Changes in fatigue characteristics of cervical paraspinal muscles with posture.

The median frequency of the myoelectric signal of the cervical paraspinal muscles was studied in 28 normal subjects in prone and sitting positions. Median frequency parameters (initial median frequency and slope of the median frequency) of the myoelectric spectrum were monitored during sustained isometric neck extensions at 20%, 50%, 80%, and 100% of maximum voluntary contractions. Force output of 100% maximum voluntary contractions was also measured in the prone and sitting positions. Reliability of the median frequency parameters for repeated trials at various force levels was computed. There were significant differences (P less than 0.05) in the median frequency parameters between the prone and sitting positions, and there was significantly higher 100% maximum voluntary contractions force production in the prone position than in the sitting position (P less than 0.05). There were also high reliability estimates for the median frequency parameters in both test positions. The cervical paraspinal muscles may use different strategies of recruiting motor units during force production in various postures of the neck. It therefore is necessary to standardize the testing procedures before the median frequency parameters can be considered objectively to measure the muscle function in the neck.