After treating time rule violations as a trivial offense over the past, tennis associations in 2018 permanently introduced an on-court serve clock to make sure that players do not exceed the time limit between points. In this study, we investigated the influence of this technological officiating aid at its first use in Grand Slam tennis at the 2018 US Open. By investigating time intervals between 2135 points at the Men's single tournament we found that the share of rule violations decreased (26.3%), but the average time did not (21.6 s) compared to previous studies. We could further confirm that the players still use this time interval for tactical reasons and to recover after longer rallies. In addition, the umpires, which do not show any reputation bias, have a significant influence on the inter-point time as well. Based on these findings we suggest introducing a serve clock that is started automatically. Further, we argue that a dynamic time limit, which takes the intensity of the previous rally into account, would be more appropriate. Players could be granted more time for recovery after longer rallies, but less time after shorter ones in order to reach the intended goals regarding pace of play.

In high-intensity interval training the interval duration can be adjusted to optimize training results in oxygen uptake, cardiac output, and local oxygen supply. This study aimed to compare these variables in two interval trainings (long intervals HIIT3m: 3 min work, 3 min active rest vs. short intervals HIIT30s: 30 s work, 30 s active rest) at the same overall work rate and training duration. 24 participants accomplished both protocols, (work: 80% power output at VO2peak, relief: 85% power output at gas exchange threshold) in randomized order. Spirometry, impedance cardiography, and near-infrared spectroscopy were used to analyze the physiological stress of the cardiopulmonary system and muscle tissue. Although times above gas exchange threshold were shorter in HIIT3m (HIIT3m 1669.9 ± 310.9 s vs. HIIT30s 1769.5 ± 189.0 s, p = 0.034), both protocols evoked similar average fractional utilization of VO2peak (HIIT3m 65.23 ± 4.68% VO2peak vs. HIIT30s 64.39 ± 6.78% VO2peak, p = 0.261). However, HIIT3m resulted in higher cardiovascular responses during the loaded phases (VO2p < 0.001, cardiac output p < 0.001). Local hemodynamics were not different between both protocols. Average physiological responses were not different in both protocols owning to incomplete rests in HIIT30s and large response amplitudes in HIIT3m. Despite lower acute cardiovascular stress in HIIT30s, short submaximal intervals may also trigger microvascular and metabolic adaptions similar to HIIT3m. Therefore, the adaption of interval duration is an important tool to adjust the goals of interval training to the needs of the athlete or patient.