Competitive athletes continuously strive to improve exercise performance and gain a competitive edge. However, in highly trained athletes, there is a limited room for improvement and therefore, novel strategies
to improve performance are crucial. While anaerobic training, such as speed endurance training (SET), is effective in improving short intense performance, when implemented in high volumes, its impact on
endurance performance remains less clear. However, as previous studies have commonly utilized a high-volume of SET in each session (>8 bouts), the minimum volume of SET required to achieve an adaptive
response is currently unclear. An alternative strategy for athletes is reduction of body weight, which might improve performance through enhancement of power-to-weight ratio. However, while rapid weight loss
strategies might improveperformance, they carry the risk of posing adverse health effects. To address these gaps, two studies were conducted investigating the effects of SET (Study I) and rapid weight loss (Study
II) on exercise performance in athletes. Study I explored the effects of six weeks of low and high volume SET, while reducing low and moderate training volume, in highly trained male cyclists on short intense and endurance exercise performance and underlying physiological mechanisms. Study II investigated the impact of 14 days of low energy availability (LEA) and additionally a three-day refueling period on exercise performance, metabolism, systemic stress, immune function, and inflammation in endurance trained female athletes. The main findings of this thesis were that Study I revealed that implementing a low-volume SET was equally effective in maintaining endurance performance and muscle metabolism as highvolume SET, whereas only the low-volume SET led to improved 4-min time trial performance. Study II revealed that female athletes undergoing 14 days of LEA impairs short intense and endurance performance, even when normalized for the reduction in body weight. Moreover, threedays of refueling were insufficient to restore endurance performance but did partly restore short intense performance. Additionally, LEA was associated with increased fat metabolism during rest and exercise and had a profound impact on immune parameters and systemic stress. Collectively, these two studies provide insights into 1) athletes can benefit from implementing low-volume SET, especially for enhancing short intense and maintain endurance performance during periods of reduced training volume, and 2) the detrimental effects of rapid weight loss and underlying the crucial importance for female athletes to have optimal energy availability for optimizing performance and health.