We tested the hypotheses that: (1) neutrophil accumulation after contraction‐induced muscle injury is dependent on the β₂ integrin CD18, (2) neutrophils contribute to muscle injury and oxidative damage after contraction‐induced muscle injury, and (3) neutrophils aid the resolution of contraction‐induced muscle injury. These hypotheses were tested by exposing extensor digitorum longus (EDL) muscles of mice deficient in CD18 (CD18^−/−; Itgb2^tm1Bay) and of wild type mice (C57BL/6) to in situ lengthening contractions and by quantifying markers of muscle inflammation, injury, oxidative damage and regeneration/repair. Neutrophil concentrations were significantly elevated in wild type mice at 6 h and 3 days post‐lengthening contractions; however, neutrophils remained at control levels at these time points in CD18−/− mice. These data indicate that CD18 is required for neutrophil accumulation after contraction‐induced muscle injury. Histological and functional (isometric force deficit) signs of muscle injury and total carbonyl content, a marker of oxidative damage, were significantly higher in wild type relative to CD18−/− mice 3 days after lengthening contractions. These data show that neutrophils exacerbate contraction‐induced muscle injury. After statistically controlling for differences in the force deficit at 3 days, wild type mice also demonstrated a higher force deficit at 7 days, a lower percentage of myofibres expressing embryonic myosin heavy chain at 3 and 7 days, and a smaller cross sectional area of central nucleated myofibres at 14 days relative to CD18−/− mice. These observations suggest that neutrophils impair the restoration of muscle structure and function after injury. In conclusion, neutrophil accumulation after contraction‐induced muscle injury is dependent on CD18. Furthermore, neutrophils appear to contribute to muscle injury and impair some of the events associated with the resolution of contraction‐induced muscle injury.

We tested the hypotheses that lengthening contractions, isometric contractions, and passive stretches increase muscle inflammatory cells (neutrophils and macrophages) and that prior conditioning with lengthening contractions, isometric contractions, or passive stretches reduces neutrophils and macrophages after subsequent lengthening contractions. Extensor digitorum longus muscles in anesthetized mice were subjected in situ to lengthening contractions, isometric contractions, or passive stretches. Six hours or 3 days after a protocol of contractions or passive stretches, neutrophils and macrophages were quantified in muscle cross sections. Three days after isometric contractions or passive stretches, neutrophils were elevated ( P < 0.05) 3.7- and 5.5-fold, respectively, relative to controls. Both macrophages and neutrophils were increased 51.2- and 7.9-fold, respectively, after lengthening contractions. Prior lengthening contractions, isometric contractions, or passive stretches reduced inflammatory cells after lengthening contractions performed 2 wk later. The major finding of this study was that passive stretches and isometric contractions elevated neutrophils without causing overt signs of injury. Because both passive stretches and isometric contractions elevated neutrophils and afforded some protection from contraction-induced muscle injury, neutrophils and/or the related inflammatory events may contribute to the induction of a protective mechanism.

Eight male cross-country runners and five male swimmers were tested four times during their collegiate seasons. Each trial corresponded to a different training load. The runners' trials were conducted before the start of organized practice (RT1), after 3 wk of increased training (RT2), 3 wk prior to the conference championship (pre-taper, RT3), and 4 d after the conference championship (post-taper, RT4). The swimmers' trials were conducted after the first 9wk of training (ST1), after completing 2 wk of hard training (ST2), after an additional 6 wk of training (pre-taper, ST3) and during a week following the conference championship (post-taper, ST4). Venous blood samples, heart rate (HR) and blood pressure (BP) were obtained after 15 min supine rest (0700 h). Serum was analyzed for Cortisol (C), total testosterone (TT), free testosterone (FT), and creatine kinase (CK). Blood samples (lactate), HR and RPE were obtained during a fixed velocity run (75% preseason VChmax) and blood samples and RPE following a 365.8 m swim (90% preseason V̇O₂max). The runners then completed a “performance run” to exhaustion (110% preseason VO₂max) and the swimmers completed maximal 22.9 and 365.8 m swims. Serum CK, C, TT, FT, and the TT:C and FT:C ratios were not significantly different among trials for the runners. Serum TT and FT were significantly (P < 0.05) lower for the swimmers at ST2 (TT 16.7±2.5; FT 85.3±8.5) compared to ST1 (TT 30.3±2.8; FT 130.2±20.9) whereas, C, TT:C or FT:C were not significantly altered. Serum CK was significantly elevated for swimmers at ST2 (135.3±20.9) and ST3 (101.7±16.7) compared to ST1 (54.12±8.2). Performance capacity in 365.8 m and 232.9 m swims was significantly reduced at ST2 and returned to preseason times at ST4. Resting HR and BP were not significantly altered for either swimmers or runners. In conclusion, C, FT: C and TT: C were not influenced by changes in training volume/intensity in swimmers or in runners. Serum TT and FT were significantly reduced and CK significantly elevated when training was substantially increased for a short time period for the swimmers (ST2). Changes in TT, FT, and CK were concomitant with decreased performance and increased global mood state. Furthermore, TT and FT were increased, and CK decreased, when swim training volume was reduced (ST4). Therefore, TT, FT and CK may be effective markers for monitoring overtraining in athletes.

We examined the influence of ibuprofen and acetaminophen on muscle neutrophil and macrophage concentrations after novel eccentric contractions. Twenty-four males (25 +/- 3 yr) were divided into three groups that received the maximal over-the-counter dose of either ibuprofen (1200 mg x d-1), acetaminophen (4000 mg x d-1), or a placebo after eccentric contractions of the knee extensors. Biopsies from the vastus lateralis were taken before and 24 h after exercise. Inflammatory cells were quantified in muscle cross-sections using immunohistochemistry. Macrophage concentrations were elevated by 1.5- to 2.5-fold (P < 0.05) at 24 h postexercise relative to preexercise concentrations, whereas neutrophil concentrations were not significantly elevated. Muscle inflammatory cell concentrations were unaffected by treatment with ibuprofen or acetaminophen when compared with placebo. Maximal over-the-counter doses of ibuprofen or acetaminophen, when administered therapeutically, do not affect muscle concentrations of neutrophils or macrophages 24 h after a novel bout of eccentric contractions.

The primary purpose of the study was to examine circulating neutrophils and monocytes and their plasma membrane expression of CD64, CD11b, and CD18 after two bouts (B1 and B2) of eccentric exercise. Subjects (n = 10) performed 25 forced-lengthened contractions of the forearm flexors on two occasions separated by 3 wk. Blood samples were obtained before exercise and at 1.5, 6, 12, 24, 48, 72, and 96 h of recovery. CD64, CD11b, and CD18 expression was determined via direct immunofluorescence and used as an indicator of neutrophil and monocyte activation. Creatine kinase activity (B1 = 1,390, B2 = 108 U/l), myoglobin (B1 = 163, B2 = 41, ng/dl), and muscle soreness and tenderness were higher (P < 0.01) after B1 compared with B2. Neutrophils at 6, 12, and 96 h were higher (P < 0.05) for B1 vs. B2. CD11b expression on neutrophils was 2.7-fold higher at 72 h for B1 vs. B2. CD64 expression on neutrophils at 72 and 96 h was 1.4- and 1.9-fold higher, respectively, for B1 vs. B2. At 72 and 96 h, CD18 and CD64 expression on monocytes was 1.3-fold higher for B1 vs. B2. The observed changes were not significantly correlated with changes in creatine kinase activity or myoglobin. In conclusion, the adaptation to eccentric arm exercise was associated with a reduction in circulating neutrophils and a lower state of neutrophil and monocyte activation.

Tumor necrosis factor-α (TNF-α) has been associated with cachexia and is known to regulate multiple inflammatory cell (neutrophil and macrophage) responses. We tested the hypothesis that neutrophils and macrophages accumulate in the extensor digitorum longus (EDL) and soleus muscles of mice after chronic TNF-α administration. Murine recombinant TNF-α (∼100 μg·kg⁻¹·day⁻¹) in vehicle solution or vehicle solution alone (sham) was administered to C57BL/6 mice for 7 days via osmotic minipumps. In EDL muscles from TNF-α-treated mice, neutrophil and macrophage concentrations were elevated seven- and threefold, respectively, compared with sham mice. Neutrophil and macrophage concentrations were also elevated five- and twofold, respectively, in solei of TNF-α- relative to sham-treated mice. Treatment with TNF-α elevated ubiquitin content by ∼25% relative to sham values for both the EDL and soleus muscles; however, these elevations were not statistically significant. No differences were observed between TNF-α- and sham-treated mice in body weight, food consumption, muscle mass, myofiber cross-sectional area, carbonyl groups, total protein content, or relative abundance of myosin heavy chain protein. Furthermore, no overt signs of muscle injury or regeneration were observed in muscles from TNF-α-treated mice in either the EDL or soleus muscles. These observations suggest that 7 days of TNF-α administration promote muscle inflammation as indicated by the accumulation of neutrophils and macrophages without overt signs of atrophy, injury, or regeneration.