Relationship between respiratory function, respiratory muscle strength, and diaphragmatic muscle thickness in sitting positions with different trunk angle

To clarify the effects of posture changes on respiratory function and respiratory muscle strength, we compared respiratory muscle strength between sitting positions with different trunk angles.

The subjects were 17 adult healthy women (age: 21.1 years, BMI: 20.2). They were seated in the edge position with the trunk in the middle position (0°), and then they were asked to hold the position with the trunk flexed at 10° or 20° or extended at 10° or 20°. Respiratory function and respiratory muscle strength [cmH₂O] were measured (AS-507, Minato Medical Science). At the same time, we used an ultrasound machine (SENON 300C) to measure the thickness of the diaphragm (thickness of diaphragm: Tdi) between the 8th and 9th ribs. As an index of diaphragmatic activity, the change in diaphragmatic muscle thickness from the resting expiration position (ΔTdi) was calculated as a percentage of the resting expiration position Tdi (ΔTdi%). Statistical analysis was performed using one-way analysis of variance and Pearson's correlation analysis (p0.05).

The mean value of vital capacity in the sitting position was 3.0L, and the % vital capacity was 84.1%. The mean maximum inspiratory muscle strength was 37.3 cmH₂O at 0° sitting position, 35.3 at 10° flexion, 39.2 at 20° flexion, 40.6 at 10° extension, and 41.6 at 20° extension. There were differences between 10° flexion and 20° flexion, there was a difference between 0° and 20° extension in the sitting position. ΔTdi[%] was 1.62% for 0° sitting position, 1.41% for 10° flexion, 1.58% for 20° flexion, 1.73% for 10° extension, and 1.47% for 20° extension, with no difference.There was a significant positive correlation between inspiratory muscle strength and ΔTdi at 20° flexion.

There are differences in inspiratory muscle strength depending on the angle of the trunk in the sitting position. As the angle of the trunk increases, there is a tendency for inspiratory muscle strength to also increase. In particular, at the flexion position, it is thought that the activity of the abdominal muscle group is suppressed, and the activity of the diaphragm is activated.

Performing inspiratory muscle strength exercises while being aware of the differences caused by the trunk angle can help achieve more efficient training results.