Cortical implication in lower voluntary muscle force production in non-hypoxemic COPD patients

Abstract

Recent studies have shown that muscle alterations cannot totally explain peripheral muscle weakness in COPD. Cerebral abnormalities in COPD are well documented but have never been implicated in muscle torque production. The purpose of this study was to assess the neural correlates of quadriceps torque control in COPD patients. Fifteen patients (FEV1 54.1±3.6% predicted) and 15 age- and sex-matched healthy controls performed maximal (MVCs) and submaximal (SVCs) voluntary contractions at 10, 30 and 50% of the maximal voluntary torque of the knee extensors. Neural activity was quantified with changes in functional near-infrared spectroscopy oxyhemoglobin (fNIRS-HbO) over the contralateral primary motor (M1), primary somatosensory (S1), premotor (PMC) and prefrontal (PFC) cortical areas. In parallel to the lower muscle torque, the COPD patients showed lower increase in HbO than healthy controls over the M1 (p<0.05), PMC (p<0.05) and PFC areas (p<0.01) during MVCs. In addition, they exhibited lower HbO changes over the M1 (p<0.01), S1 (p<0.05) and PMC (p<0.01) areas during SVCs at 50% of maximal torque and altered motor control characterized by higher torque fluctuations around the target. The results show that low muscle force production is found in a context of reduced motor cortex activity, which is consistent with central nervous system involvement in COPD muscle weakness.

Figure 1. Experimental Design.

MVC: Maximal Voluntary Contraction, SVC: Submaximal Voluntary Contraction.

Figure 2. Measurement of cortical activity by functional near-infrared spectroscopy (fNIRS).

a) fNIRS optode placement. Three receivers (black circles) and seven emitters (white circles) were set over the scalp, resulting in 9 measured channels. The crosses represent the reference points used to target primary sensory (CP₃ - CP₁), primary motor (C₃ - C₁), premotor (FC₁ - FC₃) and prefrontal cortical areas (FP₁) according to the modified international EEG 10-10 system. I_z: Inion, N_z: Nasion.b) Example of a functional near-infrared spectrospcopy oxyhemoglobin signal (fNIRS-HbO) during a maximal voluntary contraction in one subject. Hatched area represents the area under the curve of HbO (as index of neural activity).

Figure 3. HbO changes during maximal voluntary contractions over primary motor (M1), primary sensory (S1), premotor (PMC) and prefrontal (PFC) cortex areas.

* p<0.05 and ** p<0.01 significantly different from controls.

Figure 4. Performed torque versus target torque during submaximal voluntary contractions at 10, 30 and 50% of maximal quadriceps torque (Q_MVC).

NS: Non-significant difference between target and performed torque.

Figure 5. HbO changes during submaximal quadriceps contractions as a function of % of maximal quadriceps torque (Q_MVC) over primary motor (M1), primary sensory (S1), premotor (PMC) and prefrontal (PFC) cortex areas.

Values in parenthesis on the x axis indicate the mean torque performed at the given % of maximal quadriceps torque. Significant differences from rest: ^† p<0.05 ^†† p<0.01 and ^††† p<0.001. Significant differences between controls and patients: * p<0.05, ** p<0.01 and *** p<0.001.

Figure 6. HbO changes over M1 as a function of absolute torque value at 10 (triangular shape), 30 (circular shape) and 50% (square shape) of the maximal voluntary torque.