Phosphocreatine (PCr) and creatine (Cr) content were measured in the remaining part of characterized fibres. A decline in the ratio of PCr to Cr (PCr/Cr) was used as an indication of activation. The mean peak torques were, respectively, 39 (2), 72 (2) and 87 (6)% MVC. Cumulative distributions of type I and IIA fibres were significantly shifted to lower PCr/Cr ratios at all intensities (Kolmogorov-Smirnov test, P<0.05). The cumulative distribution of type IIAX fibres showed a significant leftward shift only at 87% MVC ( P<0.05). A hierarchical order of fibre activation with increasing intensity of exercise was found, with some indication of rate coding for type I and IIA fibres. Evidence for activation of type IIAX fibres was only found at 87% MVC.
This research was part of a long term interest of Professor Anthony J Sargeant in the extent to which different muscle fibre types are recruited to meet the demand for mechanical output in human exercise. The research was carried out under his direction by members of his research group in Amsterdam where JG Beltman was his PhD student.
Metabolically assessed muscle fibre recruitment in brief isometric contractions at different intensities
European Journal of Applied Physiology. 2004 Aug;92(4-5):485-92