The effect of lung volume on the co-ordinated recruitment of scalene and sternomastoid muscles in humans

Abstract

The human scalenes are obligatory inspiratory muscles that have a greater mechanical advantage than sternomastoid, an accessory muscle. This study determined scalene and sternomastoid recruitment during voluntary inspiratory tasks, and whether this activity varied with lung volume, when feedback from the lungs and inspiratory muscles would differ. If afferent feedback has a major role in determining the recruitment of the scalenes and sternomastoid, then at each lung volume, activity would be altered. Intramuscular EMG from scalene and sternomastoid muscles, and oesophageal pressure were recorded while subjects (n = 7) performed inspiratory isovolumetric ramps to maximal inspiratory pressure (MIP) and dynamic inspirations from functional residual capacity (FRC) to total lung capacity (TLC). The static inspiratory ramps were repeated at three lung volumes: FRC, FRC + tidal volume, and TLC. To determine the profile of inspiratory activation, i.e. the initial and ongoing recruitment of the muscles, the root mean square of the EMG was measured throughout the tasks. Scalene was recruited early, and EMG increased with pressure, reaching a plateau at 80% MIP. In contrast, sternomastoid activity began later, but then increased with pressure from 20 to 100% MIP. Similar profiles of activation occurred at all three lung volumes (n.s.). The ratio of sternomastoid to scalene EMG was also the same irrespective of the initial lung volume (n.s.). In dynamic inspirations, scalene and sternomastoid activation had similar stereotypical profiles to the static tasks, but scalene EMG was 15-40% greater (P < 0.05). Sternomastoid activation was the same in both tasks (n.s.). These results suggest that in voluntary tasks, scalene and sternomastoid are recruited in the order of their mechanical advantages, and that alterations in feedback related to changes in lung volume failed to alter their activation. Thus, in humans, the mechanism responsible for the differential activation of these two inspiratory muscles has an element that is preset.

Figures

Figure 1. Experimental set up and analysis of static inspiratory ramps

A, subjects were comfortably seated and breathed through a mouthpiece with a solenoid-activated shutter. Electromyographic (EMG) recordings were made with bipolar electrodes inserted into the right scalene and sternomastoid muscles. Oesophageal pressure (Poes) was recorded. B, representative recordings during a 5 s inspiratory ramp (i.e. static task) at functional residual capacity (FRC). From top to bottom, panels show Poes, and r.m.s. and raw EMG from scalene and sternomastoid. Cursors were placed at baseline and maximal Poes and the corresponding time points were denoted 0% and 100% of the ramp, respectively. Poes, scalene r.m.s. EMG and sternomastoid r.m.s. EMG were then determined at 10% time intervals (○) to give a profile of muscle activation across the task. Negative pressure is upwards in the pressure trace. Vertical calibration: 500 μV.

Figure 4. Scalene and sternomastoid activation during inspiratory ramps at different lung volumes in one subject

Data from one subject during 6 static inspiratory ramps at each lung volume: FRC, FRC +Vt, and TLC. Poes is expressed as a percentage of MIP for each lung volume. Scalene and sternomastoid r.m.s. EMG is expressed as a percentage of maximum EMG across the three lung volumes (see Methods). A, scalene activation from 0 to 100% MIP during inspiratory ramps was consistent within and between lung volumes. B, sternomastoid activity was also similar within and between lung volumes, but the profile of activation is different from that for scalene (panel A). In this subject, sternomastoid was not recruited until after ∼40% MIP.

Figure 5. The profile of activation of scalene and sternomastoid at different lung volumes

Mean ±s.e.m data from six subjects during 5 s inspiratory ramps at FRC (•), FRC +Vt (▾), and TLC (○). Poes is expressed as a percentage of MIP for each lung volume. Scalene and sternomastoid r.m.s. EMG are expressed as a percentage of maximum EMG across the three lung volumes. A, at all lung volumes, scalene EMG increased with Poes before reaching a plateau at ∼80% MIP, and a similar profile of activation was observed irrespective of initial lung volume. Scalene EMG is not 0% at 0% MIP because some subjects could not relax fully against the shutter before the ramp, especially near TLC. There was also some recruitment of scalene prior to an increase in Poes (% MIP). B, in contrast to scalene (panel A), sternomastoid recruitment is delayed. At all lung volumes, sternomastoid EMG does not increase until after 20% MIP of the inspiratory ramps. From 20 to 100% MIP, sternomastoid EMG increases similarly at FRC, FRC +Vt, and TLC. C, to compare the activity of sternomastoid to that of scalene, the ratio of sternomastoid EMG (panel B) to scalene EMG (panel A) was plotted for each lung volume (mean ±s.e.m.). As subjects could not relax their scalene before the inspiratory task, data at 0% MIP are not shown. There is no difference in sternomastoid: scalene activation during inspiratory ramps at different lung volumes.

Figure 3. Data from a single subject during static inspiratory ramps

Poes, r.m.s., and raw EMG from scalene and sternomastoid during inspiratory ramps at different lung volumes for a typical subject. Negative pressure is upwards in the pressure trace. A, during a static task at FRC, the subject generated a MIP of 104 cmH2O. Scalene was active early, at the start of the ramp, but sternomastoid recruitment was delayed. B, at FRC +Vt, the subject generated a MIP of 96 cmH2O. The recruitment of scalene and sternomastoid is similar to panel A. C, at a high lung volume, TLC, (see Methods), the subject could only generate 54 cmH2O. However, scalene and sternomastoid activity remain similar to that at FRC (panel A) and FRC +Vt (panel B).

Figure 2. Analysis of scalene and sternomastoid activation during dynamic tasks

A, representative recordings during dynamic tasks, i.e. inspiration from FRC to total lung capacity (TLC) with the shutter open. From top to bottom, panels show lung volume, Poes (negative pressure upwards) and scalene and sternomastoid r.m.s. EMG. Cursors were placed at the onset of scalene or sternomastoid activity (arrows), and at peak inspired volume (continuous vertical line). For analysis, these time points were denoted 0% and 100%, respectively, and then volume, Poes and scalene or sternomastoid EMG values were determined at 10% time intervals. For each time point (e.g. vertical dashed line) and corresponding volume (‘x’), the estimated maximal inspiratory pressure (MIP) that each subject could generate at that volume was determined (‘y’, panel B). Estimated MIP was used to normalize ΔPoes (‘z’) at that time interval. B, MIP that each subject (○) and the group (•, mean ±s.e.m,n= 7) could generate at each lung volume. For each subject, MIP was greatest at FRC (0 l), decreased at FRC + tidal volume (Vt) (∼+ 1 l; P < 0.05) and further decreased at TLC (∼+2.5 l; P < 0.05). The estimated MIP (‘y’) for each lung volume (‘x’) was calculated for each subject (○) for analysis of the dynamic tasks (panel A).

Figure 6. Activity in scalene and sternomastoid during dynamic tasks in a single subject, and comparison with activation in static inspiratory ramps

Scalene and sternomastoid EMG are expressed as a percentage of maximum EMG during inspiratory ramps (see Methods). For the dynamic tasks, pressure is expressed as a percentage of estimated MIP (MIPe; Fig. 2B, ○). For the static inspiratory ramps in panel B, EMG and Poes (% MIP) are average values from the ramp at FRC (Fig. 5A). A, scalene and sternomastoid activation during 6 dynamic breaths to TLC for a typical subject. As for static inspiratory ramps (Fig. 4), scalene and sternomastoid activity are consistent in repeated ramps, but different between muscles. Scalene is active early, but sternomastoid recruitment is delayed until ∼20% MIPe. B, mean ±s.e.m data from 6 subjects during dynamic breaths (•), compared to the mean ±s.e.m data from the static tasks at FRC (○), for 0–80 Poes (% MIP or % MIPe). In scalene, activity is greater during the dynamic tasks compared to static inspiratory tasks (top panel), but there is no difference in the activation of sternomastoid (bottom panel).