## 1 Introduction  
In swimming competitions taking place at pools, we may distinguish sprint (50-100 m), middle- (200-400 m), and long-distance races (800-1500 m) [1]. Each of the competitions places different requirements on the athletes, mainly in terms of physiology [2, 3]. As a result, success in swimming events is often determined by various factors, e.g., the effectiveness of using various energy systems [4]. In the case of short distances, good results are achieved mainly by athletes capable of coping with the efforts on an anaerobic lactic acid basis, with a high ability to tolerate this type of fatigue [5]. In addition, large body size (often associated with greater height and muscle mass) and the potential to generate high mechanical power are also important in the swimming sprint [6]. In long-distance events, the result mainly depends on efficiency of the aerobic energy supply system and the economy of swimming (e.g., low energy cost) [2]. In middle-distance races, swimming performance is determined by a combination of determinants for shorter and longer events [7]. It should be added that, apart from the aforementioned physiological factors, the level of mastery of the swimming technique has significant impact on sports result, which depends, among others, on body structure [8, 9]. Bearing these deliberations in mind, it seems reasonable to assume a diagnosis regarding the state of the swimmer's physical fitness with parallel technical control of task performance, i.e., by determining the values of selected kinematic indices. The quality of performed movements can be assessed by applying basic kinematic indices, such as stroke length (SL) and stroke rate (SR) [10]. Some authors also distinguish stroke index (SI = v \(\times\) SL), which is considered to be an exceptionally good indicator of swimming efficiency [11; 12]. The diagnosis of physiological disposition level allows for identifying factors limiting sports performance of swimmers [8]. In training practice, these diagnostics can be carried out, inter alia, by test swimming. Monitoring outcome components should, therefore, be an integral part of a planned training program. The construction of specialized water tests for swimmers is often based on those previously used in versions on land. An example of adapting land tests is tethered swimming, being an equivalent of the Wingate test [13] or the progressive swim test, analogous to the progressive run test [14]. One of the tests used to assess anaerobic capacity on land is the Running Anaerobic Sprint Test (RAST), in which the athletes cover six 35-m runs in the shortest time possible, with a 10-s interval between each repetition [15]. The maximal and mean mechanical power, as well as the fatigue index [16], are determined based on the time required to complete successive runs and the athlete's body mass. The design of this test (short efforts of maximum intensity) meets the design criteria for anaerobic capacity tests. Low costs make this test of practical use even in youth or amateur sports [16]. In this regard, the RAST test has been shown to be very useful in diagnosing indicators of anaerobic capacity, as evidenced by a high correlation between the results of this test and those obtained for the Wingate test [17]. Therefore, it seems reasonable to modify this test to the conditions of an aquatic environment and determine the relationship between its results and that for swimming, especially in the case of short- and middle-distances. The facts described above justify the research objective: the need to assess the usefulness of the modified RAST test in the form of a 6 \(\times\) 25 m (hereinafter referred to as the Swimming Anaerobic Sprint Test--SAST) in relation to swimming results, especially in teenage swimmers, among whom less technologically advanced methods are implemented. The main aim of the research was to determine the relationship between the SAST results and those obtained for the short- (100-m) and middle-distance freestyle (200-m).