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1. Rat plantaris muscles were subjected to chronic overload by the surgical removal of the soleus and most of the gastrocnemius muscles. Twelve to 16 wk later whole muscle and motor unit (ventral root dissection technique) contractile properties as well as histochemistry were determined. 2. Motor units were categorized as fast, fatigable (FF), fast, intermediate fatigue-resistant (FI), fast, fatigue-resistant (FR), and slow (S) based on contractile characteristics. Muscle fibers were identified as type I and type II according to myofibrillar ATPase staining. 3. Whole muscles demonstrated increases in wet weight, tetanic force, proportion of type I fibers, and mean cross-sectional areas of both type I and II fibers, as a result of chronic overload. 4. Tetanic tension increased by the same relative magnitude in all motor units whereas twitch tension remained unchanged. A significant change in the proportions of the motor unit types occurred in overloaded muscles, such that the latter contained higher proportions of FF and S units, and lower proportions of FI and FR units, than normal muscles. 5. The fatigue profile of a composite constructed from a summation of motor unit responses revealed that the overloaded plantaris displayed fatigue resistance similar to that of the normal plantaris for a given absolute force output. 6. Glycogen-depleted fibers of hypertrophied single motor units demonstrated uniform myofibrillar ATPase and SDH staining characteristics suggesting that metabolic adaptations among fibers of the same unit were similar after 12-16 wk of overload. 7. The finding that overload caused a uniform increase in the tetanic strength of all motor units, whereas alterations in fatigue resistance varied in degree and direction among unit types, demonstrate that these two properties are not controlled in parallel in this model. The smallest units maintain or even increase their fatigue resistance during the hypertrophic process, whereas high threshold units actually decrease in fatigue resistance.

Our aim was to quantify the overload-induced hypertrophy and conversion of fiber types (type II to I) occurring in the medial head of the gastrocnemius muscle (MG). Overload of MG was induced by a bilateral tenotomy/retraction of synergists, followed by 12-18 wk of regular treadmill locomotion (2 h of walking/running per day on 3 of 4 days). We counted all type I fibers and determined type I and II mean fiber areas in eight equidistant sections taken along the length of control and overloaded MG. Increase in muscle weights (31%), as well as in total muscle cross-sectional areas (37%) and fiber areas (type I, 57%; type II, 34%), attested to a significant hypertrophic response in overloaded MG. An increase in type I fiber composition of MG from 7.0 to 11.5% occurred as a result of overload, with the greatest and only statistically significant changes (approximately 70-100%) being found in sections taken from the most rostral 45% of the muscle length. Results of analysis of sections taken from the largest muscle girth showed that it significantly underestimated the extent of fiber conversion that occurred throughout the muscle as a whole. These data obtained on the MG, which possesses a compartmentalization of fiber types, support the notion that all fiber types respond to this model with a similar degree of hypertrophy. Also, they emphasize the complex nature of the adaptive changes that occur in these types of muscles as a result of overload.

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The purpose of the study was to examine the patterns of electromyographic (EMG) activity of the rat plantaris during loaded swimming in comparison with other locomotor activities. Five female Sprague-Dawley rats were implanted with chronic bipolar electrodes in the plantaris muscle of the left hindlimb under pentobarbital anesthesia. Characteristics of EMG bursts recorded while the conscious rat was performing treadmill walking (0.24 m/s) were stable and reproducible 10-14 days postsurgery. Following this stabilization period, records of EMG activity were obtained during walking, loaded swimming (6.5 g attached to tail), and several other locomotor tasks. Compared to walking, EMG bursts during loaded swimming were significantly higher (67%) in maximum amplitude, one-third as long in duration, and occurred at a greater rate (4.4 vs. 1.7 bursts/s, P less than 0.05). Swimming bursts were of higher amplitudes than those of all other activities examined and reached 65% of the EMG amplitude recorded following stimulation of the sciatic nerve with supramaximal voltage. The addition of a mass to the animal's tail during swimming did not increase the EMG burst amplitudes but resulted in a higher frequency of bursts. Compared with treadmill walking, loaded swimming elicited burst of high variability in amplitude. Swimming in the rat involves rapid, extensive activation of plantaris, thus providing an exercise model to study the adaptability of the neuromuscular system to prolonged activity of this type.

In this study, the fast orthograde axonal transport of radiolabeled proteins was measured to determine the effects of endurance-running training on transport velocity and amounts of transported proteins in rat sciatic motoneurons. Female rats were subjected to a progressive running-training program for 10-12 wk. Twenty-four hours after the last training session, rats underwent right L4-L5 dorsal root ganglionectomy. The next day, 20 microCi of [3H]leucine was injected bilaterally in the vicinity of the motoneuronal cell bodies supplying the sciatic nerve, to study axonal transport parameters. Results showed that peak and average transport velocities of labeled proteins were significantly (P less than 0.05) increased by 22 and 29%, respectively, in the deafferented nerves of the runners as compared with controls. Moreover, the amount of total transported protein-bound radioactivity was increased in both left (40%) and right (37%) sciatic nerves of the runners. An exhaustive exercise session reduced (P less than 0.05) peak displacement (8%) and total transported protein-bound radioactivity (36%) in the sciatic nerves of control rats, whereas no changes were noticed in trained animals. The data suggest that chronic endurance running induces significant adaptations in the fast axonal transport of labeled proteins.

We have investigated the impact of compensatory overload on the content of acetylcholinesterase (AChe) molecular forms in the rat fast-twitch medial gastrocnemius (MG). Overload was induced by way of a bilateral tenotomy of the MG's functional synergists coupled to a daily walking training program (15 m/min, 30% incline, up to 60 min per session, 12-18 wks). This latter condition ensured that the MG were used on a regular basis. In comparison to control values, overloaded MG showed 25 and 19% increases (P less than 0.05) in muscle wet weight and protein concentration, respectively. The content in AChe (activity per muscle) was also increased in these MG (28%, P less than 0.05). Sedimentation analyses revealed a general elevation in the content of AChe molecular forms, with A8, G2, and G1 displaying significant changes (35-42%, P less than 0.05). In a second group of rats, daily running training (27 m/min, 30% incline, using the same timetable) was supplemented to the compensatory overload. In this group, the additional running training led to a greater hypertrophic response as attested to by increases (P less than 0.05) in the MG wet weight (41%) and protein concentration (35%) in comparison to controls. However, total AChe content of these muscles was increased to an extent similar to that observed in the MG subjected only to compensatory overload (24%, P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

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