline to post-elt on the DL. An additional increase of 5.4% and 5.4% for the CON group was evident post-det in both directions, whilst the ELT group only showed minor improvements of 3.2% and ~1.0%, respectively (Table 5). On the NDL, both groups showed > 4.0% improvement in the posterior direction from baseline to post-elt with marginal improvements post-det (Table 6). Although similar improvements in dynamic balance performance may be attributed to the lack of training differences between groups, a potential learning effect may have been observed. However, no significant decrements were found in dynamic balance following ELT. This would suggest ELT may still be incorporated into training protocols to improve high intensity task performance without impairing dynamic balance. Furthermore, lack of group differences in the current study highlights the need to develop training protocols that can result in enhanced dynamic balance to reduce possible risk factors associated with lower extremity injury (Hrysomallis et al., 2007; McGuine & Keene, 2006; Plisky et al., 2006). Although results from the current study revealed significant improvements in static and dynamic balance following the ELT protocol, there were some limitations to the study. First, training among participants was not standardised and physical activity was self-selected. However, qualitative assessments of physical activity were provided to the participants to monitor individual training during each phase of the investigation. Additionally, each participant was continuously reminded to not make any drastic changes in training regiments that might alter their results during participation in the study. Second, individual leg

14 SPORTS BIOMECHANICS 13 length was not measured or reported to standardise reach distance on the SEBT. This might possibly explain the noticeable differences in reach distance on the SEBT for the posterior, medial and posteromedial directions between groups. However, absolute and Δ% change in reach distance were calculated in order to compare changes in each group following each phase of the investigation. Finally, participants did not participate in a standardised balance training program during the investigation. Future research should investigate balance training protocols in combination with normal strength and conditioning regimens whilst incorporating ELT. If significant improvements in balance performance following ELT are observed, this would provide further assistance to coaches and athletes seeking to optimise balance and athletic performance. Conclusion Previous studies have shown that incorporating ELT appears to be an effective supplemental training intervention for enhancing high intensity task performance. Moreover, previous research indicates that female athletes exhibiting deficits in balance performance is a key indicator of lower extremity injury. In the current study, static and dynamic balance performance improved similarly across time in both groups regardless of intervention. These findings would suggest that incorporating 3 weeks of ELT did not result in superior improvements in balance performance when compared to the control group. Disclosure statement No potential conflict of interest was reported by the authors. References Barr, M., Gabbett, T., Newton, R., & Sheppard, J. (2015). Effect of 8 days of a hypergravity condition on the sprinting speed and lower-body power of elite rugby players. The Journal of Strength & Conditioning Research, 29, Bosco, C., Rusko, H., & Hirvonen, J. (1986). The effect of extra-load conditioning on muscle performance in athletes. Medicine and Science in Sports and Exercise, 18, Bosco, C. (1985). Adaptive response of human skeletal muscle to simulated hypergravity condition. Acta Physiologica Scandinavica, 124, Bosco, C., Zanon, S., Rusko, H., Dal Monte, A., Bellotti, P., Latteri, F., & Bonomi, S. (1984). The influence of extra load on the mechanical behavior of skeletal muscle. European Journal of Applied Physiology and Occupational Physiology, 53, Bressel, E., Yonker, J., Kras, J., & Heath, E. (2007). Comparison of static and dynamic balance in female collegiate soccer, basketball, and gymnastics athletes. Journal of Athletic Training, 42, 42. Bruhn, S., Kullmann, N., & Gollhofer, A. (2004). The effects of a sensorimotor training and a strength training on postural stabilisation, maximum isometric contraction and jump performance. International Journal of Sports Medicine, 25, Chander, H., & Dabbs, N. C. (2016). Balance performance and training among female athletes. Strength & Conditioning Journal, 38, Chander, H., MacDonald, C., Dabbs, N., Allen, C., Lamont, H., & Garner, J. (2014). Balance performance in female collegiate athletes. Journal of Sports Science, 2, Filipa, A., Byrnes, R., Paterno, M., Myer, G., & Hewett, T. (2010). Neuromuscular training improves performance on the star excursion balance test in young female athletes. Journal of Orthopaedic & Sports Physical Therapy, 40,

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