1. Semper Scientific, Mission Viejo, CA 92961, US.
2. Division of Biokinesiology & Physical Therapy, Univ. of Southern California, Los Angeles, CA 90089, US.
3. Division of Biokinesiology & Physical Therapy, Univ. of Southern California, Los Angeles, CA 90089, US (Corresponding author), e-mail: powers@usc.edu