Activity of latissimus dorsi muscle during inspiratory threshold loads

Abstract

The ability of the latissimus dorsi muscle (LD) to participate as an accessory inspiratory muscle has been the subject of controversy. Electromyographic (EGM) activity of LD was evaluated in 11 healthy subjects (aged 30±2 yrs; forced expiratory volume in one second (FEV1) 106±5% predicted; maximal inspiratory pressure (PImax), 120±6 cmH2O) under different breathing conditions. The ipsilateral biceps brachii was chosen as the control muscle. The EMG was recorded from surface electrodes, but needle electrodes were also used for LD evaluation in a subset of three subjects. The EMG signal from both muscles was recorded simultaneously, rectified and integrated, with subtraction of the electrocardiographic signal. Situations evaluated were: 1) maximal voluntary contraction (MVC); 2) apnoea; and 3) breathing under progressive inspiratory threshold loads (20-100% PImax, at 20% intervals). A close relationship was evident between LD recordings from surface and needle electrodes (r=0.975). Activity of LD at baseline was 1.8±0.4% MVC, and showed a phasic increase during inspiration under loads. This change had a linear tendency and was significant for loads corresponding to 40, 60, 80 and 100% of PImax when compared to the control muscle. At this latter level, LD activity was equivalent to 32±5 % MVC (range 11-61%), whereas mean activity of the control muscle was less than 7.5% MVC. These results demonstrate that LD is progressively recruited in healthy subjects during inspiratory loading, and suggest that LD could participate as an accessory muscle for the breathing effort under specific conditions. For these reasons, LD does not appear to be an appropriate control for studies of the respiratory muscles.