Muscle regeneration following injury is dependent on the ability of muscle satellite cells to activate, proliferate and fuse with damaged fibres. This process is controlled by the myogenic regulatory factors (MRF). Little is known about the temporal relation of the MRF with the expression of known myogenic growth factors (i.e. IGF‐1) in humans following muscle damage. Eight subjects (20.6 ± 2.1 years; 81.4 ± 9.8 kg) performed 300 lengthening contractions (180 deg s−1) of their knee extensors in one leg on a dynamometer. Blood and muscle samples were collected before and at 4 (T4), 24 (T24), 72 (T72) and 120 h (T120) post‐exercise.
Mechano growth factor (MGF), IGF‐1Eaand IGF‐1EbmRNA were quantified. Serum IGF‐1 did not change over the post‐exercise time course. IGF‐1Eaand IGF‐1EbmRNA increased ∼4‐ to 6‐fold by T72 ( P< 0.01) and MGFmRNA expression peaked at T24 ( P= 0.005). MyoDmRNA expression increased ∼2‐fold at T4 ( P< 0.05). Myf5expression peaked at T24 ( P< 0.05), while MRF4and myogeninmRNA expression peaked at T72 ( P< 0.05). Myf5expression strongly correlated with the increase in MGFmRNA ( r2= 0.83; P= 0.03), while MRF4was correlated with both IGF‐1Eaand ‐Eb( r2= 0.90; r2= 0.81, respectively; P< 0.05). Immunofluorescence analysis showed IGF‐1 protein expression localized to satellite cells at T24, and to satellite cells and the myofibre at T72 and T120; IGF‐1 was not detected at T0 or T4. These results suggest that the temporal response of MGFis probably related to the activation/proliferation phase of the myogenic programme as marked by an increase in both Myf5and MyoD, while IGF‐1Eaand ‐ Ebmay be temporally related to differentiation as marked by an increase in MRF4and myogeninexpression following acute muscle damage.