Hormonal Responses after Exercise-induced Muscle Damage in Healthy Humans

Abstract

Mechanically overloaded muscle and its subsequent damage are strong stimuli for eliciting acute hormonal changes, and muscle adaptation following exercise- induced muscle damage may involve complex hormonal responses before the completion of muscle regeneration. PURPOSE: This study investigated systemic responses of thyroid-stimulating hormone (TSH), free thyroxine (fT4) and prolactin (PRL) for several days after eccentric exercise-induced muscle damage in humans. METHODS: Nine healthy men (age 25.7 ± 1.7 years, height 180.4 ± 1.7 cm, body mass 77.2 ± 2.7 kg, body mass index 23.7 ± 0.6) performed 50 maximal eccentric muscle actions using the knee extensor muscles of both legs on an isokinetic dynamometer. Blood samples were withdrawn before and at 6, 48 and 120 hrs post-exercise, and serum levels of TSH, fT4) and PRL were measured by ELISA using commercially available kits. Myoglobin (Mb) concentration and lactate dehydrogenase (LDH) activity were also evaluated as indirect markers of muscle damage. One-way ANOVA was used for statistics. RESULTS: Significant alterations in Mb and LDH were observed over time after eccentric exercise (p<0.05-0.001). Serum fT4 levels exhibited a gradual increase reaching statistical significance at 48 and 120 hrs following the muscle damaging exercise (1.20±0.05 ng/dl, 1.29±0.04 ng/dl, and 1.26±0.05 ng/dl, at 6, 48 and 120 hours after exercise, respectively, compared to 1.13±0.02 ng/dl at baseline; mean±SE, p<0.05). Both PRL and TSH showed also a gradual increase up to 33% at 48 hrs and 120 hrs post exercise, respectively, however they failed to reach statistical significance due to a large variability shown between the subjects’ responses (PRL: 23.4±3.1 ng/ml, 28.1±4.7 ng/ ml, 30.2±4.1 ng/ml and 25.7±4.6 ng/ml; TSH: 1.09±0.14 μIU/ml, 1.27±0.15 μIU/ml, 1.17±0.20 μIU/ml, and 1.33±0.17 μIU/ml, at baseline, 6, 48 and 120 hours post- exercise, respectively, mean±SE, p>0.05). CONCLUSION: The late elevated levels of TSH and PRL, and particularly of fT4, during the recovery period after muscle damage may suggest functional interactions between those hormones and muscle regeneration. Further studies are needed to characterize the mechanisms by which those hormonal responses are triggered and regulated at the systemic level during recovery after exercise-induced muscle damage.