Wavelet Analysis of Skin Blood Oscillations in Persons With Spinal Cord Injury and Able-Bodied Subjects

Schematic of experimental posture and setup. The testing posture was lying with both hips flexed to 90°. External pressure was applied to the skin by a pneumatic indentor (diameter, 25mm) aligned perpendicular to the skin over ischial tuberosity (IT). Both legs were supported by a height adjustable platform.

The wavelet transform in (A) time-frequency plane and (B) averaged spectrum of LDF signal shown on a log scale. The vertical lines indicate the outer limits of frequency interval: (1) .01 to .02Hz, (2) .02 to .06Hz, (3) .06 to .15Hz, (4) .15 to 0.4Hz, and (5) 0.4 to 2Hz, which correspond to endothelial related metabolic, neurogenic, myogenic, respiratory and cardiac activities, respectively.

Normalized absolute and relative amplitudes for the 5 frequency intervals for the resting skin over the ischial tuberosity in (A, B) the able-bodied subjects and (C, D) persons with SCI during the preloading and the 2 postloading periods (t1, t2). Significant differences are marked between postloading (t1) and preloading with *P<.05 and ^†P<.01; postloading (t2) and postloading (t1) with ^‡P<.05 and ^§P<.01. Each value was calculated as a percentage of each PRE value in the same group.

Relative amplitude for the 5 frequency intervals for the resting skin over the ischial tuberosity in the able-bodied subjects and persons with SCI during the (A) preloading and (B) postloading periods (t1). *Significant differences are marked between persons with SCI and able-bodied subjects (P<.05).

Typical examples of time-averaged wavelet transform calculated from signals measured in the resting skin over the ischial tuberosity in (A) an able-bodied subject and (B) a person with SCI. The vertical lines indicate the outer limits of each frequency interval from 1 to 5, which correspond to endothelial related metabolic, neurogenic, myogenic, heart, and respiratory activities, respectively.