Characteristics of speed skaters' physical fitness distribution in 3000m competitions
First, we discovered that the amount of change in peak force (PF) values in the IMTP test was a significant reference indicator in the follow-up investigation. The larger the change in PF over the same period of time, the more outstanding the explosive quality. The shorter the time spent in equal PF changes, the stronger the isometric force [13–14]. Since long-distance speed skating requires athletes to maintain a high speed of uniform motion with the help of continuous pedal power of the lower limbs [15], which is very physically demanding for athletes, so a reasonable distribution of speed and physical energy in each phase is an important element of long-distance speed skating techniques and tactics [16]. Second, we discovered that the individual variability of test parameters such as PF, RFD, and IMP was greater in the early half of the test period for the three subject athletes than in the second half. This indicates that there is significant individual variability in the anterior segment of the athletes' lower limb power generation. This difference may be due to the more pronounced difference in the athletes' starting strength monitoring values and the gradual decay of the effect in the later period of monitoring. Athletes need to have good strength endurance and speed endurance to maintain a small drop differential after reaching maximum speed in order to keep the energy expenditure reduced [17]. But the problem of excessive drop differential has existed for a long time among speed skaters, so how to maintain the front speed advantage in the backstretch skating in athletes' competition is especially crucial to improve the competitive level of speed skaters [18].
In addition, comparing the 3000m speed characteristics of world-class athletes can be seen (Fig. 6). After reaching the maximum speed at the end of the 1st lap, the S02 athletes could not maintain the high-speed skating, and the speed dropped significantly in the latter part of the skating. While the world-class athletes could maintain the higher speed and longer time, while the drop rate was lower. Moreover, world-class athletes have fully developed bodies with tall proportions [19], outstanding explosive power and speed endurance, and have the advantage of both physical characteristics and athletic ability compared to S02 athletes [20]. China elite athletes have no significant differences in physical form and athletic quality, and even the advantages of athletic quality are not outstanding. There is also a lot of room for improvement in terms of technical and tactical level, special ability and athletic quality developed and cultivated by later training, so it is very important to find the athletic quality and technical shortcomings of elite athletes to become a top athlete and quickly improve the quality of training and athletic level.
The law of lower limb muscle isometric strength characteristics on the effect of speed
Isometric force refers to the force generated by a muscle contraction while maintaining a constant length, allowing muscle tension to reach its maximum capacity [21]. Although no external work is performed since the body does not move, the muscle still requires a significant amount of energy. Because active contraction generates tension, leading to the elongation of the elastic component of the muscle to resist the applied load. Isometric strength of the muscle serves as a foundation for maintaining specific human posture and athletic performance [22].
The results of the study showed that the isometric strength of the lower limb muscles of the three speed skaters influenced not only the start-up acceleration in the first half of the Competition, but also the speed retention and speed endurance qualities in the second half of the Competition [23]. According to the survey, world-class speed skating long-distance athletes have a good stamina distribution strategy in the 3000m competition. They have the characteristics of fast starting speed, strong mid-range speed maintenance ability and small back-range speed reduction difference [24–25]. However, most athletes have an elevated level of energy metabolism in the body during the middle and late stages due to the alternation between the anaerobic and aerobic energy supply systems, as well as a significant increase in the rate of lower limb work. Due to the slower rise in heart rate, the athlete's oxygen demand will exceed the maximum oxygen uptake thus creating an oxygen debt. This leads to insufficient oxygen and energy supply to local muscle tissues and lactic acid accumulation in the local muscles of the lower limbs. At the same time, the problem of local tissue hypoxia, lactic acidosis, decreased ability and frequency of lower limbs to do work, occurs. Therefore, long distance speed skaters should not only have good absolute lower limb strength to improve the efficiency of skating [26], but also need to have sustained lower limb explosive strength and speed endurance to maintain the skating rhythm. And the lower limb muscles should have good lactic acid tolerance and fatigue resistance to counteract the physiological reactions that occur during skating, all of these lower limb muscle characteristics play a vital role in skating.
In fact, the speed skating technique is characterized by the requirement for the athlete to achieve short and intensive push-offs while stroking [27] and to maintain a relatively stable posture, which requires a high level of lower limb strength. In speed skating, the total power output of an athlete during competition is determined by the product of work per stroke and stroke frequency. Therefore, the key factor affecting speed is the amount of work done by the athlete in each stroke [28], making it crucial for the athlete to perform work efficiently. While most sports require athletes to push forward using an opposite direction, in speed skating, athletes need to push sideways to generate forward propulsion due to the unique characteristics of ice friction [29–30]. Therefore, the performance of speed skaters largely depends on their ability to generate muscle power, and athletes must use proper movements and techniques to deliver maximum force to their skating motion. Studies have shown that the hip extensors can produce maximum muscle power during the explosive skating phase, while the muscle power of the hip, knee, and ankle joints decrease as skating speed increases, ultimately impacting the power output of each aspect of skating performance [31]. Another study pointed out that strengthening the rapid contraction ability of the leg muscles would improve the specific power of the athletes' stirrups [32]. Therefore, the explosive power of the lower limbs is crucial for speed skaters.
From the biomechanical point of view, the power output of speed skaters is mainly used to overcome air resistance and ice friction [33]. Athletes can reduce the drag coefficient by changing their posture to reduce the windward area and keeping their bodies in a streamlined posture. So, maintaining a smaller hip and knee angle and a lower squat angle during the skating phase is more beneficial to athletes' resistance reduction. However, Low squat angles can easily cause muscle compression in the lower limbs of athletes. This results in reduced blood flow to the lower extremities, leading to local muscle hypoxia and premature exercise fatigue. At the same time, insufficient lower limb strength will also lead to deformation of the backstroke and compensatory phenomena. So that the body cannot maintain a continuous low sitting skating posture, increasing air resistance and friction. Therefore, in order to maintain a good stance on the ice it is also necessary to improve the endurance of the lower limb muscles.
Isometric strength training of lower limb muscles
Speed skating is a type of high-speed competition [34]. In the first half of the athletes use anaerobic glycolytic system to supply a large proportion of energy. And the latter part of the skating will be converted to aerobic metabolism for energy supply [35]. So, a large amount of lactic acid produced by anaerobic metabolism will be created and accumulated in the muscles in the early part of the competition. It has been shown that the lower lactate indicates, the higher efficiency of work done by the athlete at the same speed. If the lactate rise is nearly the same, the skating will be faster This demonstrates that the greater the economy of movement due to specialized strength and skating technique [36]. As speed skaters pay more attention to the improvement of aerobic and anaerobic capacity, the development of endurance qualities and flexibility qualities in their daily training is very evident. They relatively neglected lower extremity specific strength training, especially the development of certain specific muscle strength types in the lower extremities. The results of this study show that the isometric strength of the lower limb muscles is very important for the skating ability of speed skaters. Thus, the key to enhancing an athlete's skating performance is to exercise their lower limb isometric strength. There are many lower limb strength and explosive power training methods, such as deep squats, jumps, and other conventional training methods, such as isometric contraction training and plyometric training, as training methods that have been adopted in recent years to improve the rapid strength ability of the lower limbs, can be applied to daily speed skating special physical training [37].
Firstly, isometric contraction training is divided into static and dynamic contraction training [38]. Static contraction training involves sustained muscle contraction against a fixed object such as a wall or training device. Dynamic contraction training is performed by contracting against a moving object, such as holding the weight at a certain point and then relaxing back to the origin. Dynamic contraction training should be used when the muscle contraction strength is lower than the maximum contraction strength, contraction duration is generally 5 to 20s, etc., High intensity lasts 3s athletes in the isometric contraction training is recommended to start from a lower level of strength, gradually progressive, which is conducive to the development of muscle strength. In the isometric contraction training should be noted in the maximum degree of muscle contraction to naturally hold your breath, to avoid holding your breath caused by blood pressure. The exercises should be performed at different levels of joint mobility to maximize the training of the participating muscles.
Furthermore, plyometric training (PT) is an extension of the impact training method used by Verkhoshansky in the former Soviet Union to improve muscle explosive power [39]. It emphasizes different practice means to complete jumping down - jumping up - and then jumping up quickly within the specified height, which can show a variety of training forms such as rotary jump, straddle jump, side skating step jump and single-leg jump (Fig. 7). The muscle contraction form of super-length training fits well with the sports mechanics characteristics of fast power generation required of athletes in modern competitive sports [40]. So, it is used in physical training to develop the qualities of maximal strength, fast power and explosive power of athletes with obvious effects, and it has become a common training method for lower limb strength training. It has been found that plyometric training can increase the stiffness of muscle tendons and improve the economy of movement of athletes thus improving their competitive performance, but no significant changes in lactate threshold and maximal oxygen uptake occurred in athletes after training [41]. It has also been found that weight-bearing plyometric training can significantly increase peak hip and ankle moments, peak joint power, joint stiffness, and thus high lower limb muscle strength and explosive power, as well as improve neuromuscular adaptations by reducing hip antagonist muscle activation, and this type of training method was found to be well suited for competitive events [42]. The use of plyometric training methods for speed skating physical training can not only develop the lower limb specific explosive power of athletes and thus improve the frequency and effect of skating, but also improve the stiffness and stability of the hip and ankle joints of athletes. Thus, the rationalization of the movement is improved and the efficiency of the stirrups is improved. It is an efficient training method for speed skaters with specific lower body strength worth promoting.