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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.

The purpose of this study was to investigate whether acute passive stretching (APS) reduced maximal isometric voluntary contraction (MVC) of the plantar flexors (PF) and if so, by what mechanisms. The PF in 15 female volunteers were stretched for 10 min (5 x 120 s) by a torque motor to within 2 degrees of maximum dorsiflexion (D) range of motion (ROM). MVC with twitch interpolation, maximal Hoffmann reflex (H(max)) and compound action potentials (M(max)) were recorded at 20 degrees D. Stretch reflexes (SR) were mechanically induced at 200 degrees s(-1) between 0 degrees and 10 degrees ( )D and SR torque and EMG amplitude were determined. All tests were assessed pre- (pre) and post-APS (post-test(1)). MVC, SR, and M(max) were again assessed after additional stretch was applied [mean 26 (1) degrees D; post-test(2)] to test if the optimal angle had been altered. EMG was recorded from soleus (SOL), medial gastrocnemius (MG) and tibialis anterior (TA) using bipolar surface electrodes. APS resulted in a 27% decrease in mean peak passive torque (P0.05). MVC and SR torque were 7% (P0.05) and 13% lower at post-test(1) (P0.05), respectively. SR EMG amplitude of SOL and MG was reduced by 27% (P0.05) and 22% (P0.05), respectively. The H(max)/M(max) EMG and H(max)/M(max) torque ratios were unchanged at post-test(1). At post-test(2), MVC and SR EMG recovered to pre-APS values, while the SR and M(max) torque increased by 19% and 13%, respectively (P0.05). The decrease in MVC during post-test(1) was attributed to changes in the mechanical properties of PF and not to reduced muscle activation.

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

Eleutherodactylus coqui differs from most other anuran amphibians in that it has internal fertilization. This requires close apposition of male and female cloacae during amplexus, achieved by a reverse hindleg clasp (RHLC) where—rather than the male clasping the female with his forelimbs, as occurs in most anurans—the female adducts her hindlimbs around the male. Since normal amplexus in anurans is associated with enlarged forelimb muscles in males of some species, we asked whether the RHLC of E. coqui was associated with enlarged hindlimb adductors in the female or with other sexually dimorphic myological specializations. Several features, such as muscle fiber number, size, type, and overall cross-sectional area were compared between male and female adult E. coqui for two muscles essential to the RHLC and a control hindlimb muscle not used in the behavior. No sexual dimorphism was found that could be specifically related to the RHLC. While muscles in all cases were larger in the females than in the males, this was accounted for by the larger body size of females in this species.

Nonexercising muscles appear to be metabolically active during exercise. Animal models for this purpose have not been established. However, we have been able to teach animals to run on their forelimbs while their hindlimbs are suspended above the treadmill with no visible limb movement. To document that indeed this mode of exercise does not provoke additional muscle activity, we have compared the levels of neural activation of the soleus and plantaris muscles using a computer analysis of the electromyographic interference pattern, recorded from bipolar fine wire electrodes implanted across each muscle. Via computer analyses of the electromygraphic interference patterns the frequencies and amplitudes of motor unit action potentials were obtained. The data were sampled during 20 s of every minute of observation. Comparisons were made in four conditions: (i) resting on the treadmill while bearing weight on the hindlimbs (normal rest), (ii) running on the treadmill (15 m/min, 8% grade) on all four limbs (normal exercise), (iii) resting while the hindlimbs were suspended in a harness above the treadmill (suspended rest), and (iv) exercising with the forelimbs (15 m/min, 8% grade) while the hindlimbs were suspended above the treadmill (suspended exercise). All four experimental conditions were carried out for 90 min each and were performed by each animal. The results clearly show that muscle activities (frequencies and amplitudes), when the hindlimbs are suspended above the treadmill, at rest or during exercise, are lower than the activities in these same muscles when the animals are at rest, supporting only their body weight. Activities in the same muscles during exercise were from 300 to 2000% greater than during hindlimb suspension. Despite these large differences in muscle activities, comparable quantities of glycogen are lost from these hindlimb suspended (nonexercising) muscles during exercise and when these same muscles are performing the exercise. The physiologic data indicate that suspending the hindlimbs of a rat during forelimb exercise provides a convenient animal model for the study of nonexercising muscle during exercise. In fact the soleus and plantaris activities during suspended exercise are less than during normal rest.Key words: soleus, plantaris, nonexercising muscle, electromyography.

The effect of biofeedback training on ankle function was studied in young children with cerebral palsy (CP) during a pilot study and a 6-week follow-up study. Patients underwent range of motion (ROM) and ankle dorsiflexor (DF) strength training in a laboratory 3 days per week, receiving auditory and visual feedback. An at-home programme used portable EMG units to help train DF muscle recruitment on the remaining days. Independent outcome measures included DF strength, active ROM, and tapping ability pretraining and 6 weeks and 14 months posttraining. Tapping ability increased significantly in the trained leg posttraining. It then fell significantly at the 6-weeks posttraining test, but remained significantly higher than pretest levels, and returned to pretraining levels by 14 months. Passive ROM was unchanged, but active ROM increased significantly in the trained leg. DF strength increased in most children posttraining in both the pilot and main study. Increased motor-unit recruitment is believed to explain the increases in DF strength and active ROM. These results suggest that biofeedback training can improve ankle function, and the implications for gait are discussed.

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.

The effects of hind limb suspension on the development of dystrophic muscles was studied in five dystrophic hamsters (CHF 147, formally UMX 7.1) from age 20 days for 5 months. Their histochemical and contractile properties were compared to five aged-matched controls. Twitch and tetanic tensions (Po) were reduced in treated soleus by 22% and 32% and in plantaris by 29% and 39%, respectively; these reductions were proportional to their smaller cross-sectional areas. Twitch duration and half-relaxation times were not altered in experimental soleus muscle but the mean time to 50% of Po was 19 ms faster than the controls. Experimental soleus and plantaris had higher percentages of type II fibers, i.e., 63% and 97% compared with 37% and 93% in controls, respectively. A similar trend was seen in gastrocnemius and extensor digitorum longus muscles. Areas of both fiber types were reduced in all muscles, but only statistically significant differences in type I areas were seen in soleus and type II areas in the other three muscles. The percentages of fibers with centronuclei in whole cross sections, determined at different positions along the muscle lengths, were reduced by 30% in soleus and extensor digitorum longus. The areas of fiber necrosis were also reduced in experimental extensor digitorum longus and plantaris. These changes in nonweight-bearing muscles support the hypothesis that contractile activity can influence the development of dystrophic properties.