## 1 Introduction  
The literature presents evidence that intensities characterized as vigorous or close to VO2 Max stand out for promoting rapid physiological adaptations in a shorter time of exposure to exercise [1; 2; 3; 4]. Workouts popularly known as high-intensity interval training (HIIT) [1; 2; 3; 4] have become an interesting alternative that counteracts one of the barriers related to adherence to regular training programs, in this case, the time spent in each session. Thus, these workouts reverberate an improvement in the time-efficiency relationship [5]. However, while this exercise mode appears to be efficient in the physiological aspect [6; 7; 8], when in exaggerated doses or when incompatible with the training status [9], it appears to provide negative affective responses [10; 11], which could encourage evasion. On the other hand, the self-selection of workloads, according to the practitioner's preference, seems to be an interesting alternative [9; 12; 13; 14]. Since self-regulation of the magnitude of the external load is based on interoceptive and cognitive interactions, supported by previous experiences, this condition would enable an increase in self-efficacy [15] and the achievement of a positive affective valence, without the need for fixed patterns of intensities or elaborate prescriptions. The meta-analysis produced by [PERSON], [PERSON], and [PERSON] [16] presents the idea that self-selected exercise vs. imposed prescription have different influences on affective responses, and that the \"equal intensity\" condition would demonstrate better affective responses in the self-selected exercise condition, based on the premise of greater autonomy [16]. The authors also point out that intensities above the ventilatory threshold seem to induce worse responses regarding affect. Based on this understanding, the \"Dual Model\" Theory seems to be the foundation for understanding such responses [16]. Additionally, it is worth noting that negative affective responses seem to reverberate in anxiety symptoms [17]. Numerous studies have observed the behavior of affective responses to self-selected and imposed work intensities [8; 13; 18; 19] in athletes [20; 21], sedentary [15; 22; 23; 24], and overweight/obese individuals [25]. However, there are no known studies that have investigated affective and anxiety responses in middle-aged people who practice recreational physical exercise, nor in the face of stimuli with a maximum time trial characteristic, i.e., an open task. In this case, adherence is not the heart of the matter but rather the understanding of the nature of the stimulus, as well as the possible deleterious psychophysiological effects, such as on anxiety levels. Furthermore, the volume load (VL) and training impulse (TRIMP) responses to a self-selected protocol are apparently unknown. Thus, we do not know, in the \"let it run freely\" condition, whether VL and the TRIMP performed for the self-selected protocol could be significantly attenuated compared to the imposed programming, since these would not be practitioners engaged in high performance. Therefore, the aim of the present study was to investigate the affective responses of middle-aged recreational runners to running stimuli with imposed and self-selected velocity (time trial characteristics), as well as the effects on aerobic performance variables (VL and TRIMP). Secondarily, as a means of observing possible stressful conditions of exercise, anxiety responses were also determined. Finally, as a tertiary outcome, the level of association between VO2 Max and anxiety scores, as well as between TRIMP and the post-exercise feeling scale, was investigated. Our primary hypothesis (H1) is based on the construct that self-selected speed stimuli will enable ideal adjustments to effort regulation, and consequently, better affective responses, in addition to equal results on aerobic performance parameters.