Improvements in exercise performance: effects of carbohydrate feedings and diet

In an effort to determine the effects of carbohydrate (CHO) feedings immediately before exercise in both the fasted and fed state, 10 well-trained male cyclists [maximum O2 consumption (VO2 max), 4.35 +/- 0.11 l/min)] performed 45 min of cycling at 77% VO2 max followed by a 15-min performance ride on an isokinetic cycle ergometer. After a 12-h fast, subjects ingested 45 g of liquid carbohydrate (LCHO), solid carbohydrate confectionery bar (SCHO), or placebo (P) 5 min before exercise. An additional trial was performed in which a high-CHO meal (200 g) taken 4 h before exercise was combined with a confectionery bar feeding (M + SCHO) immediately before the activity. At 10 min of exercise, serum glucose values were elevated by 18 and 24% during SCHO and LCHO, respectively, compared with P. At 0 and 45 min no significant differences were observed in muscle glycogen concentration or total use between the four trials. Total work produced during the final 15 min of exercise was significantly greater (P less than 0.05) during M + SCHO (194,735 +/- 9,448 N X m), compared with all other trials and significantly greater (P less than 0.05) during LCHO and SCHO (175,204 +/- 11,780 and 176,013 +/- 10,465 N X m, respectively) than trial P (159,143 +/- 11,407 N X m). These results suggest that, under conditions when CHO stores are less than optimal, exercise performance is enhanced with the ingestion of 45 g of CHO 5 min before 1 h of intense cycling.(ABSTRACT TRUNCATED AT 250 WORDS)     

Muscle glycogenolysis during differing intensities of weight-resistance exercise

Skeletal muscle glycogen metabolism was investigated in eight male subjects during and after six sets of 70% one repetition maximum (1 RM, I-70) and 35% 1 RM (I-35) intensity weight-resistance leg extension exercise. Total force application to the machine lever arm was determined via a strain gauge and computer interfaced system and was equated between trials. Compared with the I-70 trial, the I-35 trial was characterized by almost double the repetitions (13 +/- 1 vs. 6 +/- 0) and half the peak concentric torque for each repetition (12.4 +/- 0.5 vs. 24.2 +/- 1.0 Nm). After the sixth set, muscle glycogen degradation was similar between I-70 and I-35 trials (47.0 +/- 6.6 and 46.6 +/- 6.0 mmol/kg wet wt, respectively), as was muscle lactate accumulation (13.8 +/- 0.7 and 16.7 +/- 4.2 mmol/kg wet wt, respectively). After 2 h of passive recovery without caloric intake, muscle glycogen increased by 22.2 +/- 6.8 and 14.2 +/- 2.5 mmol/kg wet wt in the I-70 and I-35 trials, respectively. Optical absorbance measurement of periodic acid-Schiff-stained muscle sections after the 2 h of recovery revealed larger absorbance increases in fast-twitch than in slow-twitch fibers (0.119 +/- 0.024 and 0.055 +/- 0.024, P = 0.02). Data indicated that when external work was constant, the absolute amount of muscle glycogenolysis was the same regardless of the intensity of resistance exercise. Nevertheless the rate of glycogenolysis during the I-70 trial was approximately double that of the I-35 trial.     

Muscle water and electrolytes following varied levels of dehydration in man.

In an effort to assess the effects of dehydration on the content of water and electrolytes (Na+, K+, Cl-, and Mg2+) in plasma and muscle tissue, eight men exercised in the heat (39.5 degrees C, 25%). Blood urine, and muscle biopsy samples were obtained before exercise and after the subjects had reduced their body weight by 2.2, 4.1, and 5.8%. On the average, plasma and muscle water (H2Om) contents were found to decline 2.4 and 1.2% for each percent decrease in body weight. Muscle sodium (Na+m) and chloride (Cl-m) content remained unchanged with dehydration, while muscle magnesium (Mg2+m) declined 12% as a result of the 5.8% dehydration. In terms of intracellular concentrations, K+i increased 7.2 and 10.6% at the 2.2 and 4.1% dehydration levels, respectively. Calculations of the resting membrane potential suggest that the water and electrolyte losses observed in these studies do not significantly alter the excitability of the muscle cell membrane.     

Progressive resistance training reduces myosin heavy chain coexpression in single muscle fibers from older men.

The purpose of this study was to examine myosin heavy chain (MHC) and myosin light chain (MLC) isoforms following 12 wk of progressive resistance training (PRT). A needle biopsy was taken from the vastus lateralis to determine fiber-type expression [ATPase (pH 4.54) and MHC/MLC] in seven healthy men (age = 74.0 +/- 1.8 yr). Subjects were also tested for 1-repetition maximum (1-RM), pre- and posttraining. The progressive knee extensor protocol consisted of three sets at 80% of 1-RM 3 days/wk for 12 wk. Freeze-dried, single muscle fibers were dissected for MHC and MLC analysis and then subjected to SDS-PAGE and silver staining, pre- and posttraining. MHC expression increased in the I (10.4%; P < 0.05) and decreased in I/IIa (9.0%; P < 0.05), I/IIa/x (0.9%; P < 0.05), and IIa/x (8.9%; P < 0.05) isoforms, with no change in the IIa and IIx isoforms, pre- vs. posttraining (total fibers = 3,059). The MLC(3f)-to-MLC(2) ratio did not change with the PRT in either the MHC I or MHC IIa isoforms (total fibers = 902), pre- to posttraining. ATPase fiber distribution did not significantly differ following training (I: 50. 4 +/- 6.7 vs. 51.9 +/- 7.9, IIa: 36.8 +/- 5.3 vs. 41.1 +/- 7.0, IIb: 12.8 +/- 5.6 vs. 7.0 +/- 4.0%; pre- vs. posttraining, respectively). 1-RM increased (51.9%; P < 0.05) from pre- to posttraining. The PRT provide a stimulus for alterations in MHC isoforms, which demonstrated a decrease in all hybrid isoforms and an increase in MHC I expression (not found in the ATPase results), unlike the MLC ratio (3:2), which was not altered with training.     

Effect of 17days of bed rest on peak isometric force and unloaded shortening velocity of human soleus fibers

The purpose of this study was to examine the effect of prolongedbed rest (BR) on the peak isometric force(P_o) and unloaded shorteningvelocity (V_o)of single Ca²⁺-activated musclefibers. Soleus muscle biopsies were obtained from eight adult malesbefore and after 17 days of 6° head-down BR. Chemicallypermeabilized single fiber segments were mounted between a forcetransducer and position motor, activated with saturating levels ofCa²⁺, and subjected to slacklength steps. V_owas determined by plotting the time for force redevelopment vs. theslack step distance. Gel electrophoresis revealed that 96% of the pre-and 87% of the post-BR fibers studied expressed only the slow type Imyosin heavy chain isoform. Fibers with diameter >100 µm made uponly 14% of this post-BR type I population compared with 33% of thepre-BR type I population. Consequently, the post-BR type I fibers(n = 147) were, on average, 5%smaller in diameter than the pre-BR type I fibers(n = 218) and produced 13% lessabsolute P_o. BR had no overalleffect on P_o per fibercross-sectional area(P_o/CSA), even though halfof the subjects displayed a decline of 9-12% inP_o/CSA after BR. Type Ifiber V_oincreased by an average of 34% with BR. Although the ratio of myosinlight chain 3 to myosin light chain 2 also rose with BR, there was nocorrelation between this ratio andV_o for either thepre- or post-BR fibers. In separate fibers obtained from the originalbiopsies, quantitative electron microscopy revealed a 20-24%decrease in thin filament density, with no change in thick filamentdensity. These results raise the possibility that alterations in thegeometric relationships between thin and thick filaments may be atleast partially responsible for the elevatedV_o of the post-BRtype I fibers.