17 The power of the stance phase head and tibia acceleration in the frequency domain was determined CHIR 99021 by calculating the power spectral density (PSD) using a square window. Examination of the acceleration signals collected over the entire stance phase follows the periodic assumptions of Fourier analysis and allows for examination of frequencies below 15 Hz.45 and 46 The PSD was performed on frequencies 0 to the Nyquist frequency (FN) and normalized to 1 Hz bins. 14 and 22 After binning, the PSD was normalized in order for the sum of the powers from 0 to FN to be

equal to the mean squared amplitude of the data in the time domain. Examining the PSD results revealed two primary peaks or local maxima for the tibial and head acceleration signal power in both RF and FF running that were outside of the lower (4–8 Hz) and higher (10–20 Hz)

ranges previously identified selleck products in the literature for RF running. 13, 14, 15 and 17 As a result, we expanded the lower and higher frequency ranges investigated to 3–8 Hz and 9–20 Hz, respectively, to more appropriately include the dominating frequency components of each footfall pattern. The frequency at which peak power occurred within the lower and higher frequency range of the tibial (TPFlow, TPFhigh) and head (HPFlow, HPFhigh) acceleration signal was determined. Signal power magnitude in the frequency domain was quantified by the integral of the signal power contained in the lower and higher frequency ranges in the tibial (TSMlow, TSMhigh) and head (HSMlow, HSMhigh) acceleration signals. 15 A transfer function has been previously used to determine the degree of shock attenuation in human running by calculating the ratio of each frequency bin between the tibial and head signal14, 15, 19 and 22 (i.e., the transmissibility of each frequency component21). The transfer function was calculated across all frequencies from 0 to FN to determine the degree of shock attenuation occurring between the tibia to the head by: Shockattenuation=10×log10(PSDhead/PSDtibia) For each frequency, the transfer function calculated the gain or attenuation, in decibels,

between the tibia and head signals. Positive values indicated a gain, or increase in signal strength, and negative Ketanserin values indicated attenuation, or decrease in signal strength. A gain in lower frequency components is typically a result of changes in head vertical velocity and voluntary segment motion during the stance phase whereas negative values indicate attenuation in signal power as the impact shock travels through the body.17 and 22 Shock attenuation magnitude was quantified by the integral of the transfer function result within the lower (ATTlow) and higher frequency ranges (ATThigh). For the lower and higher frequency ranges, tibial and head peak signal power and signal magnitude were averaged across all stance phases of each participant and then across group.