## 4 Discussion
study, which may suggest that exercise intensity in our study was not sufficiently challenging to induce EIH. However, the fact that our participants were not accustomed to arm aerobic exercise, rating the exercise as \"very severe\" (i.e., 7/10 on the Borg scale), with lactate accumulation above the OBLA point at the end of the exercise, may imply that a higher intensity would be hard to maintain and may lead to exercise termination in a short time. The evidence from these two previous studies may suggest that intensity and not duration is more important for inducing EIH following upper-limb aerobic exercise, but this needs further examination. The aerobic exercise intensity required to induce hypoalgesia has been investigated in many studies [6; 23; 33; 34; 35], and various intensity methods were tested on the lower limbs. In detail, [PERSON] et al. [6] found that 20 min of lower-limb pedaling at an intensity of 70% of the predicted maximal HR led to greater increase in PPT compared to 50% of the predicted maximal HR. However, the same degree of decrease in THP ratings was observed at both intensities. [PERSON] and [PERSON] [23] found a decrease in THP ratings only after interval training at an intensity of 85% of the reserved HR, which was not demonstrated from aerobic exercise at an intensity of 70% of the HR reserve (HRR). Moreover, at both intensities, no change in PPT measures were reported. Another study by [PERSON] et al. [35] determined exercise intensity by the HR measured at the anaerobic threshold. They demonstrated that aerobic exercise for 20 min at an intensity 20% higher than the anaerobic threshold HR resulted in a significant increase in HPT and pain tolerance, compared to HR lower than the anaerobic threshold. Similarly, [PERSON] and [PERSON] [34] reported an increase in PPT pain threshold after an effort of 70% of maximum oxygen consumption but not at 30%. [PERSON] et al. [33] examined the change in pressure pain intensity after aerobic exercise on a treadmill and found a decrease in pain intensity after exercise at an intensity of 75% of the maximal oxygen consumption, but no significant change was identified in response to exertion at 50% of maximal oxygen consumption. In addition, a local increase in PPT at 70% HRR but not at 50% was reported [36]. In contrast to these studies which indicate that EIH is related to higher intensities (70% of aerobic capacities), there are studies which indicate that lower intensities have a beneficial effect on pain sensitivity. For example, a recent meta-analysis which examinedthe effect of exercise on pain thresholds in healthy individuals found that moderate aerobic exertion, defined as 11-14/20 on the Borg scale (fairly light to somewhat hard), had a greater effect on the pain threshold compared to other intensities (less or more intense) [7]. Taken together, the inconsistency seen in various studies of the hypoalgesic effects following aerobic exercise, i.e., with different intensities and durations required to achieve EIH effects, explains the fact that there is no optimal recommended protocol and emphasizes the need for further research. In the present study, the target HR was determined at the intensity of 60% of maximal HR based on previous studies, indicating that maximum effort in the upper limbs is obtained at a lower HR than in the lower limbs [17, 37]. We therefore assumed that this intensity would allow untrained individuals to maintain arm aerobic exercise for 20 min. At this intensity, lactate was higher than the OBLA point, and the exercise was rated as hard. Nevertheless, this intensity may not be sufficient to induce hypoalgesic effects, even when it is rated as severe, and the effort involves exertion of a smaller muscle mass. EIH has been tested in different body locations in order to test the involvement of local and central mechanisms.