(A) Clinical scoring of behaviour, mobility, weight loss, mouse grimace, evidence of joint inflammation and duration of joint swelling (as outlined in supplementary material, Table S2) in TNF‐Tg/11βKO mice versus TNF‐Tg controls between 5 and 9 weeks of age (N = 6 per group). (B) Rate of corticosterone generation by ex vivo biopsies of whole quadriceps muscles isolated from WT, TNF‐Tg, 11βKO and TNF‐Tg mice on an 11βKO background following incubation for 16 h with DHC, as determined by scanning thin‐layer chromatograms (N = 6 per group). (C) Total quadriceps muscle weight normalized to total body weight in WT, TNF‐Tg, 11βKO and TNF‐Tg mice on an 11βKO background (N = 6 per group). (D) Representative images of quadriceps muscle sections taken from WT and 11βKO mice on either control or TNF‐Tg backgrounds stained with Gill's haematoxylin and eosin (scale bars: 50 µm) (N = 6 per group). (E–G) Average fibre size (AU), and quantification of small (<30 µm) and large (≥20 µm) fibres (AU) determined with Image J in paraffin sections of quadriceps muscles isolated from WT mice, TNF‐Tg mice, and TNF‐Tg mice on an 11βKO background (N = 6 per group). (H–J) Levels of mRNA (ΔCt) for human TNF‐α and the murine proinflammatory genes Il6 and Tnf in WT mice, TNF‐Tg mice and TNF‐Tg mice on an 11βKO background at 9 weeks (N = 6 per group). Values are expressed as mean ± SE. Statistical significance was determined with one‐way anova with a Dunnett post hoc analysis. ND, not detected; NS, not significant. *p < 0.05, **p < 0.005, ***p < 0.0005.