Force Control Is Related to Low-Frequency Oscillations in Force and Surface EMG

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.

Motor control differs for increasing and releasing force

“…We typically select the nondominant limb because the task would be more novel to both young and older adults compared with the dominant limbs (Chen et al 2012;Moon et al 2014;Onushko et al 2013). Each experimental session lasted ϳ1 h. Before the experimental session, participants were familiarized to the task and performed five practice trials.…”

“…We used this visual feedback to eliminate the effects of visuomotor corrections on force control Moon et al 2014;Tracy 2007).…”

“…6B). Low-frequency oscillations in muscle activity have been associated with reduced force control (Moon et al 2014). Motor unit modulation from 0 to 4 Hz, often referred to as common drive (De Luca 1985;De Luca and Erim 2002;Farina et al 2002), appears to be different for force increase and release.…”

“…Motor unit modulation from 0 to 4 Hz, often referred to as common drive (De Luca 1985;De Luca and Erim 2002;Farina et al 2002), appears to be different for force increase and release. Specifically, low-frequency oscillations in muscle activity (bursting) increase during force release and likely contribute to increased force variability (Moon et al 2014;Negro et al 2009;Yoshitake and Shinohara 2013). In summary, the control strategy used by the central nervous system appears to be different during force release.…”

“…Furthermore, there is evidence that the modulation of multiple motor units also differs for young and older adults . There is evidence that force control is strongly related to the modulation of multiple motor units (Farina et al 2002;Moon et al 2014;Onushko et al 2013;Shinohara 2011). In this study, therefore, we examine the activation of multiple motor units when participants increase and release force.…”