Introduction
Because the lymphatics in skeletal muscles are unable to contract spontaneously the lymph flow from skeletal muscles is dependent on external compression of the lymphatics, brought about especially by muscle contractions. During exercise muscles contract repeatedly and at varying forces and frequencies. This presentation gives an overview of the interplay between skeletal muscle contractions and subsequent lymph flow dynamics during various types of voluntary exercise.
Methods
Lymph flow was assessed as the fractional clearance rate of intramuscularly injected 99Tc-labeled human serum albumin (2% albumin, 10-60 MBq, 0.1-0.15 ml/injection) from either vastus lateralis or biceps brachialis muscles of healthy subjects (Havas et al. 1997). The experiments, which were approved by the Ethics Committee of Jyväskylä Central Hospital, included either prolonged periods of exercise (running or bicycling) or several 10 min periods of local muscle contractions (elbow flexions) at different forces and frequencies of contraction. Values given here are means of 12 muscles or more.
Results
Albumin clearance was similar from resting vastus lateralis and biceps brachialis muscles at upright position (0.08 %/min). During prolonged exercise (90-120 min at 60 % of VO2max) albumin clearance first increased by 3-4 fold (to 0.29 %/min) but thereafter gradually decreased during the course of exercise towards a level of less than half of the initial peak value (0.12 %/min) despite of constant exercise intensity. During elbow flexions at constant frequency of 0.16 Hz the albumin clearance rate was similar during contractions at 10 and 20 % force levels (0.18 and 0.21 %/min, respectively), but significantly higher during contractions at 30 and 40 % force levels (0.31 and 0.32 %/min, respectively). In bicycle ergometry at intensities of 20-80 % of VO2max the clearance rate peaked at 40 -50 % intensity level. At low pedalling resistance (18 N) the clearance rate doubled as the pedaling rate was increased from 50 to 90 rpm ( 0.12 vs. 0.24 %/min, respectively). However, at higher pedalling resistance (32 N) the clearance rate was independent of the pedalling rate (0.22 %/min at 50, 75 and 90 rpm).
Conclusions
The magnitude of muscle contraction induced increase in lymph flow is partly dependent on the force of contraction, probably secondary to contraction force related increases in blood flow and intramuscular pressure, which can enhance lymph formation and induce more efficient lymphatic pumping. When the force of contraction is low, the lymph flow seems to be directly related to the frequency of contractions. On the other hand, at higher force of contraction the clearance during one contraction was reversely related to contraction frequency, suggesting that the time for filling of lymphatics between contractions was limiting the total lymph flow. The attenuation of lymph flow during the course of prolonged exercise suggests that the interstitial milieu of skeletal muscles may be in a state of dynamic change even during a steady state exercise.
References
Havas et al.1997. J Physiol 504,233-239.