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First published online September 16, 2005
Journal of Experimental Biology 208, 3761-3770 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01825

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Does daily activity level determine muscle phenotype?

John A. Hodgson¹, Roland R. Roy^1,*, Norio Higuchi³, Ryan J. Monti¹, Hui Zhong¹, Elena Grossman² and V. Reggie Edgerton^1,2

¹ Brain Research Institute, University of California Los Angeles, Los Angeles, CA 90095-1761, USA
² Department of Physiological Science, University of California Los Angeles, Los Angeles, CA 90095-1761, USA
³ Faculty of Political Science and Economics, Matsusaka University, Matsusaka, Japan

View larger version (27K): [in a new window]   Fig. 1. Examples of EMG data recorded during daily cage activity. Raw EMG (gray lines) and smoothed rectified EMG (black lines) are shown during periods of sustained (posture-like) activity (A) and phasic (locomotor-like) activity (B). Sol, soleus, MG, medial gastrocnemius, TA, tibialis anterior, VL, vastus lateralis.

View larger version (29K): [in a new window]   Fig. 2. (A) Mean integrated EMG (IEMG) and (B) mean duration of EMG h-1 over a 24 h recording period for six rats. The rats were in darkness between 9:00 h and 21:00 h daily (shaded bars on abscissa). Values are means ± 1 S.D. Abbreviations as in Fig. 1.

View larger version (24K): [in a new window]   Fig. 3. (A) Mean daily integrated EMG (IEMG), (B) mean EMG burst amplitude, and (C) mean daily duration of EMG over an entire 24 h period for six rats. The dark and light portions of the bars in (A) and (C) represent the dark and light periods. (D) Mean daily EMG amplitude over the entire 24 h period, i.e. including all periods of inactivity. Values are means ± 1 S.D. Abbreviations as in Fig. 1. s, m, t and v, significantly different from soleus, MG, TA and VL, respectively.

View larger version (28K): [in a new window]   Fig. 4. (A) Distributions of the daily EMG amplitudes for the four muscles from one rat over a 24 h recording period (thick line). Note that the ordinate is a logarithmic scale. The lower maximum amplitude of the soleus was verified from the raw EMG data and was typical for all rats. The thin black line shows the distributions predicted for 20 min and the light gray shaded area 10 min of locomotor-like activity. These predictions were based on the distribution of EMG amplitudes recorded during a bout of phasic EMG activity (see locomotor-like activity in Fig. 1). (B) Graphs of the total time that EMG activity exceeded the amplitude level identified on the abscissa. Results from the six rats are plotted as separate curves to show the consistency across animals. Note that the Sol muscle had higher amplitudes for a greater amount of time and that the TA clearly had lower amplitudes for a lesser amount of time compared to the other muscles studied. The vertical dotted line in both (A) and (B) highlights an EMG amplitude of 0.25 mV, to illustrate the duration of activity at this relatively low amplitude. See text for details.

View larger version (13K): [in a new window]   Fig. 5. (A) A survey of normal daily muscle activity from rat, cat, monkey and human muscles and (B) the percentage of type I (slow) muscle fibers. (C) The relationship between muscle fiber type and the duration of daily activity. The species are identified by the color of the bars and symbols and the muscles are denoted by symbol shape. The phenotype data are from the following sources: rats (Ariano et al., 1973; Armstrong and Phelps, 1984; Delp and Duan, 1996), cats (Ariano et al., 1973; Braund et al., 1995; Talmadge et al., 1996), Rhesus monkeys (Grichko et al., 1999; Roy et al., 1991a) and humans (Saltin and Gollnick, 1983). Additional EMG data are taken from the following sources: rats (Alford et al., 1987; Blewett and Elder, 1993), cats (Alaimo et al., 1984; Hensbergen and Kernell, 1997, 1998), Rhesus monkey (Hodgson et al., 2001) and humans (Edgerton et al., 2001; Kern et al., 2001). Abbreviations as in Fig. 1.

View larger version (22K): [in a new window]   Fig. 6. A diagram illustrating the hypothetical relationship between EMG amplitude, muscle fiber (black) and motor unit (blue) recruitment, and the duration of EMG activity (red) observed in the present study. See text for details.

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