1. Seventy-two percent of all athletic stress fractures are the result of running, with nearly half occurring in the distal end of the tibia. PURPOSE: The purpose of this study was to investigate the relationship between the level of body weight (BW) in an unloader treadmill and tibial acceleration (TA). Research reports that increased vertical ground reaction force during running is positively correlated to peak TA. METHODS: Fifteen collegiate cross country runners (9M; 20.4 ± 2.4 yrs; 60.1 ± 12.6 kg) granted informed consent and participated in the present study. Following a 10-minute familiarization run on an unloading differential air pressure treadmill each subject started at 100% BW and ran at a consistent velocity for three minutes during each of nine stages which progressively decreased at 5% intervals until 60% BW for a total of 37 minutes. During the last 30 seconds of each stage, TA and heart rate (HR) were measured. TA was assessed with a skin-mounted uniaxial accelerometer attached to the lower third of the anterior tibia and data were recorded via computer data collection system and processed with custom-written data processing code. HR data was collected via a heart rate monitor. A repeated measures analysis of variance with Bonferroni post-hoc comparison was used to analyze the data. Significance level was set apriori at p<0.05. RESULTS: There were no significant differences between TA from 100% BW (10.59g) to any level of unloading (60%-95%). Mean peak tibial acceleration initially increased with the levels of unloading (95%-75%) before dropping below 100% BW TA levels at 70%. Mean peak-to-peak TA was significantly less (p= 0.021) at 60% BW (16.55g) as compared to 100% BW(18.56g). CONCLUSION: Tibial accelerations are not directly related to level of BW on an AGTM. PRACTICAL APPLICATIONS: Based on the present study, a runner rehabilitating from a tibial stress fracture/reaction should begin rehabilitation at BW levels below 60% BW to maintain stress levels below normal. By running at these reduced BW levels, the runner better maintain aerobic fitness while still rehabilitating from a tibial stress fracture as opposed to traditional rehabilitation protocol which is rest. ACKNOWLEDGEMENTS: Dr. Anna Price for statistical analysis. Brendan J. Rickert, Matthew F. Moran, Beau K. Greer Department of Physical Therapy and Human Movement Science, Sacred Heart University, Fairfield, CT Contact Information: rickertb@sacredheart.edu TIBIAL ACCELERATION IN DISTANCE RUNNERS IN REDUCED BODY WEIGHT CONDITIONS ABSTRACT INTRODUCTION RESULTS PRACTICAL APPLICATION CONCLUSIONS REFERENCES ACKNOWLEDGMENTS Presented at the 2013 NSCA National Conference Las Vegas, Nevada Fifteen collegiate cross-country team runners volunteered for this study. All subjects were free of lower-extremity injuries for at least six months prior to the study and had a minimum of four years of running experience. All subjects were weighed measured for height, and self-reported their leg dominance and current training history (Table 1). Leg dominance was determined by asking each subject their preferred kicking leg. All experimental procedures were approved by the Sacred Heart University Institutional Review Board. Subjects were informed of the experimental procedures and all granted their informed consent. 1. Alter-G. Alter-G Anti-Gravity Treadmill. Freemont, CA. 2012. Available at: http://www.alter- g.com/http://www.alter- g.com/ 2. Mizrahi J, Verbitsky O, Isakove E. Fatigue-related loading imbalance on the shank in running: a possible factor in stress fractures. Annals of Biomedical Engineering. 2000 28.463-469. 3. Derrick TR, Hamill J, Caldwell GE. Energy absorption of impacts during running at various stride lenghs. Medicine & Science in Sports & Exercise. January 1998. 30(1) 128-135. 4. Hamill J, Derrick TR, Holt KG. Shock attenuation and stride frequency during running. Human Movement Science. 1995. 14. 45-60. 5. Milner C, Ferber R, Pollard C, Hamill J, Davis I. Biomechanical Factors Associated with Tibial Stress Fracture in Female Runners. Medicine & Science In Sports & Exercise. February 2006;38(2):323-328. 6. ACSM’s Guidelines for Exercise Testing and Prescription: Eight Edition. 2010. China. 7. Moran, MF, Rickert BJ: Effect of Body Weight Support on Spatiotemporal Running Mechanics. Presenting at ACSM Annual Meeting, San Francisco, May 2012. 8. Kim B. Ground Reaction Forces and Bone Parameters in Females with Tibial Stress Fracture. Medicine & Science In Sports & Exercise. March 2004;36(3):397-404. 9. Cavanagh PR and Lafortune MA. Ground reaction forces in distance running. Journal of Biomechanics. 1980. Vol. 13, 397-406. 10. Figueroa MA, Manning J, Escamilla P. Physiological Responses to the AlterG Anti-Gavity Treadmill. International Journal of Applied Science and Technology.2011. Vol 1; 6. Twenty-four to 65% of all runners report a running-related injury (RRI) every year. 2 Runners experience 2-2.5 times their body weight (BW) upon impact during the stance phase of running. 4 Fifty percent of all stress fractures occur in the distal end of the tibia. 5 Typically, a tibial stress fracture requires 4-6 weeks of recovery time before training can resume. 8 The AlterG treadmill (TM) artificially reduces BW through differential air pressure (DAP). 1 For rehabilitation, DAP TM have beneficial outcomes because they reduce ground reaction forces, muscular activity, and metabolic cost. 9,10 The purpose of this study was to investigate the relationship between the level of BW in an AlterG TM and tibial acceleration (TA). PARTICIPANTS EXPERIMENTAL METHODS Height (m) 1.7 ± 0.2 Mass (kg)60.1 ± 12.6 Age (years)20.4 ± 2.4 Running Experience (years)8.1 ± 3.1 Self-Reported Running Miles Per Week49 ± 16 Leg Dominance (Right Leg)N= 14 Warm up Stage 1 Stage 2 Stage 3 Stage 4 Stage 5 Stage 6 Stage 7 Stage 8 Stage 9 BW %100% 95%90%85%80%75%70%65%60% Duration (Min)10333333333 Figure 1: AlterG Treadmill All subjects wore their own training shoes and did not run on their respective testing day. Each data collection session started with a 10-minute warm- up on the AG TM at 0% incline and 100% BW. This 10-minute run was intended to acclimatize the participant to the AG TM as well as to determine a speed that was associated with 75% of their estimated maximum heart rate [ (206.9-(0.67x age) x 0.75) 6 ]. After the warm-up, the subject ran at the same speed throughout each of the nine testing stages (Table 2). Each stage lasted 3-min and BW percentage was decreased 5% at the end of each stage. BW was not randomized due to a previous finding that subjects who went from reduced BW to 100% BW had higher rate of perceived exertion (RPE) and HR at the same BW conditions when compared to the group of subjects that decreased unloading. 7 The accelerometer was placed on the non-dominant leg, which was opposite of their dominant leg (Figure 2). A second testing period (n=2) was completed three weeks after the first testing period to examine the reliability of the testing protocol. A Repeated Measures ANOVA revealed several significant differences between variables measured. Mean peak tibial shock at 60% BW was noticeably less than all levels of BW 70% to 95% BW, however, there was no significant (p= 0.058) difference between 60% BW and 100% BW (Figure 3). Additionally, there was not a linear relationship between level of unloading and tibial acceleration. Mean Peak to Peak (PP) tibial acceleration at 60% BW was significantly different from 100% (p= 0.021) (Figure 4). SR decreased in a linear pattern as BW percentage was reduced, however SR did not decrease in a 1-to-1 relation to decreasing BW% (Figure 6). Mean SR at 100% BW was 1.45 strides per second (SPS) which was reduced to 1.33 SPS at 60% BW. HR also decreased from 100% BW to 60% BW in a similar fashion as SR. Mean HR was 150 BPM at 100% BW and was reduced to 129 BPM at 60% BW (Figure 5). Table 1: Subject data Table 2: Testing Protocol Figure 2: Accelerometer in its housing chamber fixed to the tibia Figure 3: Mean Tibial AccelerationFigure 4: Mean Peak to Peak Tibial Acceleration Figure 5: Mean Heart RateFigure 6: Mean Stride Thank you to Dr. Anna Price for statistical analysis assistance. The AG TM is a new tool in performance enhancement and rehabilitation. Mean peak tibial acceleration and mean peak to peak acceleration was reduced at 60% BW as compared to 100% BW which may be an important threshold for those returning from a tibial stress fracture. With the reduction in BW, stride rate and heart rate both decrease. This is important to note for use in high level athletes as the idea behind this more aggressive rehabilitation protocol is to stimulate training conditions while allowing bone remodeling to exceed bone resorption. Additionally a change in stride rate could results in unfavorable change in running biomechanics. It is anecdotally suggested to maintain normal stride rate when running in reduced BW conditions Though this study had few significant results in mean peak tibial acceleration and mean peak to peak acceleration, it is platform to be used for future research studies and training and rehabilitation protocols. Future research on the relationship of stride rate and mean peak tibial acceleration is needed to more accurately assess the effect of unloading.