The increase in plasma interleukin-6 (IL-6) appears dependent on factors such as exercise intensity, duration, muscle mass recruited, and mode (concentric vs eccentric) (for review see Febbraio & Pederson, 2002). Particularly during the later stages of prolonged endurance exercise the increase in plasma IL-6 is most pronounced. The late appearance in the plasma following prolonged exercise suggests that a component of the rise in plasma IL-6 may be via damage to skeletal muscle. A single bout of eccentric exercise produces significant muscle damage. However, the same eccentric exercise bout performed some weeks afterward shows minimal damage to the active muscle (McHugh et al. 1999). Therefore, the aim of this experiment was to investigate the plasma IL-6 response with a single eccentric exercise bout, and 5 weeks later, with another identical “repeated” bout of eccentric exercise, to assess the contribution of skeletal muscle damage to the IL-6 response.
Fifteen inactive males volunteered for this study. Following ethical approval subjects were randomly assigned to a concentric exercise group (CON; n=7) or an eccentric exercise group (ECC; n=8). Subjects performed two bouts of walking exercise separated by 5 weeks (B1 & B2) on a motor driven treadmill at a constant speed (5 km.hr-1) for 90 min. CON walked on the flat (0° decline) while ECC walked downhill (14° decline). Forearm venous samples were collected at regular intervals Pre-, during and post-exercise for determination of plasma IL-6 (R&D systems ELISA kit). Maximal voluntary isometric contraction (MVC) of the quadriceps muscle group, and delayed onset muscle soreness (DOMS) were determined at the same time points, except during exercise. A 2-way ANOVA for repeated measures and Student-Newman-Keuls post hoc test was used to assess significance with p<0.05.
MVC and DOMS showed no muscle damage in either B1 or B2 in CON. However, there was significant muscle damage in ECC post B1 only (not B2). Similarly, plasma IL-6 was elevated in ECC only (towards the end of B1), and peaked immediately post-exercise. All changes returned to baseline levels within 7 days.
In this study exercise intensity was not different between B1 & B2 for both CON and ECC. With no muscle damage or change in plasma IL-6 in B2, yet elevated plasma IL-6 in ECC in B1, it appears that exercise-induced muscle damage contributes almost exclusively to the increase in IL-6 reported following under these low intensity walking exercise conditions.
Febbraio, M.A. & Pedersen, B.K. (2002) FASEB Journal 16, 1335-1347.
McHugh M.P., Connolly, D.A.J., Eston, E.J. & Gleim, G.W. (1999) Sports Medicine 18, 157-170.