The aim of this study was to analyze the error structure in propulsive movements with regard to its influence on monofin swimming speed. The random cycles performed by six swimmers were filmed during a progressive test (900m). An objective method to estimate errors committed in the area of angular displacement of the feet and monofin segments was employed. The parameters were compared with a previously described model. Mutual dependences between the level of errors, stroke frequency, stroke length and amplitude in relation to swimming velocity were analyzed. The results showed that proper foot movements and the avoidance of errors, arising at the distal part of the fin, ensure the progression of swimming speed. The individual stroke parameters distribution which consists of optimally increasing stroke frequency to the maximal possible level that enables the stabilization of stroke length leads to the minimization of errors. Identification of key elements in the stroke structure based on the analysis of errors committed should aid in improving monofin swimming technique.

• The monofin swimming technique was evaluated through the prism of objectively defined errors committed by the swimmers.

• The dependences between the level of errors, stroke rate, stroke length and amplitude in relation to swimming velocity were analyzed.

• Optimally increasing stroke rate to the maximal possible level that enables the stabilization of stroke length leads to the minimization of errors.

• Propriety foot movement and the avoidance of errors arising at the distal part of fin, provide for the progression of swimming speed.

• The key elements improving monofin swimming technique, based on the analysis of errors committed, were designated.