Abstract
Motor unit synchronization was estimated from the surface electromyograms (EMG) of the first dorsal interosseus muscle of human volunteers by a simplified surface-EMG technique (Milner-Brown et al. 1973, 1975). Single motor units were identified from intramuscular recordings and were used to obtain a spike-triggered average of the surface-EMG. The discharge rate of a reference motor unit was controlled at two levels (high and low), and the effect of motor unit activity on the surface-EMG estimate of synchronization was studied in 56 motor units. The surface-EMG estimate of motor unit synchronization was significantly higher when the reference motor unit discharged at the high rate than when it discharged at the low rate. A regression analysis indicated that the synchronization ratio calculated from the surface EMG was significantly correlated with the level of EMG activity in the muscle. Motor unit synchronization was also estimated from surface-EMG measurements that were derived by computer simulation. The simulation permitted manipulation of motor unit activity (discharge rate and recruitment) with a complete absence of synchrony among the units in the pool. The stimulated surface-EMG index was influenced by an artifact associated with signal rectification, and this effect changed non-monotonically with motor unit activity. Furthermore, the increase in the motor unit activity reduced the signal-to-noise ratio of the spike-triggered surface EMG average, and consequently decreased the sensitivity of the surface-EMG index as an estimate of motor unit synchronization. We conclude that the simplified surface-EMG method (Milner-Brown et al. 1973, 1975) does not provide a useful index of motor unit synchronization due to its inability to accurately distinguish the synchronization from methodological effects related to a rectification artifact and variation in the signal-to-noise ratio.
This is a preview of subscription content, log in via an institution to check access.
Similar content being viewed by others
References
Baker JR, Davey NJ, Ellaway PH, Friedland CL (1992) Short-term synchronization of motor unit discharge during weak isometric contraction in Parkinson's disease. Brain 115:137–154
Bendat JS, Piersol AG (1986) Random data analysis and measurement procedures, 2nd edn. Wiley, New York
Coyle EF, Feiring DC, Rotkis TC, Cote RW III, Roby FB, Lee W, Wilmore JH (1981) Specificity of power improvements through slow and fast isokinetic training. J Appl Physiol 51:1437–1442
Datta AK, Farmer SF, Stephens JA (1991) Central nervous pathways underlying synchronization of human motor unit firing studied during voluntary contractions. J Physiol 432:401–425
Datta AK, Stephens JA (1990) Synchronization of motor unit activity during voluntary contraction in man. J Physiol 422:397–419
Dengler R, Wolf W, Birk P, Struppler A (1984) Synchronous discharges in pairs of steadily firing motor units tend to form clusters. Neurosci Lett 47:167–172
Dietz V, Bischofberger E, Wita C, Freund H-J (1976) Correlation between the discharges of two simultaneously recorded motor units and physiological tremor. Electroencephalogr Clin Neurophysiol 40:97–105
Duchateau J, Hainaut K (1984) Isometric or dynamic training differential effects on mechanical properties of a human muscle. J Appl Physiol 56:296–301
Elble RJ, Randall JE (1976) Motor-unit activity responsible for 8- to 12-Hz component of human physiological finger tremor. J Neurophysiol 39:370–383
Enoka RM, Robinson GA, Kossev AR (1989) Task and fatigue effects on low threshold motor units in a human hand muscle. J Neurophysiol 62:1344–1359
Fuglevand AJ, Winter DA, Patla AE, Stashuk D (1992) Detection of motor unit action potentials with surface electrodes: influence of electrode size and spacing. Biol Cybern 67:143–153
Fuglevand AJ, Winter DA, Patla AE (1993) Models of recruitment and rate coding organization in motor-unit pools. J Neurophysiol 70:2470–2488
Hamm TM, Roscoe DD, Reinking RM, Stuart DG (1985) Detection of synchrony in the discharge of a population of neurons. I. Development of a synchronization index. J Neurosci Methods 13:37–50
Kirkwood PA, Sears TA (1978) The synaptic connections to intercostal motoneurons as revealed by their average common excitation potential. J Physiol 275:103–134
Komi PV (1986) Training of muscle strength and power: interaction of neuromotoric, hypertrophic, and mechanical factors. J Sports Med 7:10–15
Lippold OCJ, Redfearn JWT, Vuco J (1957) The rhythmical activity of groups of motor units in the voluntary contraction of muscle. J Physiol 137:473–487
Milner-Brown HS, Stein RB, Yemm R (1973) The contractile properties of human motor units during voluntary isometric contractions. J Physiol 228:285–306
Milner-Brown HS, Stein RB, Lee RG (1975) Synchronization of human motor units: possible roles of exercise and supraspinal reflexes. Electroencephalogr Clin Neurophysiol 38:245–254
Moore GP, Perkel DH, Segundo JP (1966) Statistical analysis and functional interpretation of neuronal spike data. Annu Rev Physiol 28:493–522
Nordstrom MA, Miles TS, Türker KS (1990) Synchronization of motor units in human masseter during a prolonged isometric contraction. J Physiol 426:409–421
Nordstrom MA, Fuglevand AJ, Enoka RM (1992) Estimating the strength of common input to human motoneurons from the cross-correlogram. J Physiol 453:547–574
Person RS, Kudina LP (1968) Cross-correlation of electromyograms showing interference patterns. Electroencephalogr Clin Neurophysiol 25:58–68
Powers RK, Vanden Noven S, Rymer WZ (1989) Evidence of shared, direct input to motoneurons supplying Synergist muscles in humans. Neurosci Lett 102:76–81
Roscoe DD, Hamm TM, Reinking RM, Stuart DG (1985) Detection of synchrony in the discharge of a population of neurons. II. Implementation and sensitivity of a synchronization index. J Neurosci Methods 13:51–64
Sale DG (1987) Influence of exercise and training on motor unit activation. Exerc Sport Sci Rev 15:95–151
Sale DG (1988) Neural adaptation to resistance training. Med Sci Sports Exerc 20:S135-S145
Schmied A, Ivarsson C, Fetz EE (1993) Short-term synchronization of motor units in human extensor digitorum communis muscle: relation to contractile properties and voluntary control. Exp Brain Res 97:159–172
Sears TA, Stagg D (1976) Short-term synchronization of intercostal motoneurone activity. J Physiol 263:357–381
Taylor A (1962) The significance of grouping of motor unit activity. J Physiol 162:259–269
Tesch PA, Hjort H, Balldin UI (1983) Effects of strength training on G tolerance. Aviat Space Environ Med 54:691–695
Yue G, Fuglevand AJ, Nordstrom MA, Enoka RM (1992) Motor unit synchronization assessed from surface EMG increases with discharge rate of reference motor unit. Soc Neurosci Abst 18:1406
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Yue, G., Fuglevand, A.J., Nordstrom, M.A. et al. Limitations of the surface electromyography technique for estimating motor unit synchronization. Biol. Cybern. 73, 223–233 (1995). https://doi.org/10.1007/BF00201424
Received:
Accepted:
Issue Date:
DOI: https://doi.org/10.1007/BF00201424