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It has been suggested that decreasing muscle pH because of the accumulation of lactic acid is a fatigue factor during high-intensity exercise [1][2][3]. Lactic acid production in muscle releases lactate (Lac Ϫ ) and hydrogen ions (H ϩ ) within the physiological pH range. Therefore the accumulation of lactic acid causes metabolic acidosis. The decrease of intracellular pH brings about muscle fatigue through several mechanisms such as decreasing skeletal muscle tension, relaxation [4], and the inhibition of phosphofructokinase activity [5]. However, the body has an ability to restrain the decrease of intracellular pH, which is known as buffering capacity. It has been suggested that a higher buffering capacity can better stabilize the intracellular pH and better enhance the capability for high-intensity exercise performance.Most intracellular buffering within the skeletal muscle is accomplished by proteins, dibasic inorganic phosphate, bicarbonate, and carnosine [6]. A large amount of the histidine-containing dipeptide carnosine (␤-alanyl-L-histidine) has been shown to be present within the skeletal muscle of most vertebrate species [7]. It has been suggested that the carnosine significantly contributes to the physicochemical Japanese Journal of Physiology, 52, 199-205, 2002 Key words: carnosine, buffering capacity, acid-base balance, fiber-type distribution, high-intensity exercise. Abstract:The histidine-containing dipeptide carnosine (␤-alanyl-L-histidine) has been shown to significantly contribute to the physicochemical buffering in skeletal muscles, which maintains acid-base balance when a large quantity of H ϩ is produced in association with lactic acid accumulation during high-intensity exercise. The purpose of the present study was to examine the relations among the skeletal muscle carnosine concentration, fiber-type distribution, and high-intensity exercise performance. The subjects were 11 healthy men. Muscle biopsy samples were taken from the vastus lateralis at rest. The carnosine concentration was determined by the use of an amino acid autoanalyzer. The fiber-type distribution was determined by the staining intensity of myosin adenosinetriphosphatase. The high-intensity exercise performance was assessed by the use of 30-s maximal cycle ergometer sprinting. A significant correlation was demonstrated between the carnosine concentration and the type IIX fiber composition (rϭ0.646, pϽ0.05). The carnosine concentration was significantly correlated with the mean power per body mass (rϭ0.785, pϽ0.01) during the 30-s sprinting. When dividing the sprinting into 6 phases (0-5, 6-10, 11-15, 16-20, 21-25, 26-30 s), significant correlations were observed between the carnosine concentration and the mean power per body mass of the final 2 phases (21-25 s: rϭ0.694, pϽ0.05; 26-30 s: rϭ0.660, pϽ0.05). These results indicated that the carnosine concentration could be an important factor in determining the high-intensity exercise performance.

We examined hormonal responses to slow movement exercise involving concentric (CON) and eccentric (ECC) actions. Nine men performed knee extension exercises: (1) low-intensity exercise with slow CON contractions (5-1; 5 s for CON and 1 s for ECC); (2) low-intensity exercise with slow ECC contractions (1-5; 1 s for CON and 5 s for ECC); (3) low-intensity exercise with slow CON and ECC contractions (3-3; 3 s for each contraction); and (4) high-intensity exercise at normal velocity (1-1; 1 s for each contraction). Lactate concentration was significantly higher after the 5-1 than after the 1-5 (P < 0.05). Slow movement exercises significantly raised the concentrations of plasma epinephrine, serum growth hormone, and free testosterone (P < 0.05). Serum growth hormone concentration increased to a greater extent after the three slow movement trials compared with the normal movement trial (1-1). However, serum cortisol concentration was significantly higher after the 5-1 than after the 1-5 and 1-1 (P < 0.05). Average V(O)(2) throughout the exercise session (divided by the time to complete exercise session) was significantly higher in the 1-1 (P < 0.05), with no significant difference among the slow movement trials. In conclusion, low-intensity exercises with slow movement acutely increased anabolic hormone concentrations regardless of the time to complete CON and ECC actions. In contrast, low-intensity exercise with slower ECC action stimulated smaller changes in lactate and cortisol compared with low-intensity exercise with slower CON action.

Acute and long-term effects of resistance-training regimens with varied combinations of high- and low-intensity exercises were studied. Acute changes in the serum growth hormone (GH) concentration were initially measured after 3 types of regimens for knee extension exercise: a medium intensity (approximately 10 repetition maximum [RM]) short interset rest period (30 s) with progressively decreasing load ("hypertrophy type"); 5 sets of a high-intensity (90% of 1RM) and low-repetition exercise ("strength type"); and a single set of low-intensity and high-repetition exercise added immediately after the strength-type regimen ("combi-type"). Postexercise increases in serum GH concentration showed a significant regimen dependence: hypertrophy-type > combi-type > strength-type (p < 0.05, n = 8). Next, the long-term effects of periodized training protocols with the above regimens on muscular function were investigated. Male subjects (n = 16) were assigned to either hypertrophy/combi (HC) or hypertrophy/ strength (HS) groups and performed leg press and extension exercises twice a week for 10 weeks. During the first 6 weeks, both groups used the hypertrophy-type regimen to gain muscular size. During the subsequent 4 weeks, HC and HS groups performed combi-type and strength-type regimens, respectively. Muscular strength, endurance, and cross sectional area (CSA) were examined after 2, 6, and 10 weeks. After the initial 6 weeks, no significant difference was seen in the percentage changes of all variables between the groups. After the subsequent 4 weeks, however, 1RM of leg press, maximal isokinetic strength, and muscular endurance of leg extension showed significantly (p < 0.05) larger increases in the HC group than in the HS group. In addition, increases in CSA after this period also tended to be larger in the HC group than in the HS group (p = 0.08). The results suggest that a combination of high- and low-intensity regimens is effective for optimizing the strength adaptation of muscle in a periodized training program.

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