Researchers investigating the mechanism of strength reduction of sewing threads analyzed the effect of fabric tightness and such selected thread properties as size, coefficient of yarn-metal friction, twist direction, number of plies, type of fiber, and fiber denier. Experiments demonstrated that the abrasion resistance of sewing thread is a key factor influencing the thread's ability to retain strength during sewing. Structural parameters, such as fiber length, fineness, number of plies, and twist, also influenced strength retention after sewing. Fabric tightness only affected strength reduction in certain threads under specific conditions. Changes in elongation at break of the thread depended on loss in initial modulus and therefore the loosening of the structure, which depended in turn on twist and other structural properties of the thread. 10 refs.