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A group of 12 sedentary medical students (1 man and 11 women aged 21-27 years) participated in a strength training programme for the trunk muscles lasting 18 weeks. The maximal isometric flexion and extension forces of the trunk muscles were measured before the training and at 18 weeks by dynamometer. The cross-sectional area of the back muscles, i.e. erector spinae, multifidus and psoas muscles, was measured from magnetic resonance images (spin echo sequence TR/TE 1500/80, slice thickness 10 mm) obtained at the L4-L5 disc level before the training, at 11 and 18 weeks. During training, no significant change in the body mass or body fat content was found. Muscle forces or muscle cross-sectional area were not related to body mass. There was a significant increase in both trunk muscle cross-sectional area (psoas muscle P< 0.001 and back muscles P < 0.01) and trunk muscle forces (flexion and extension forces P < 0.01) during the training but no direct association between the muscle cross-sectional area and strength of the flexors and extensors was detected before or after the training.

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