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The purpose of this study was to determine whether a program of regular sprint exercise training alters the functional properties or protects against the development of fatigue in fast- and slow-twitch rat skeletal muscle. The training program consisted of 6 sprints of 4.5-min duration at 40 m/min and 15% slope with 2.5-min rest intervals, performed 5 days/wk for 6 wk. The exercise program significantly increased (P less than 0.05) citrate synthase activity (mumol X g-1 X min-1) in the predominantly type I soleus (SOL) from 28 +/- 2 to 44 +/- 2; the type IIb superficial region of the vastus lateralis (SVL) from 10 +/- 1 to 16 +/- 1; and the type IIa deep region of the vastus lateralis (DVL) from 34 +/- 2 to 53 +/- 2. Phosphofructokinase activity (mumol X g-1 X min-1) also increased with training in the SOL (17 +/- 1 vs. 23 +/- 1) and the DVL (64 +/- 5 vs. 79 +/- 5). Sprint training reduced (P less than 0.05) the contraction time (CT) (111 +/- 7 vs. 92 +/- 3 ms) and the one-half relaxation time (118 +/- 3 vs. 104 +/- 2 ms) in the slow-twitch soleus. The exercise program also induced a decreased CT in the fast-twitch extensor digitorum longus (EDL), but significance was limited to the P less than 0.1 level. Muscle fatigue was produced by electrical stimulation at 45 trains/min and either 15 trains/min in SOL or 10 trains/min in the EDL and SVL for 1, 5, or 10 min.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of thyrotoxicosis on the contractile properties and development of muscle fatigue in the slow soleus (SOL) and fast extensor digitorum longus (EDL) muscles were examined in rats given 3 mg of L-thyroxine and 1 mg of L-triiodothyronine per kilogram of diet for 6 weeks. The hormone treatment produced significant decreases in the contraction time, one-half relaxation time, and twitch tension in the SOL, while the peak rate of tension development (+ dP/dt) and decline (- dP/dt) in this muscle were elevated. Additionally, the force-frequency curve was shifted to the right and, thus, resembled the curve of a normal fast-twitch muscle. In contrast, the contractile properties of the fast EDL were relatively unaltered by the hormone administration. Thyrotoxicosis also changed the SOL response to contractile activity as twitch tension, + dP/dt, and - dP/dt remained high, and a faster decline in muscle glycogen and an increase in lactate occurred compared to control muscles. These results clearly demonstrate a preferential effect of thyroid hormone on slow compared to fast skeletal muscle.

The effect of hind-limb immobilization on selected lysosomal enzyme activities was studied in rat hind-limb muscles composed primarily of type I, IIA, or IIB fibers. Following immobilization, acid protease and acid phosphatase both exhibited significant (P less than 0.05) increases in their activity per unit weight in all three fiber types. Acid phosphatase activity increased at day 14 of immobilization in the three muscles and returned to control levels by day 21. Acid protease activity also changed biphasically, displaying a higher and earlier rise than acid phosphatase. The pattern of change in acid protease, but not acid phosphatase, closely parallels observed muscle wasting. The present data therefore demonstrate enhanced proteolytic capacity of all three fiber types early during muscular atrophy. In addition, the data suggest a dependence of basal hydrolytic and proteolytic activities and their adaptive response to immobilization on muscle fiber composition.

Biopsies of lumbar multifidus muscles were obtained at operation on seventeen patients aged from fifteen to fifty-eight with lumbar spinal derangement, and further material was taken from the cadavers of three subjects aged from nineteen to fifty-one. Sections were prepared to show the presence of ATPase activity, so distinguishing Fast from Slow types of muscle fibre. The normal mosiac pattern arising from the intermingling of fibres from Fast and Slow motor units was seen in sections from cadaveric material and from many of the biopsies. With age and limited lumbar flexibility, the Fast fibres became relatively smaller but with increasing variation in size, suggesting a reduced capacity for phasic activity. The presence of positive root signs was associated with a greater proportion of Slow fibres, and in some patients with the occurrence of atrophied Fast fibres, giving rise to differences in the populations of the two fibres in neighbouring fascicles. The results suggest that multifidus adopts an increasingly postural role with advancing age and with disabling lesions of the lumbar spine.

The purpose of the present study was to examine the effects of treadmill running on the oxidative capacity of regenerated rodent skeletal muscle. The soleus, plantaris, and gastrocnemius muscles were removed under general anesthesia from one hindlimb of young female rats (N = 15). The visible "white" portion of the latter muscle was minced into 1 mm3 fragments and autotransplanted leaving both the tibial and sural nerves intact. Six animals were treadmill run daily beginning 4 days post-surgery. Intensity and duration were progressively increased such that by day 45 the animals were running 1 h at 21.5 m . min-1 up a 15% grade, The regenerated muscle homogenates had a significantly lower capacity for maximal pyruvate + malate oxidation and smaller mass relative to the contralateral gastrocnemius. The exercise program significantly increased these parameters, tending to normalize the values. Some factor associated with exercise stimulated recovery from the muscular trauma.

The contractile and selected biochemical properties of fast- and slow-twitch skeletal muscle were studied at 9, 18, and 28 months of age in sedentary and regularly exercised rats. The isometric twitch duration was prolonged with aging in both the fast- and slow-twitch muscle. This effect was primarily due to a prolonged one-half relaxation time (1/2RT), which developed late in life. Regular exercise tended to further prolong the twitch duration, particularly in the slow-twitch soleus. Surprisingly, twitch and tetanic tension (Po), peak rate of tension development and decline, and the maximal shortening velocity were all unaltered between 9 and 28 months of age. Furthermore, regular exercise (running or swimming) had little or no effect on these properties. The prolonged 1/2RT with aging could not be explained by a decreased rate of Ca2+ sequestration by the sarcoplasmic reticulum, as the rate of Ca2+ uptake measured in muscle homogenates was unaltered in any of the muscles studied between 9 and 28 months. The degree of muscle fatigue (decline in Po) with 30 min of contractile activity in the slow-twitch soleus was not affected by aging. However, lactate reached two-fold higher levels and glycogen fell to considerably lower levels in the muscles of the old rats. This suggests an increased glycolysis and glycogen utilization during contractile activity in aged rats.