Activation of the human diaphragm during a repetitive postural task

1. The co-ordination between respiratory and postural functions of the diap hragm was investigated during repetitive upper Limb movement. It was hypoth esised that diaphragm activity would occur either tonically or phasically i n association with the forces from each movement and that this activity wou ld combine with phasic respiratory activity. 2. Movements of the upper limb and ribcage were measured while standing sub jects performed repetitive upper limb movements 'as fast as possible'. Elec tromyographic (EMG) recordings of the costal diaphragm were made using intr amuscular electrodes in four subjects. Surface electrodes were placed over the deltoid and erector spinae muscles. 3. In contrast to standing at rest, diaphragm activity was present througho ut expiration at 78 +/- 17% (mean +/- S.D.) of its peak inspiratory magnitu de during repeated upper limb movement. 4. Bursts of deltoid and erector spinae EMG activity occurred at the Limb m ovement frequency (similar to 2.9 Hz). Although the majority of diaphragm E MG power was at the respiratory frequency (similar to 0.4 Hz), a peak was a lso present at the movement frequency. This finding was corroborated by ave raged EMG activity triggered from upper limb movement. In addition, diaphra gm EMG activity was coherent with ribcage motion at the respiratory frequen cy and with upper limb movement at the movement frequency. 5. The diaphragm response was similar when movement was performed while sit ting. In addition, when subjects moved with increasing frequency the peak u pper limb acceleration correlated with diaphragm EMG amplitude. These findi ngs support the argument that diaphragm contraction is related to trunk con trol. 6. The results indicate that activity of human phrenic motoneurones is orga nised such that it contributes to both posture and respiration during a tas k which repetitively challenges trunk posture.