619 RESULTS: During the downhill conditions, Ve increased (61.1 to 65.6 L/min; p=.017) and HR increased (141 to 151 bpm; p=.010) from fastest cadence to slowest cadence, with no difference in energy cost or VO2 among the conditions (both p>.05). As cadence increased among the downhill conditions, step length decreased and step width increased (both p<.05). GRF values progressively increased in the downhill conditions from fastest to slowest cadence (16.7 to 17.5 Nm; p=.029). Among ankle, knee and hip powers, knee power values increased from fastest to slowest cadence (9.5 to 12.2 Watts; p=.023). CONCLUSIONS: When compared to the control condition, running downhill with a progressively faster cadence increases cardiopulmonary and biomechanical responses, but not metabolic responses. The findings suggest that runners maintain energetic cost among different cadence conditions by making adjustments to muscle activation patterns when running downhill. This muscle recruitment concept could be tested using similar cadence conditions described here during flat, uphill and downhill running. 2185 Board #337 June 2, 2:00 PM - 3:30 PM Kinetic Response to Stride Frequency Perturbations During Treadmill Running Joshua P. Bailey, Leland Barker, Kendell Galor, Michael Soucy, John A. Mercer, FACSM. University of Nevada, Las Vegas, Las Vegas, NV. Email: bailey69@unlv.nevada.edu (No relationships reported) As a runner changes stride frequency, ground reaction forces likely change. There is a large body of research on ground reaction forces during running overground; however, there is very little research of ground reaction forces while running on a treadmill. PURPOSE: To investigate the effect of stride frequency perturbations on kinetic events during treadmill running. METHODS: Participants (n=8; 24.9 ± 4.2 yrs; 1.73 ± 0.09 m; 73.3 ± 13.4 kg) determined preferred treadmill running speed while running on an instrumented force treadmill (Bertec, OH). Preferred stride frequency (PSF) was measured and participants ran a total of 7 conditions, each representing a different stride frequency perturbation (PSF, PSF ± 5%, ± 10%, ± 15%). Run conditions were 5 minutes with 4 - 30 second data collection occurring every other 30 seconds after 1 minute. Data were processed via custom Matlab code identifying 15 right foot stance periods for analysis. Kinetic variables (active peak (FZ2), percent of stance phase of FZ2 (FZ2%) and peak loading rate), braking impulse (BIM), propulsive impulse (PIM) and stance period were analyzed using repeated measure ANOVAs across perturbation conditions (α=0.05). Due to the reduced frequency of occurrence of impact peak (FZ1) during the higher stride frequency perturbations (+5%,+10%&+15%), FZ1s were analyzed comparing PSF to reduced SF perturbations (-10% & -15%) with multiple paired sample t - tests . RESULTS: Peak loading rate, FZ2 and BIM were not significantly different across SF perturbations (p>0.05). Stance time was significantly different across perturbations (p<0.001). Stance time during PSF was longer (0.260s) than both PSF+10% (0.235s) and PSF+15% (0.228s) (p0.05). PIM was significantly different (p<0.001). Increased SF perturbations were significantly lower than decreased SF perturbations (p<0.05). CONCLUSIONS: During treadmill running, SF perturbations affected running kinetics by reducing the occurrence of FZ1 during increased SF perturbations and increases in FZ1 magnitude at reduced SF. 2186 Board #338 June 2, 2:00 PM - 3:30 PM Effective Force Application Between Various Stride Frequencies While Running at Constant Velocity Leland Barker, Josh Bailey, Michael Soucy, Kendall Galor, John Mercer, FACSM. UNLV, Las Vegas, NV. (Sponsor: Dr. John Mercer, FACSM) Email: barkel1@unlv.nevada.edu (No relationships reported) Effective force application (EFA) is calculated as a ratio of horizontal force to resultant force (EFA=F H /F Res x100)), where EFA is the effectiveness of force application. EFA has been measured in sprinting, but not running at submaximal constant velocities using different stride frequencies. PURPOSE: To examine EFA while running a constant velocity using different stride frequencies. METHODS: Participants (n=6, 69.7± 14.8 kg) ran at their preferred running speed at ±5%, ±10%, and ±15% of their preferred stride frequency on a force instrumented treadmill (Bertec, Columbus, OH). Participants ran for 5 minutes at each stride frequency (intermittently directed by metronome), where 4-30s trials were collected with the metronome off for each trial collection. EFA was calculated for 20 consecutive strides with the average EFA over stance, during braking, and during propulsive phases. A repeated measures ANOVA was performed to analyze the differences in EFA, EFA Brake, and EFA Propulsive (α=0.05). RESULTS: EFA, EFA Brake, and EFA Propulsive were not different (p>0.05) between stride frequencies. Mean EFA of 0%, +5%, +10%, +5%, -5%, -10%, -15% EFA values were 6.81 ±2.20, 6.32 ±1.21, 5.77 ±1.47, 5.56 ±1.47, 6.23 ±1.41, 6.44 ±1.73, 5.95 ±1.79, respectively. Mean EFA Brake of 0%, +5%, +10%, +5%, -5%, -10%, -15% EFA values were -8.3 ±1.4, -8.67 ± 1.5, -8.87 ±1.79, -8.61 ±1.89, -8.63 ±1.36, -8.72 ±1.27, -8.65 ±1.06, respectively. Mean EFA Propulsive of 0%, +5%, +10%, +5%, -5%, -10%, - 15% EFA values were 19.8 ±1.50, 20.63 ±1.51, 19.83 ±1.25, 19.45 ±1.12, 20.15 ±0.84, 20.33 ±.59, 20.01 ±0.77, respectively CONCLUSION: EFA was not influenced by stride frequency changes during constant velocity, even when braking and propulsive EFA phases were analyzed separately. Since EFA calculation is analogous to angle of force application, despite variations in stride frequency and stride length, the angle of force is maintained when running velocity is constant. More studies are required to determine if EFA can be intentionally manipulated to optimize performance. 2187 Board #339 June 2, 2:00 PM - 3:30 PM Relationships Between Natural Cadence And Vertical Load Rates In Injured And Healthy Runners. Erin Futrell 1 , Adam Tenforde 2 , Steve T. Jamison 2 , Irene S. Davis, FACSM 2 . 1 MGH Institute of Health Professions, Boston, MA. 2 Spaulding National Running Center, Cambridge, MA. (Sponsor: Irene Davis, FACSM) Email: efutrell@mghihp.edu (No relationships reported) Vertical ground reaction force load rates have been linked with running injuries. Increasing cadence has been shown to reduce load rates. However the relationship between natural cadence and load rates has not been examined. PURPOSE:To examine the relationship between natural running cadence and vertical load rates in both healthy and injured runners. It was expected that as cadence increases, load rates would decrease. METHODS: Two groups of runners were examined. One group included healthy runners (n=32, 25M, ages 18-54yrs) and the other included injured runners (n=93, 45M, ages 15-65yrs) being seen in a running clinic. Vertical ground reaction force (VGRF) and cadence (CAD) were recorded as participants ran on an instrumented treadmill at a self- selected speed (x=2.6 m/s ±0.12). The vertical average and instantaneous load rates (VALR, VILR) of the VGRF were calculated and correlated with CAD. This was done for the healthy and injured groups separately and then combined. Additionally, CAD, VALR and VILR were compared between the healthy and injured groups. Significance set at p<0.05. RESULTS: CAD was not significantly correlated with either VALR or VILR for either group (Table). Furthermore, there were no differences in CAD between the healthy and injured groups. VALR and VILR were significantly higher in the injured group by 28% and 16% respectively. Copyright © 2016 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited.