Abstract
Force variability during constant force tasks is directly related to oscillations below 0.5 Hz in force. However, it is unknown whether such oscillations exist in muscle activity. The purpose of this paper, therefore, was to determine whether oscillations below 0.5 Hz in force are evident in the activation of muscle. Fourteen young adults (21.07±2.76 years, 7 women) performed constant isometric force tasks at 5% and 30% MVC by abducting the left index finger. We recorded the force output from the index finger and surface EMG from the first dorsal interosseous (FDI) muscle and quantified the following outcomes: 1) variability of force using the SD of force; 2) power spectrum of force below 2 Hz; 3) EMG bursts; 4) power spectrum of EMG bursts below 2 Hz; and 5) power spectrum of the interference EMG from 10–300 Hz. The SD of force increased significantly from 5 to 30% MVC and this increase was significantly related to the increase in force oscillations below 0.5 Hz (R 2 = 0.82). For both force levels, the power spectrum for force and EMG burst was similar and contained most of the power from 0–0.5 Hz. Force and EMG burst oscillations below 0.5 Hz were highly coherent (coherence = 0.68). The increase in force oscillations below 0.5 Hz from 5 to 30% MVC was related to an increase in EMG burst oscillations below 0.5 Hz (R 2 = 0.51). Finally, there was a strong association between the increase in EMG burst oscillations below 0.5 Hz and the interference EMG from 35–60 Hz (R 2 = 0.95). In conclusion, this finding demonstrates that bursting of the EMG signal contains low-frequency oscillations below 0.5 Hz, which are associated with oscillations in force below 0.5 Hz.
Highlights
Force control is essential in many activities of daily living
They determined bursting of muscle activity by low-pass filtering the rectified EMG signal at 5 Hz. They found that force variability was temporally related to the EMG burst signal. These findings indicate that the EMG burst can be used to identify the lowfrequency oscillations in muscle activity that contribute to force variability
This finding is in line with our previous work demonstrating that an increase in force variability is related to an increase in force oscillations from 0–0.5 Hz
Summary
Force control is essential in many activities of daily living. The inability to control force is quantified as increased force variability and has functional consequences such as diminished capacity to execute accurate movements [1,2,3]. Previous studies have demonstrated that the variability of the force output is related to oscillations in force below 4 Hz [6,7,8]. Clearly demonstrate that oscillations in force below 0.5 Hz contribute significantly to force variability [4,5]. This is demonstrated by a positive and strong association between force variability and force oscillations ,0.2 Hz for both young and older adults. The importance of this study is to further our understanding on how the central nervous system oscillates multiple motor units to control the force output
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