Approximately 150 volunteers (military and civilians) will complete this experiment. A sample size of 120 volunteers is recommended for characterizing a sample reference population and constructing reference intervals (Fraser, 2001). A sample size of 150 will allow for the potential loss of data due to equipment or handling errors, and 'bad' samples resulting from volunteer non-compliance. A subset of as many as 50 volunteers will undergo repeat testing (3 days; ~3 hours) for the purpose of determining within-subject variance.