| This dissertation consisted of three studies. The first study aimed to examine the reliability of the soleus (SOL) Hoffmann reflex (H-reflex) amplitude recording during erect, slumped, and slouched sitting using generalizability theory. The reliability coefficients of the SOL H-reflex amplitude recordings ranged from .970 to .971 for erect sitting, ranged from .979 to .980 for slumped sitting, and were equal to .943 for slouched sitting. Although increasing the number of traces and sessions enhanced the reliability, recording 5 traces of the SOL H-reflex amplitude in a single session during any of the 3 sitting postures was sufficient to obtain acceptable reliability.;The purpose of the second study was to investigate the effect of erect, slumped and slouched sitting, and prolonged sitting on lumbosacral nerve root function by recording the SOL H-reflex amplitude. Thirty participants were assigned to two groups based on their daily sitting duration (prolonged sitting group and controls). The difference among the examined positions and the difference between the two groups in the amplitude were statistically examined. The results showed significant amplitude facilitation in slouched sitting, indicating less spinal loading and minimal nerve root compression. Furthermore, it was found that the SOL H-reflex amplitudes in the prolonged sitting group were significantly less than in the control group, possibly due to some compression force over the nerve roots inside the intervertebral foramina.;The third study aimed to examine the difference in ischial load during erect, slumped and slouched sitting and to assess its relationship to the SOL H-reflex. The ischial pressure and H-reflex amplitude were recorded from 30 healthy participants during the three sitting postures and the relationships between the two outcomes were statistically assessed. Although the SOL H-reflex amplitude demonstrated a high degree of sensitivity to changes in the ischial load in sitting position, no significant relationship between ischial load and the amplitude was found. It was concluded that the magnitude of spinal loading is not only influenced by the upper body weight transmitted via the vertebral column to the ischial tuberosities, but other factors such as spinal alignment and back muscles activity may also play major role in such influence. |
