Surface Electromyography to Quantify the Gradient of Neural Drive to the Human Parasternal Intercostal Muscles
Authors List
Anna L Hudson1,2,3, Billy L Luu2,3, Simon C Gandevia2,3 & Jane E Butler2,3
1Flinders University, 2Neuroscience Research Australia and 3University of New South Wales.
There is a rostrocaudal gradient of neural drive to the human parasternal intercostal muscles with earlier onset of muscle activity in the rostral interspaces compared to the caudal spaces (Gandevia et al 2006; doi.org/10.1113/jphysiol.2005.101915). This gradient is matched to the respiratory effectiveness of the muscles, which optimises the efficiency of lung ventilation. Electromyographic activity (EMG) was measured previously using an intramuscular needle electrode. The aim of this study was to determine if the gradient of neural drive in the human parasternal intercostal muscles can be evaluated with non-invasive, surface recordings of EMG. We recruited 20 healthy participants (10 females, aged 28.2 ± 7.4 years, BMI 23.0 ± 2.6; mean ± SD). EMG over the 1st-to-5th right parasternal intercostal muscles and electrocardiogram (ECG) were measured with bipolar surface electrodes. Participants breathed for 10 minutes through a mouthpiece (wearing a nose clip) attached to a pneumotachometer and pressure and CO2 sensors.
Off-line EMG recordings were filtered to remove ECG artefact using an independent component analysis and wavelet transform technique. The onset of inspiratory EMG was measured from waveform averages (range 44 -165 breaths) by a single investigator, blinded to the muscle. The parasternal intercostal muscles were imaged with ultrasound to estimate muscle depth, relative to the skin surface (for n=18 participants). During eupnoea, the average inspiratory time was 2.30 ± 0.88 s, tidal volume was 0.69 ± 0.23 l and inspiratory mouth pressure was -1.18 ± 0.29cmH2O. The onset of inspiratory
EMG activity differed between parasternal intercostal spaces (p<0.001, RM ANOVA; Fig 1). The distance between the skin and parasternal intercostal muscle was smallest for the 1st interspace (1.47 ± 0.27 cm) and largest for the 4th space (1.94 ± 0.40 cm). There was no effect of sex on the muscle depth or EMG onset times.
Waveform averages of surface EMG can indicate differences in the timing of inspiratory parasternal intercostal muscle activity during eupnoea. The onset times in the 4th and 5th interspaces are variable, consistent with the smaller EMG signal amplitude in these muscles (Gandevia et al 2006). These data show that surface recordings are a feasible method to measure the gradient of neural drive to the human parasternal intercostal muscles. This method may be particularly useful in vulnerable populations such as chronic lung disease and spinal cord injury.
Funding: NHMRC (Australia), Flinders Foundation
Anna L Hudson1,2,3, Billy L Luu2,3, Simon C Gandevia2,3 & Jane E Butler2,3
1Flinders University, 2Neuroscience Research Australia and 3University of New South Wales.
There is a rostrocaudal gradient of neural drive to the human parasternal intercostal muscles with earlier onset of muscle activity in the rostral interspaces compared to the caudal spaces (Gandevia et al 2006; doi.org/10.1113/jphysiol.2005.101915). This gradient is matched to the respiratory effectiveness of the muscles, which optimises the efficiency of lung ventilation. Electromyographic activity (EMG) was measured previously using an intramuscular needle electrode. The aim of this study was to determine if the gradient of neural drive in the human parasternal intercostal muscles can be evaluated with non-invasive, surface recordings of EMG. We recruited 20 healthy participants (10 females, aged 28.2 ± 7.4 years, BMI 23.0 ± 2.6; mean ± SD). EMG over the 1st-to-5th right parasternal intercostal muscles and electrocardiogram (ECG) were measured with bipolar surface electrodes. Participants breathed for 10 minutes through a mouthpiece (wearing a nose clip) attached to a pneumotachometer and pressure and CO2 sensors.
Off-line EMG recordings were filtered to remove ECG artefact using an independent component analysis and wavelet transform technique. The onset of inspiratory EMG was measured from waveform averages (range 44 -165 breaths) by a single investigator, blinded to the muscle. The parasternal intercostal muscles were imaged with ultrasound to estimate muscle depth, relative to the skin surface (for n=18 participants). During eupnoea, the average inspiratory time was 2.30 ± 0.88 s, tidal volume was 0.69 ± 0.23 l and inspiratory mouth pressure was -1.18 ± 0.29cmH2O. The onset of inspiratory
EMG activity differed between parasternal intercostal spaces (p<0.001, RM ANOVA; Fig 1). The distance between the skin and parasternal intercostal muscle was smallest for the 1st interspace (1.47 ± 0.27 cm) and largest for the 4th space (1.94 ± 0.40 cm). There was no effect of sex on the muscle depth or EMG onset times.
Waveform averages of surface EMG can indicate differences in the timing of inspiratory parasternal intercostal muscle activity during eupnoea. The onset times in the 4th and 5th interspaces are variable, consistent with the smaller EMG signal amplitude in these muscles (Gandevia et al 2006). These data show that surface recordings are a feasible method to measure the gradient of neural drive to the human parasternal intercostal muscles. This method may be particularly useful in vulnerable populations such as chronic lung disease and spinal cord injury.
Funding: NHMRC (Australia), Flinders Foundation
