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The aim of this study was to evaluate the suitability of ultrasonography for the quantification of gastrocnemius muscle architecture in healthy young children. The variation and reliability of measurement of muscle thickness, pennation angle and fibre length of the medial gastrocnemius were determined, using stationary and portable ultrasound machines, in 13 boys and eight girls aged 4–10. Ultrasound images were obtained from each leg, in duplicate, with the ankle at 90°, then at maximal plantar flexion, with the two machines within the same session. The same set of 16 scans was repeated in four children 4–6 weeks later. The mean muscle thickness, pennation angle and fibre length differed between ankle positions and between legs. Measurements obtained using the two machines established similar values with no significant differences in absolute values and coefficients of variation (CV). For duplicate images taken during the same session for the same leg, ankle position and machine, the CV and intraclass correlation coefficients (ICC) ranged, respectively, from 2·1% to 3·1% and 0·94–0·98 for muscle thickness, from 4·1% to 6·0% and 0·85–0·96 for pennation angle and from 4·5% to 6·3% and 0·87–0·96 for fibre length. Corresponding values for variables for the same child measured on two separate occasions were within the same ranges, all being similar to reliability data reported previously for adult muscle. Muscle thickness, pennation angle and fibre length of the medial gastrocnemius can therefore be quantified reliably, using either a stationary or portable ultrasound machine, in healthy young children. [ABSTRACT FROM AUTHOR]

Intramuscular injections of botulinum toxin A (Btx-A) and exercise are used in the treatment of muscle spasticity in children with cerebral palsy. However, little is known about the biological changes within muscle subsequent to Btx-A-induced paralysis and how the combination of Btx-A and exercise might affect the growing muscle. The wet mass, myosin heavy chain (MHC) composition, and titin content of the juvenile rat gastrocnemius muscle were determined 3 weeks after Btx-A injections and subsequent voluntary wheel-running exercise. Btx-A increased the proportion of type IIa (+121%) and IIx (+65%) MHC while decreasing the proportion of type IIb MHC (−51%) and reducing the titin content (−18%). Exercise did not amplify or reduce the changes induced by Btx-A. Thus, we conclude that although the sarcomeric stability of paralyzed muscle might be impaired, moderate mechanical loading does not seem to affect paralyzed muscle protein composition. Muscle Nerve, 2008 [ABSTRACT FROM AUTHOR]

Intramuscular injections of the paralytic botulinum neurotoxin A (Btx) and physical exercise are used in the treatment of chronic spasticity in children with cerebral palsy. We tested whether Btx-induced paralysis and/or exercise training would have differential effects on the expression of mechanosensing and signalling genes implicated in the adaptive remodelling of skeletal muscle. Juvenile (29-day-old) male rats were injected with Btx or saline (NoBtx) into the right gastrocnemius and housed in standard cages (NoEx) or with running wheels (Ex), for 3 weeks ( n= 6 per group). The mRNA expression of nine sarcomere-associated genes in the medial gastrocnemius was then determined by quantitative reverse transcriptase-polymerase chain reaction. The Btx-injected muscles weighed 50% less than NoBtx muscles, but Ex had no effect on the wet mass of Btx or NoBtx muscles. Atrogenic MuRF1, sarcomeric Titin and myogenic MyoD were upregulated (2-fold) with the elimination of contractile activity in Btx muscle. Expression of CARP, Ankrd2 and MLP was increased with mechanical stimuli associated with Btx (5- to 10-fold) or Ex (2- to 4-fold). Expression of CARP and Ankrd2 increased synergistically in Btx–Ex muscle (≥20-fold), indicating that these genes may be sensitive to passive stretch of the sarcomeric I-band region of titin to which their proteins bind. Tcap, Myopalladin and Atrogin1 were not, or were no longer responsive to the altered mechanical stimuli after 3 weeks of Btx or Ex. The expression of Ankrd2, CARP and MLP may thus be enhanced by passive stretch within the Btx-paralysed and/or exercising gastrocnemius and contribute to adaptations, other than muscle mass, in juvenile rats. [ABSTRACT FROM AUTHOR]

The myogenic regulatory factor MyoD plays an important role in embryonic and adult skeletal muscle growth. Even though it is best known as a marker for activated satellite cells, it is also expressed in myonuclei, and its expression can be induced by a variety of different conditions. Several model systems have been used to study the mechanisms behind MyoD regulation, such as exercise, stretch, disuse, and denervation. Since MyoD reacts in a highly muscle-specific manner, and its expression varies over time and between species, universally valid predictions and explanations for changes in MyoD expression are not possible. This review explores the complex role of MyoD in muscle plasticity by evaluating the induction of MyoD expression in the context of muscle composition and electrical and mechanical stimulation. Muscle Nerve, 2008 [ABSTRACT FROM AUTHOR]

The aims of this study were to characterize the pattern of voluntary activity of young rats in response to resistance loading on running wheels and to determine the effects of the activity on the growth of six limb skeletal muscles. Male Sprague–Dawley rats (4 weeks old) were housed individually with a resistance running wheel (R-RUN, n= 7) or a conventional free-spinning running wheel (F-RUN, n= 6) or without a wheel, as non-running control animals (CON, n= 6). The torque required to move the wheel in the R-RUN group was progressively increased, and the activity (velocity, distance and duration of each bout) of the two running wheel groups was recorded continuously for 45 days. The R-RUN group performed many more, shorter and faster bouts of running than the F-RUN group, yet the mean daily distance was not different between the F-RUN (1.3 ± 0.2 km) and R-RUN group (1.4 ± 0.6 km). Only the R-RUN resulted in a significantly ( P < 0.05) enhanced muscle wet mass, relative to the increase in body mass, of the plantaris (23%) and vastus lateralis muscle (17%), and the plantaris muscle fibre cross-sectional area, compared with CON. Both F-RUN and R-RUN led to a significantly greater wet mass relative to increase in body mass and muscle fibre cross-sectional area in the soleus muscle compared with CON. We conclude that the pattern of voluntary activity on a resistance running wheel differs from that on a free-spinning running wheel and provides a suitable model to induce physiological muscle hypertrophy in rats. [ABSTRACT FROM AUTHOR]