Immediate Circulating Cortisol Responses Differ Between Two Functionally Divergent Localized Resistance-Type Plantar...

Practical localized resistance tasks involving functionally divergent muscle groups may produce different systemic biochemical responses, although neither task isolates a single muscle.

In this exploratory, nonrandomized, order-balanced within-participant crossover study, 102 young adults completed bilateral seated plantar flexion and unilateral elbow flexion, with 51 participants assigned by alternation to each condition sequence. The analysis-ready biochemical dataset contained complete observations for 102 participants. Blood was collected before and 2-3 min after each condition. We analyzed plasma glucose and lactate and serum total cholesterol, HDL-C, LDL-C, triglycerides, creatine kinase, lactate dehydrogenase, GGT, and cortisol. Linear mixed-effects models included exercise condition, sampling time, study period, assigned sequence, and their respective interactions with sampling time as fixed effects. Participant and participant-specific study visit were included as random intercepts where supported by the model. The exercise condition × time interaction remained the primary contrast, and Holm correction was applied across the ten predefined biochemical outcomes. Linear mixed-effects models included exercise condition, sampling time, study period, assigned sequence, and their respective interactions with sampling time as fixed effects. Participant and participant-specific study visit were included as random intercepts where supported by the model. The exercise condition × time interaction remained the primary contrast, and Holm correction was applied across the ten predefined biochemical outcomes.

After adjustment for period and sequence and Holm correction across the ten primary interaction tests, cortisol remained the only statistically significant exercise condition × time interaction (149.219 nmol·L^-1, 95% CI 62.338 to 236.099; unadjusted p = 0.000821; Holm-adjusted p = 0.008). Glucose showed a negative interaction estimate (-7.013 mg·dL^-1, 95% CI -12.238 to -1.788; unadjusted p = 0.009), but did not remain significant after Holm correction (adjusted p = 0.078). No period × time or sequence × time effect remained statistically significant after correction. Sensitivity analyses were not fully concordant with the primary model, supporting cautious interpretation of the cortisol finding.

These findings describe immediate responses to two non-equivalent multi-muscle tasks and cannot be attributed to selective soleus or biceps-brachii activation, fiber-type composition, or muscle-specific substrate use.