***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: This study was undertaken to investigate physiological adaptation with two endurance-training periods differing in intensity distribution . In a r and omized crossover fashion , separated by 4 wk of detraining , 12 male cyclists completed two 6-wk training periods : 1 ) a polarized model [ 6.4 ( ±1.4 SD ) h/wk ; 80 % , 0 % , and 20 % of training time in low- , moderate- , and high-intensity zones , respectively ] ; and 2 ) a threshold model [ 7.5 ( ±2.0 SD ) h/wk ; 57 % , 43 % , and 0 % training-intensity distribution ] . Before and after each training period , following 2 days of diet and exercise control , fasted skeletal muscle biopsies were obtained for mitochondrial enzyme activity and monocarboxylate transporter ( MCT ) 1 and 4 expression , and morning first-void urine sample s were collected for NMR spectroscopy-based metabolomics analysis . Endurance performance ( 40-km time trial ) , incremental exercise , peak power output ( PPO ) , and high-intensity exercise capacity ( 95 % maximal work rate to exhaustion ) were also assessed . Endurance performance , PPOs , lactate threshold ( LT ) , MCT4 , and high-intensity exercise capacity all increased over both training periods . Improvements were greater following polarized rather than threshold for PPO [ mean ( ±SE ) change of 8 (±2)% vs. 3 (±1)% , P < 0.05 ] , LT [ 9 (±3)% vs. 2 (±4)% , P < 0.05 ] , and high-intensity exercise capacity [ 85 (±14)% vs. 37 (±14)% , P < 0.05 ] . No changes in mitochondrial enzyme activities or MCT1 were observed following training . A significant multilevel , partial least squares-discriminant analysis model was obtained for the threshold model but not the polarized model in the metabolomics analysis . A polarized training distribution results in greater systemic adaptation over 6 wk in already well-trained cyclists . Markers of muscle metabolic adaptation are largely unchanged , but metabolomics markers suggest different cellular metabolic stress that requires further investigation 
    Output:
    
Metabolite shifts , even after strenuous exercise , typically return to near pre-exercise levels after one day of recovery .