Muscle Fatigue during High-Intensity Exercise in Children :

1. The recovery of power output and muscle metabolites was examined following maximal sprint cycling exercise. Fourteen male subjects performed two 30 s cycle ergometer sprints separated by 1.5, 3 and 6 min of recovery, on three separate occasions. On a fourth occasion eight of the subjects performed only one 30 s sprint and muscle biopsies were obtained during recovery. 2. At the end of the 30 s sprint phosphocreatine (PCr) and ATP contents were 19.7 +/- 1.2 and 70.5 +/- 6.5% of the resting values (rest), respectively, while muscle lactate was 119.0 +/- 4.6 mmol (kg dry wt)-1 and muscle pH was 6.72 +/- 0.06. During recovery, PCr increased rapidly to 65.0 +/- 2.8% of rest after 1.5 min, but reached only 85.5 +/- 3.5% of rest after 6 min of recovery. At the same time ATP and muscle pH remained low (19.5 +/- 0.9 mmol (kg dry wt)-1 and 6.79 +/- 0.02, respectively). Modelling of the individual PCr resynthesis using a power function curve gave an average half-time for PCr resynthesis of 56.6 +/- 7.3 s. 3. Recovery of peak power output (PPO), peak pedal speed (maxSp) and mean power during the initial 6 s (MPO6) of sprint 2 did not reach the control values after 6 min of rest, and occurred in parallel with the resynthesis of PCr, despite the low muscle pH. High correlations (r = 0.71-0.86; P 0.05). 4. These data suggest that PCr resynthesis after 30 s of maximal sprint exercise is slower than previously observed after dynamic exercise of longer duration, and PCr resynthesis is important for the recovery of power during repeated bouts of sprint exercise.

Needle biopsies from the vastus lateralis of 13 six-year-old Swiss children were analyzed for muscle fiber type populations and morphometrical characteristics. No significant differences existed between the males and females for fiber type distribution, maximum oxygen consumption, or any of the ultra-structural parameters investigated. The vastus lateralis muscle consisted of 19.7% fast twitch glycolytic (FG) fibers, 21.5% fast twitch oxidative glycolytic (FOG) fibers, and 5,9,% slow twitch oxidative (SO) fibers. Maximum oxygen consumption averaged 45.2 ml/kg min-1 when the subjects were considered as a single group. Morphometrically, it was found that the mean volume density of the central mitochondria was 5.54%, the mitochondrial/myofibrillar volume ratio was 6.68%, and the intracellular lipid volume was 0.46%. There was a significant correlation (r=0.69) between the mitochondrial volume density and the distribution of SO fibers as determined histochemically. It was concluded that the fiber type distribution pattern and ultrastructure of skeletal muscle in six-year-old children was not different from normal adult tissue.