## 1 Introduction  
The rate of force development (RFD) is a commonly used measure of explosive strength in humans [1]. RFD is being increasingly used for assessments in sport science [2], physical therapy and rehabilitation [3], as well as in in studies investigating the basic biomechanics and physiology of human movement [1, 4]. A thorough review of the RFD literature was published by [PERSON] et al. [1], summarizing the multiple underlying mechanisms as well as several methodological considerations pertaining to RFD assessments. It stressed that attempting to achieve the maximal force and RFD within the same contraction may not be optimal [1]. This recommendation was based on studies (summarized further in the following paragraphs) comparing the instructions given to participants to contract as \"hard\" as possible (no regard to the RFD), as \"hard and as fast\" as possible (emphasis on peak force, but with a fast start), and as \"fast and as hard\" as possible (emphasizing the quick RFD, but also sustaining the effort to achieve peak force). The instruction should emphasize the \"fast\" aspect, in order to obtain the maximal RFD. The first study to compare the RFD between \"hard-and-fast\" and \"fast-and-hard\" instructions reported that the latter elicited a slightly higher peak RFD (\(\sim\)8-12%) in handgrip muscles [5]. A subsequent study reported a similar finding for the ankle, elbow and index-finger muscles, with the differences in the RFD between the instructions ranging from 23% to 53%, depending on the muscle group [6]. The larger difference in comparison to the former study could be attributed to slightly different instructions, as [PERSON] et al. [6] instructed the participants only to attain peak force and allowed for individual strategies in exerting maximal force (for the \"hard-and-fast\" instruction). In contrast, [PERSON] et al. [5] instructed the participants to contract as hard and as fast as possible. Differences in the peak RFD (\(\sim\)21%) between \"hard-and-fast\" and \"fast-and-hard\" instructions have been recently shown for the knee extension task [7]. Note that these studies only analyzed the peak RFD. A study using an ankle dorsiflexion task reported a \(\sim\)20-30% higher RFD for the \"fast-and-hard\" contraction, although only for the peak RFD and the RFD in the 50 and 100 ms time windows, whereas the differences were reversed in the 200-400 ms time windows [8]. A recent study reported that the RFD was higher throughout the first 250 ms for the \"fast-and-hard\" compared to \"hard-and-fast\" instructions, for the isometric squat [9]. Another study compared \"hard-and-fast\" to \"fast-and-hard\" instructions in the leg press and elbow flexion task and also reported a substantially higher (\(\sim\)27-32%) peak RFD in both tasks for the \"fast-and-hard\" instruction [10]. This study also reported moderate to high correlations (r = 0.55-0.78) between the peak RFD values from each instruction [10]. A later electromyography study by the same research group demonstrated that the \"fast-and-hard\" instruction elicits a larger agonist activation at the beginning of the contraction (compared to the \"hard-and-fast\" instruction), while the antagonist co-activation was unaffected by the instruction [11]. Higher EMG activity could be related to the increased motor unit firing rate [12] or speed of recruitment of motor units [13]. While the emphasis on the \"fast\" execution of the contraction is needed to obtain the highest RFD, all of the aforementioned studies instructed the participants to focus on the fast start start, but to sustain high force contraction in the subsequent 1-4 s as well. Moreover, some studies instructed the participants that the peak force is of no concern during the fast instructions [5] and termed the instruction as \"fast\" instead of \"fast-and-hard\". This is probably not optimal, considering the well-known linear relationship between peak force and RFD [14].