Interstitial muscle lactate, pyruvate and potassium dynamics in the trapezius muscle during repetitive low-force arm movements, measured with microdialysis

Aim: Local muscle metabolic responses to repetitive low-force contractions and to intense static contractions were studied by microdialysis in humans. Methods: Microdialysate and electromyography (EMG) were sampled from the trapezius muscle, mixed venous blood samples were taken and perceived exertion was rated (0-9) before and during 20 min of standardized repetitive arm movement (REP), 60 min recovery (R1), and 10 min 90degrees sustained arm position (SUS) at 20% maximum voluntary contraction, followed by 60 min recovery (R2) in six healthy male participants (28-33 years). Results: Average muscle activity was 8+/-2% of EMG(max-RMS) (mean+/-SEM) during REP and 22+/-5% of EMG(max-RMS) during SUS. Perceived exertion increased from 0 to 3.2+/-0.5 during REP and from 0 to 8.5+/-0.3 during SUS. During REP interstitial muscle lactate increased from 2.1+/-0.2 to 2.9+/-0.2 mmol L-1 (P<0.001) and returned to the baseline level during R1, while dialysate [K+] increased from 3.8+/-0.2 to 4.7+/-0.2 mmol L-1 (P<0.002) and returned to 3.8 +/- 0.2 mmol L-1 during R1. In contrast, plasma lactate and [K+] remained unchanged. During SUS interstitial muscle lactate increased from 2.3+/-0.2 to 3.3+/-0.3 mmol L-1 (P<0.003), increased further to 6.5+/-1.3 mmol L-1 post-exercise (P<0.001) and returned to baseline levels during R2. Dialysate [K+] increased from 3.9+/-0.2 to 4.6+/-0.2 mmol L-1 (P<0.05) and returned to baseline level during R2. Plasma lactate increased significantly during SUS whereas plasma [K+] was unchanged. During REP and SUS interstitial pyruvate was unchanged but increased in the post-exercise period proportional to the exercise intensity. Conclusions: The microdialysis technique was effective in revealing muscle metabolic events that were not found systemically. Furthermore, the trapezius muscle showed an anaerobic metabolism during low-force contraction, which could indicate inhomogeneous muscle activation.