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Kristin M. Mendez, Alexander W. Parsons, Khase E. Willis, Kevin K. McCully, FACSM. University of Georgia, Athens, GA.

BACKGROUND: The diaphragm is the main inspiratory muscle of the body and important to overall respiratory health. Our lab developed a noninvasive and nonvolitional method to measure diaphragmatic endurance. The purpose of this study was to evaluate the potential effects of abdominal mass, manipulated by body position, on test outcomes. METHODS: Male and female participants (n=5, age: 26.2 ± 6.5) were tested on three separate days. All were of relatively healthy body mass indexes (25.07 ± 3.8). Day 1: anthropometric measurements were taken, and phrenic nerve stimulation familiarization was performed. Day 2: participants were placed in a 10-degree head up tilt. Day 3: participants were placed in a 10-degree head down tilt. Diaphragmatic endurance was measured on each day. The phrenic nerve was stimulated using point electrodes, causing hemi-diaphragmatic contractions. Electrical stimulation current was set at the highest intensity (mA) a person could tolerate. If too much concurrent brachial plexus activation was observed, the stimulation was decreased to an intensity that still produced a vigorous diaphragmatic contraction but reduced arm movement. A tri-axial accelerometer was placed on the participant’s abdomen to capture the acceleration of twitch contractions. The endurance test consisted of 5 minutes of stimulation at a frequency of 5 Hz. The Endurance Index (EI) represents the percentage of diaphragm acceleration that remained after 5 minutes. RESULTS: While not statistically significant, position differences did exist. Data reflected our hypothesis that EIs would be higher in the head up position with a mean value of 88.5% (5.3) compared to the supine and head down positions with mean values of 77.6% (8.0) and 78.6% (7.6), respectively. No main effect was found for body position with a repeated measures analysis of variance (Wilks Λ=.230, F2,3=5.021, p=.110) There was a large effect size between head up and both supine (d=1.6) and head down positions (d=1.5). A small effect size was seen between supine and head down (d=.1). CONCLUSIONS: Body position has the potential to influence diaphragm endurance index. This suggests abdominal mass may be a confounding factor when performing the endurance test. Performing the test with a 10-degree head up tilt may reduce the impact of abdominal mass in young, healthy individuals. Additional testing is needed with a larger sample size.

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