Sensorimotor interactions within the context of muscular fatigue – an example

Mihai TARATA1, John SMITH2
1Medical Informatics, University of Medicine and Pharmacy, Bul. Maresal Antonescu 62, Craiova, RO-1100, Romania, mihai@umfcv.ro
2 Another University, 1000 New Street, City, Country, email@emaildom.xx

Abstract. The paper uses results from original experiments on fatiguing isometric exercise in humans, as an illustrative background for an insight into the different mechanisms involved in fulfilling the motor task with a minimal cost.
The paper also aims to estimate to what extent SEMG can be used to explore  the sensorimotor interactions in exhausting isometric exercise, perhaps not at an individual neuronal level, but on a larger scale, which allows trends to be traced in the motor behavior in the context of the developing fatigue.

1.   Introduction

During the voluntary contraction of a muscle, two mechanisms work together to produce force: the firing frequency, and the recruitment of the motor units. Increasing the firing frequency, or the number of recruited motor units, contributes to increasing the force. It has been reported that during sustained isotonic isometric contractions the firing rate of the motor units decreases as a function of time [2]. 
The following shows to what extent the fatigue can be explained from information provided by the SEMG.

2.   Materials and methods 

Motivated subjects participated, each group aged between 22 and 40. A pair of  Ag/AgCl, 15 mm diameter surface EMG disk electrodes, …
a second similar pair of electrodes were placed on the Brachioradialis muscle of  the same arm, under the same technical conditions. 
The power density spectra were computed via zero padding to 1024 point FFT (Fast Fourier Transform) for each data buffer, to allow the interpolation for a finer frequency resolution.

2.1 The parameters

PSD: The PSD of a stationary random process xn is estimated via the discrete-time Fourier transform:
                                                    (1)
where FFT is the Fourier transform of the specific sequence. 
MF: MF is the frequency, which separates the power spectrum into two equal surface regions, and has been proved as a reliable estimator of the muscle fatigue [11,12,13].

3.   Results and discussion

Within the functional frame that we have described for a sustained isometric exercise we distinguish between a macro and a micro behavior. 
This shows a progressive recruitment of new motor units (increased activation) in the both muscles (Figures 1,2), which is better shown in the PSD of the mechanical vibrations of the wrist, proving also an increased activity in the higher frequency domain.

 Figure 1. The evolution of the PSD EMG with time, on contour plot, for    the Brac hioradialis muscle, together with the evolution of the resultant force at the wrist, RMS and MF of the EMG. 

All these are the proof of the dominant role of the central intervention in the optimization of the system.

4.   Conclusion

The above results show that the SEMG can be used to explore the sensorimotor interactions 
…
The technique also helps in understanding the interaction between muscles, or groups of muscles and the inner activation mechanisms during sustained exercise.

References

[1] C.J. De Luca, Control  properties of motor units, J. of Experimental Biology 115(1985) 125-136.
[2] F.B.Stulen, C.J. De Luca, Frequency parameters of the myoelectric signal as a measure of muscle conduction velocity, IEEE Trans. Biomed. Eng., vol. BME-28 (1981)  515-522.
…
 [6] E. Henneman, Relation between size of neurons and their susceptibility to discharge, Science 126(1957) 1345-1346.
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 [13] M. Tarata, Monitoring the Evolution of the Muscular Fatigue, Via New Parameters developed from the EMG Signal,  In: Proceedings of the  ECSAP'97 The First European Conference on Signal Analysis and Predic- tion, Prague, Czech Republic, 24-27 June 1997,  ISBN 80-7080-282-0, ICT Press, Praha, 1997,  p. 431-434
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 [16] U. Windhorst,  G.  Boorman, Overview: potential role of segmental motor circuitry in muscle fatigue, In: S.C. Gandevia et al. (ed.), Fatigue, .Plenum Press, New York, 1995, pp. 241-258.