# Effects of Introducing Rest Intervals in Functional Fitness Training
## 1 Introduction  
High-intensity functional training or Functional Fitness Training (FFT) is a new exercise modality prioritizing functional multi-joint movements, executed in multiple planes of movement, at high intensity and with different durations, that can be adapted to any level of fitness. FFT is also characterized by eliciting more muscle recruitment than more traditional exercise, as it consists of whole-body exercises (squats, deadlifts, cleans, snatches, pull ups, vertical jumps, etc.) executed at high intensity as a circuit [1]. FFT was first mentioned in the scientific literature in 2014 [2]. In this early work, [PERSON] et al. (2014) [2] soughtto find high-intensity training options that would save time over traditional moderate-intensity aerobic plus resistance training programs yet provoke a similar level of enjoyment and adherence to the exercise program. These findings could be the consequence of a self-selected intensity of exercise and also of the high variety of FFT exercises and training protocols [2]. FFT has often been compared to high-intensity interval training (HIIT). This last training modality is characterized by bouts of high-intensity work interspersed with very short rest periods, leading to fitness and health improvements in less time than weekly exercise recommendations [3; 4]. However, as HIIT frequently involves aerobic training intervals, its health benefits may not be as great as those offered by a combined aerobic and resistance training program [5]. HIIT focuses on unimodal exercises such as running, pedaling, rowing, etc., completed as short high-intensity bouts to improve aerobic capacity and cardiorespiratory fitness without promoting significant muscle mass gains [6; 7]. In contrast, FFT exercises are multimodal, whole-body functional exercises with universal motor recruitment patterns in multiple movement planes [2; 8; 9] conducted at high intensity as a circuit. These exercises pursue improvements in both muscle strength and cardiovascular fitness [10]. Another difference between HIIT and FFT is that the latter lacks prescribed rest intervals [1]. Many FFT workouts consist of completing a set number of repetitions in the shortest time possible (\"rounds for time\") or completing within a set time frame as many repetitions as possible (\"AMRAP\") of a set of exercises [1; 11]. Hence, rest or recovery intervals are introduced by athletes \"as needed\" depending on their physical fitness level or the nature of the FFT workout. This means that a FFT workout may be completed with no rest periods at all. This contrasts with most HIIT protocols, in which both work and rest intervals are preset [1; 6; 7]. A few studies have compared the effects of HIIT and FFT on performance. For instance, [PERSON] et al. (2015) [12] compared a program consisting of a multimodal circuit, similar to FFT, featuring strength and metabolic-component exercises with a unimodal HIIT training program of rowing. Both groups completed 6 weeks of training 3 days per week, and each session consisted of 6 rounds of 60 s all-out intervals followed by 3 min of passive recovery. While both the multimodal circuit training program (MM-FFT) and rowing HIIT groups showed similar gains in aerobic threshold (13% vs. 12%), VO\({}_{\text{2 max}}\) (7-5%), and power and anaerobic capacity (15% vs. 12% and 18% vs. 14%, respectively), only the MM-FFT group showed significant improvement in the kg lifted in a one repetition maximum (1 RM) of squat, deadlift, and overhead press, as well as significant increases in broad jump muscular power and muscular endurance, documenting improved muscle performance in MM-FFT compared to rowing HIIT [12]. Hence, one of the characteristics of many FFT is the lack of preset rest intervals. In their study, [PERSON] et al. (2015) [12] prescribed rest intervals in the MM-FFT program to match work-rest times of both programs such that they differed only in the type of exercises conducted (multimodal vs. unimodal rowing).