Recently, it has been shown that the extramuscular connective tissue (ECT) is likely involved in delayed onset muscle soreness (DOMS). Therefore, the aim of the present study was to investigate the effects of maximal trunk extension eccentric exercise (EE) on ECT thickness, self-reported DOMS, ECT stiffness, skin temperature, and possible correlations between these outcomes. Healthy adults (n = 16, 29.34 ± 9.87 years) performed fatiguing EE of the trunk. A group of highly active individuals (TR, n = 8, > 14 h of sport per week) was compared with a group of less active individuals (UTR, n = 8, < 2 h of sport per week). Ultrasound measurements of ECT thickness, stiffness with MyotonPro and IndentoPro, skin temperature with infrared thermography, and pain on palpation (100 mm visual analog scale, VAS) as a surrogate for DOMS were recorded before (t₀), immediately (t₁), 24 h (t₂₄), and 48 h (t₄₈) after EE. ECT thickness increased after EE from t₀ to t₂₄ (5.96 mm to 7.10 mm, p = 0.007) and from t₀ to t₄₈ (5.96 mm to 7.21 mm, p < 0.001). VAS also increased from t₀ to t₂₄ (15.6 mm to 23.8 mm, p < 0.001) and from t₀ to t₄₈ (15.6 mm to 22.8 mm, p < 0.001). Skin temperature increased from t₁ to t₂₄ (31.6° Celsius to 32.7° Celsius, p = 0.032) and t₁ to t₄₈ (31.6° Celsius to 32.9° Celsius, p = 0.003), while stiffness remained unchanged (p > 0.05). Correlation analysis revealed no linear relationship between the outcomes within the 48-hour measurement period. The results may confirm previous findings of possible ECT involvement in the genesis of DOMS in the extremities also for the paraspinal ECT of trunk extensors. Subsequent work should focus on possible interventions targeting the ECT to prevent or reduce DOMS after strenuous muscle EE.

• Eccentric exercise increases the thickness of lumbar extramuscular connective tissue.

• The results may confirm previous findings on the involvement of fascia in DOMS.

• A focus on fascial interventions for prevention or reduction of DOMS could be promising.