The aim of this study was to determine whether the Ca²⁺-induced conformational changes (switch) of the skeletal troponin complex (sTn) rate limit the kinetics contraction and/or relaxation of skeletal muscle. Previous studies investigated the kinetics of the Ca²⁺-controlled sTn switch using either isolated sTn or reconstituted thin filaments in the absence of ATP-dependent cross-bridge turnover, i.e., in the absence of the process of contraction regulated by the switch. Here we measure the Ca²⁺-induced sTn switch kinetics in psoas myofibrils which provide the intact contractile sarcomeric structure. To measure switch kinetics, we labelled sTn with the fluorescent probes IANBD (N-((2-(iodoacetoxy)ethyl)-N-methyl)amino-7-nitrobenz-2-oxa-1,3-diazole) or IAF (5-iodoacetamidofluorescein). The labelled sTn was biochemically incorporated into the myofibrils whereby the endogenous sTn was replaced. To determine the kinetics of the Ca²⁺-induced switch-on of sTn, the myofibrils were mixed with different calcium concentrations ([Ca²⁺]) in a stopped-flow apparatus detecting the kinetics of the fluorescence change. For switch-off kinetics, the myofibrils were shortly activated at high [Ca²⁺]and then the [Ca²⁺] was rapidly reduced by mixing the myofibrils with the Ca²⁺-chelator BAPTA. These switch-kinetics were compared with the kinetics of the Ca²⁺-controlled force development and relaxation which were measured on single myofibrils in a mechanical setup. These comparisons show that the kinetics of the switch-on of sTn is much faster than the kinetics of force development whereas the one of the switch-off is similar fast as force relaxation. The latter finding is different to the situation in cardiac myofibrils in which the switch-off of cardiac Tn (cTn) is faster than force relaxation (Solzin et al 2007). Additionally, the kinetics of the switch-off of sTn in psoas myofibrils is slowed-down under rigor conditions suggesting that rigor-bridges delay the switch-off.