## 1 Introduction  
Acute dynamic and isometric resistance exercise and aerobic exercise reduce pain sensitivity in healthy [1, 2] and chronic pain [3, 4] subjects. This is manifested experimentally as a decrease in pain ratings and an increase in pain thresholds and tolerance; a phenomenon termed exercise-induced hypoalgesia (EIH) [5]. The hypoalgesic effects of EIH have been found to be associated with the intensity and duration of aerobic exercise [5, 6]. To date, almost all studies investigating the EIH effect of aerobic exercise have examined the large muscle groups of the lower extremities [2, 5, 7, 8, 9]. However, in some clinical conditions (e.g., paraplegia [10], lower-limb amputation [11], post-total knee replacement surgery or injuries), when exercise with lower extremities is limited, upper-limb exercise is recommended in order to improve functioning or fitness. However, to our knowledge, only two studies have examined the effect of aerobic exercise of the upper limbs on experimental pain sensitivity. These studies found a decrease in pain sensitivity manifested as an increase in pain thresholds [12, 13]. However, in order to investigate the hypoalgesic effects, it is necessary to evaluate supra-threshold processing by estimating the pain magnitude rating [14]. Different physiological and metabolic responses have been reported from conducting upper-limb versus lower-limb aerobic exercise. Specifically, arm exercise has elicited a lower maximum rate of oxygen consumption (VO\({}_{2}\)) (34% less VO\({}_{2}\)) [15], a lower anaerobic threshold (AT) [15, 16], and a lower maximal heart rate (HR) [17] compared to leg exercise. In order to maintain the same power output with a smaller muscle mass, one needs to work harder, and the rate of perceived exertion (RPE) evaluated by the Borg scale is higher. Accordingly, during upper-limb exercise, a higher RPE has been reported at a lower HR compared to leg exercise [18] The use of RPE to control for a given exercise intensity across different exercise modalities is useful. However, the physiological responses as related to the RPE vary according to the exercised muscle size [19]. Focusing on HR, [PERSON] et al. [17] found that the maximal HR in an upper-limb cranking exercise was about 90% of the maximal HR of lower-limb pedaling. Similarly, the maximal VO2 during the upper-limb exercise only reached about 70% of the VO2 achieved with the lower-limb exercise. These differences challenge the determination of the target HR for aerobic exercise based on the maximal HR formulas of Tanaka [20] or the formula of 220-age [21]. Indeed, it has been reported that the prediction equations for maximal HR significantly overestimate the true maximal HR when performing upper-limb exercise [22]. Furthermore, the lactate accumulation point was found to occur at a lower maximal HR with upper-limb versus lower-limb aerobic exercise (61% vs. 76%, respectively) [16]. Based on these physiological differences between aerobic exercise conducted by the lower vs. upper limbs, the question arises as to whether the EIH effects previously reported with lower-limb exercise will also be achieved during upper-limb aerobic exercise of moderate to high intensity. This study aimed (1) to experimentally test the acute effects of a single aerobic exercise of the upper limbs on pain threshold and pain intensity in healthy subjects, and (2) to compare the HR, RPE, workload (power) and the hypoalgesic effects of two protocols controlling for intensity: one adjusted to the target HR (THR) of 60% of heart rate reserve and the other based on a rating of 7/10 on Borg's scale. Considering the differences between aerobic activity in the upper limbs compared to the lower limbs, we aimed to explore if EIH is observed following upper extremity exercise, which may serve as a tool to improve physical function and to reduce pain in those who are limited to perform an exercise with the lower limbs.