The effects of sprint training on the contractile properties of human muscle fibres obtained by needle biopsy were investigated. Individual fibres were mechanically skinned and activated by Ca(2+)- and Sr(2+)-buffered solutions at pH 7.1, and allocated to distinct populations on the basis of their contractile characteristics. The majority of fibres sampled pre-training could be separated into the three major fibre groups: Populations I (24/70, 34%), II (25/70, 36%) and III (18/70, 26%), which exhibited characteristics similar to those of histochemically classified type I, IIA and IIB fibres, respectively. The remainder (3/70, 4%) represented another fibre group, with intermediate characteristics. The muscle fibres were also activated by Ca2+ at a reduced pH of 6.6, to mimic the intracellular acidification that occurs during intense exercise. Lowering pH increased the threshold for contraction by Ca2+, reduced Ca2+ sensitivity, and increased the steepness of the force-pCa relationship, in all fibres sampled from the three major fibre groups. Maximum force was not significantly reduced in any fibre population. In the post-training sample, the three major fibre types were present in different proportions: Populations I (10/52, 19%), II (20/52, 38.5%) and III (11/52, 21%). Three other fibre groups sampled in low numbers exhibited contractile characteristics intermediate between Population I and Population II. Following sprint training all of the three main fibre populations exhibited higher thresholds for contraction by, and lower sensitivities to, Sr2+ but not Ca2+, compared with the fibres sampled pre-training. Maximum force was significantly lower in Population II fibres after sprint training. At pH 6.6, post-trained Population III fibres exhibited even lower Ca2+ sensitivity, with concomitant increases in the threshold for contraction and force-pCa curve steepness.