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Tonic firing behavior in soleus muscle of unrestrained rats aged 7 to ≥100 days was studied by chronic single-motor-unit and gross-electromyographic (EMG) recordings. Median motor-unit firing frequency at 10 days was 19–26 Hz and did not change appreciably after this time, whereas interval-to-interval firing variability was reduced with age. Two units with median frequencies 40 and 59 Hz were encountered in one 7-day-old rat. Integrated rectified gross EMG developed from being phasic only to predominantly tonic during the second and third postnatal week. From the end of the third week, rather short tonic periods with irregular amplitude were replaced by longer lasting constant-amplitude periods. Quantitatively, median duration of gross-EMG activity episodes more than doubled, while 90th-percentile values for episode duration increased 19-fold, from 7.4 s at 7 days to 140 s in adults. The main part of this increase took place after 22 days. Previous work in adult rats has indicated that descending monoaminergic innervation is essential for maintained tonic motoneuron activity, which probably is caused by depolarizing plateau potentials. Such innervation of the lumbar spinal cord matures gradually to an adult pattern and density ∼3–4 wk after birth. The present results, describing a concurrent considerable development of tonic firing behavior, support and extend these findings.

Glycogen decrements have been observed in non-exercising muscles during exercise. We therefore investigated whether the degraded glycogen was retained within the muscle in the form of glycolytic intermediates, or whether it was effluxed from the non-exercising muscles. For these studies a suspension harness was used to unload the hindlimb muscles at rest and during exercise [McDermott et al. (1987) J Appl Physiol 63:1275–1283]. Concentrations of glycogen and glycolytic intermediates glucose 6-phosphate, fructose 6-phosphate, fructose 1,6-bisphosphate, glycerol 3-phosphate, and lactate) were measured in non-exercising and exercising muscles (soleus, plantaris, red and white gastrocnemius) during a 90-min exercise bout 15 m/min, 8% grade). On-line electromyographic analysis showed that the contractile activity in the non-exercising muscles was markedly lower than in the exercising muscles. Similar decrements in muscle glycogen levels were observed in both the non-exercising and exercising muscles at the end of the 90-min, exercise bout (P

When the histochemical and contractile properties of infant muscles change postnatally, and what influence muscle function has on these changes, were the focus of this study. Contractile properties were measured in the plantaflexor (PF) and dorsiflexors (DF) of 19 newborns and 36 infants aged 5–16 months. Infants were tested between one and four times at monthly intervals. Measurements included maximal twitch tension (Pt), time to peak tension (TPT), and half-relaxation times (1/2RT). TPT was similar in PF (77 ms) and DF (73 ms) at birth, remained unchanged in DF, and slowed in PF to 110–120 ms between 9 and 12 months. Type I distributions were determined at autopsy in fetal through to adult muscles. Completion of differentiation occurred in soleus at about the age that contractile properties slowed and infants started to use these muscles more. A trend of higher percentages of type I distributions was also noted in children than either newborns or adults in other muscles. The implications of these findings and the clinical use of these methods for evaluating peripheral neuromuscular function is discussed. © 1993 John Wiley & Sons, Inc.

The objective of this study was to determine whether increased contractile activity is beneficial or detrimental to developing dystrophic muscle. Hamsters (20-days-old) were gradually introduced to running at a speed of 14 m/min at 10% grade for 2 (T2) or 4 (T4) h/d for 4 weeks, 5 d/wk. Histological and fiber type properties were determined in the soleus (SOL), plantaris (PL), and extensor digitorum longus and contractile properties in SOL and PL from 5 animals/group, including 5 controls. Experimental animals had normal body and muscle mass. Training for 2 h/d had little effect on SOL contractile properties, whereas 4 h/d resulted in significant increases in force, percentage of type I fibers, and type I hypertrophy. Force also increased in PL. Muscle necrosis was reduced in SOL (T2 and T4) and unchanged in PL. In conclusion, endurance training generally had a beneficial or, at least, no detrimental effect on developing dystrophic muscles.