Superficial motor units are larger than deeper motor units in human vastus lateralis muscle

Previous studies have suggested that regionalization may occur for human motor units, whereby smaller motor units are located in deeper parts of the muscle and larger motor units are located in more superficial portions. We examined this possibility in the human vastus lateralis muscle using macro‐EMG (electromyography) to estimate motor unit size. The sample consisted of nine individuals from whom 114 motor units were recorded at forces ranging between 5% and 60% MVC. Peak‐to‐peak macro‐EMG amplitude was well correlated with macro area (Spearman rho = 0.96). There was a statistically significant inverse relationship between recording depth and macro peak‐to‐peak amplitude (rho = −0.402, p < 0.001). We conclude that there is a nonrandom distribution of motor units in human muscle, with larger motor units located in more superficial regions and smaller units located in deeper regions. Clinicians who monitor motor unit activity need to recognize that a representative sample of motor unit recordings should include motor units from both deeper and more superficial regions of muscle. Muscle Nerve, 2005

[1]  H P Clamann,et al.  Activity of single motor units during isometric tension , 1970, Neurology.

[2]  D. Downham,et al.  Differences in fiber number and fiber type proportion within fascicles. A quantitative morphological study of whole vastus lateralis muscle from childhood to old age , 1992, The Anatomical record.

[3]  D. Downham,et al.  Distribution of different fibre types in human skeletal muscles A statistical and computational study of the fibre type arrangement in m. vastus lateralis of young, healthy males , 1984, Journal of the Neurological Sciences.

[4]  G Kamen,et al.  Motor unit discharge behavior in older adults during maximal-effort contractions. , 1995, Journal of applied physiology.

[5]  G C Elder,et al.  Variability of fiber type distributions within human muscles. , 1982, Journal of applied physiology: respiratory, environmental and exercise physiology.

[6]  D. Hostler,et al.  Fiber type composition of the vastus lateralis muscle of young men and women. , 1999, The journal of histochemistry and cytochemistry : official journal of the Histochemistry Society.

[7]  B. Essén-Gustavsson,et al.  Spatial distribution of fiber types within skeletal muscle fascicles from Standardbred horses , 2002, The Anatomical record.

[8]  Á. Cobos,et al.  Muscle fibre types in the suprahyoid muscles of the rat , 2001, Journal of anatomy.

[9]  E. Stålberg,et al.  Association of muscle strength and electrophysiological measures of reinnervation in diabetic neuropathy , 1998, Muscle & nerve.

[10]  D. Kernell,et al.  Fibre type regionalisation in lower hindlimb muscles of rabbit, rat and mouse: a comparative study , 2001, Journal of anatomy.

[11]  V. Edgerton,et al.  Muscle fibre type populations of human leg muscles , 1975, The Histochemical Journal.

[12]  A. Kimura,et al.  Macro-EMG and motor unit recruitment threshold: differences between the young and the aged , 1994, Neuroscience Letters.

[13]  E Stålberg,et al.  Macro EMG in healthy subjects of different ages. , 1982, Journal of neurology, neurosurgery, and psychiatry.

[14]  F Pernus,et al.  Arrangement of fiber types within fascicles of human vastus lateralis muscle , 1991, Muscle & nerve.

[15]  E Stålberg,et al.  Macro EMG, a new recording technique. , 1980, Journal of neurology, neurosurgery, and psychiatry.

[16]  F Pernus,et al.  Histochemical and morphometric characteristics of the normal human vastus medialis longus and vastus medialis obliquus muscles. , 1995, Journal of anatomy.

[17]  F. Richmond,et al.  In‐series fiber architecture in long human muscles , 1993, Journal of morphology.

[18]  R. Turek,et al.  Muscle fibre variation in the gluteus medius of the horse. , 1985, Equine veterinary journal.

[19]  M. Johnson,et al.  Data on the distribution of fibre types in thirty-six human muscles. An autopsy study. , 1973, Journal of the neurological sciences.

[20]  A. Kimura,et al.  Macro-EMG and motor unit recruitment threshold. , 1993, Muscle and Nerve.

[21]  Thorstensson.  Muscle strength, fibre types and enzyme activities in man. , 1976, Acta physiologica Scandinavica. Supplementum.

[22]  J Lexell,et al.  Distribution of different fibre types in human skeletal muscles. 2. A study of cross-sections of whole m. vastus lateralis. , 1983, Acta physiologica Scandinavica.

[23]  E. Kararizou,et al.  Distribution of muscle fibre types in human skeletal muscle fascicles: an autopsy study of three human muscles. , 1995, Functional neurology.

[24]  E. Kararizou,et al.  Size and proportion of fiber types in human muscle fascicles. , 1996, Clinical neuropathology.

[25]  D. Kernell,et al.  Muscle regionalization. , 1998, Canadian journal of applied physiology = Revue canadienne de physiologie appliquee.

[26]  M. Sjöström,et al.  Distribution of different fibre types in human skeletal muscles. I. Method for the preparation and analysis of cross-sections of whole tibialis anterior , 1983, The Histochemical Journal.

[27]  B. Nordgren,et al.  The quadriceps femoris muscle in 20–70‐year‐old subjects: Relationship between knee extension torque, electrophysiological parameters, and muscle fiber characteristics , 1989, Muscle & nerve.

[28]  H. Blau,et al.  Development of muscle fiber types in the prenatal rat hindlimb. , 1990, Developmental biology.

[29]  D. Kernell,et al.  Proximo-distal organization and fibre type regionalization in rat hindlimb muscles , 2004, Journal of Muscle Research & Cell Motility.

[30]  B. Picard,et al.  Regional variations of muscle fibre characteristics in m. semitendinosus of growing cattle , 1997, Journal of Muscle Research & Cell Motility.

[31]  D. Downham,et al.  Distribution of different fibre types in human skeletal muscles Fibre type arrangement in m. vastus lateralis from three groups of healthy men between 15 and 83 years , 1983, Journal of the Neurological Sciences.

[32]  Probabilities associated with counting average motor unit firing rates in active human muscle. , 1998, Canadian journal of applied physiology = Revue canadienne de physiologie appliquee.

[33]  I. Gath,et al.  In situ measurement of the innervation ratio of motor units in human muscles , 2004, Experimental Brain Research.

[34]  W. Nix,et al.  Single fiber macro versus concentric trigger macro EMG: A comparison of methods , 1992, Muscle & nerve.

[35]  H. Kryvi,et al.  Distribution patterns of muscle fibre types in major muscles of the bull (Bos taurus) , 2005, Anatomy and Embryology.

[36]  F. Richmond,et al.  Marked non-uniformity of fiber-type composition in the primate suboccipital muscle obliquus capitis inferior , 1999, Experimental Brain Research.

[37]  Á. Cobos,et al.  Muscle fibre types and their distribution in the biceps and triceps brachii of the rat and rabbit , 1998, Journal of anatomy.

[38]  P. Schantz,et al.  Muscle fibre type distribution, muscle cross-sectional area and maximal voluntary strength in humans. , 1983, Acta physiologica Scandinavica.