Differences in gait electromyographic activity in stroke and healthy subjects using Discrete Wavelet Transform
Hemiplegic gait is a common outcome of stroke. Hence, to assess asymmetric control of poststroke gait, this work analyzed time-frequency domain (TFD) of surface electromyography (sEMG). Methodology: 9 chronic stroke patients (SP) and 10 healthy controls (HC) were compared using Discrete Wavelet Transform (DWT) to study the electrophysiology underlying tibialis anterior (TA) activity. DWT was applied to filtered sEMG signals during 10 walking trials. Percent of energy within frequency bands (25Hz bandwidth, 25 to 300Hz) was compared in stance (ST) and swing (SW) phases using Wilcoxon’s test (dominant vs. nondominant in HC, paretic vs. nonparetic in SP) and Mann-Whitney U-test (paretic, nonparetic vs. HC). Results: The energy at each frequency band may reflect the number and type of recruited motor units (MUs). For dominant vs. nondominant HC, a marked distribution along gait was found: low-frequency MUs prevailed in ST (25-50Hz), while slow (50-75Hz) and fast (200-225Hz) did in SW for effective foot clearance. Yet, differences in HC were observed, probably due to subjects’ variability or other effects (i.e., nondominant required more energy for the same task in ST). This physiological distribution was altered in SP. For paretic vs. HC, paretic ST had lower energy. In SW, it increased at 25-50Hz, but decreased at 75-100 and 150-300Hz. This overall reduction explains the typical TA weakness in stroke. Energy changes in bands suggest alterations in MUs recruitment: 50-75Hz MUs that sustain HC ST were replaced by slower MUs in paretic (25-50Hz). Also, faster MUs were reduced in paretic ST (75-125, 175-275Hz) and SW (75-100, 150-300Hz). For nonparetic vs. HC, energy was lower in 25-50Hz, but it increased in 75-99, 125-150 and 175-200Hz, likely because of nonparetic mechanisms that compensate the inability to activate the affected TA. During SW, nonparetic showed a rise in low frequencies (25-50, 50-75Hz) and a reduction in 175-250Hz, contrary to the HC pattern of MUs during SW. For paretic vs. nonparetic, energy in paretic was lower in all bands. In SW, energy in paretic increased in 25-50 and 225-300Hz bands and was reduced in 50-75 and 150-200Hz. Thus, paretic relies on the slowest (25-50Hz) and fastest (225-300Hz) MUs, rather than the 50-75Hz (HC and nonparetic) or 200-225Hz (HC) and 150-174Hz (nonparetic) ones. Conclusion: DWT of TA showed altered spectral attributes in stroke gait. The energy distribution in HC was modified in paretic and nonparetic, which may indicate a pathological alteration in the ratio of recruited MUs of each class during gait. In paretic SW this change is pronounced, possibly due to the occurrence of foot drop. Energy distribution characterized poststroke gait and asymmetries of dominant vs. nondominant and paretic vs. nonparetic. In the future, these findings will be compared with Continuous Wavelet Transform and new features will be extracted from TFD to provide a deeper description of neuromuscular altered mechanisms.