## 2 Materials and Methods  
Using a single-cohort, within-subject observational study design with pre-/post-measures, we investigated the 4-week training effect on physical capacity, strength, and aerobic measures. Physically active men and women were recruited to complete a four-week, different-day concurrent training program. Their strength was assessed using three repetition max (3 RM) tests for the bench press, leg press, and isometric mid-thigh pull (IMTP), while their dynamic performance was assessed through a countermovement jump (CMJ). Their aerobic capacity (VO\({}_{2}\)max) was determined on a cycling ergometer, using a continuous incremental exercise test. Previous research in concurrent training [5; 8; 10; 12; 14] suggests that a sample size of \(n=20\) participants is required for a comprehensive investigation. We sought to conduct a pilot exploratory study, and 12 physically active participants were recruited (Table 1) through convenience sampling. The inclusion criteria required potential participants to self-identify as \"apparently healthy\" using an adult pre-exercise screening questionnaire (Exercise Sport Science Australia, Adult Pre-exercise Screening System), and they were not to identify as elite athletes. Participants with external commitments (e.g., training programs, sports) were accepted; however, their external training minutes were recorded. The participants were advised to maintain their current diet and not to take any additional supplements or NSAIDs for the duration of their study involvement. All the sessions were supervised by a researcher (GB), and informed consent was gained, with ethical approval by the Institutional Human Research Ethics Committee from Curtin University (HREC2022-0300). Each participant attended 19 sessions over the course of six weeks, consisting of familiarisation, testing (pre- and post-), and sixteen training sessions (RT, \(n=8\) and HIIT, \(n=8\)) over four weeks. In the first week, during familiarisation and following the attainment of informed consent, all the exercises used in the intervention were demonstrated and attempted by the participants to familiarise them with the movements. The seat settings for the HIIT sessions on the cycling ergometer (Concept 2, 430952017, Burleigh Heads, QLD, Australia) were determined as the height that allowed for slight knee flexion at the bottom pedalling position, and these settings were then recorded for subsequent use. On the second (week 2) and last (week 6) visits, all the performance tests were conducted in a standardised order from the smallest muscle contraction time to the longest/continual (CMJ, IMTP, bench press, leg press, and VO2 max). The CMJ and IMTP testing were both performed using portable force plate (AMTI, BP600600-2000, 9953M, Watertown, MA, USA) sampling at 500 Hz and operated using BioAnalysis (v2.2, AMTI, Watertown, MA, USA) software. Prior to performing the CMJ, the participants completed a standardised warm-up of three unloaded submaximal jumps at approximately 80% of their self-determined effort, followed by three minutes of rest. The participants then performed three unloaded, maximal-effort jumps with three minutes of rest between repetitions, with the best repetition used for the statistical comparison. The primary variables of interest with the CMJ were the jump height (cm) achieved, determined utilising the flight time calculation (11), and the peak propulsive force (PF). When performing each CMJ, the participants were instructed to begin from a standing position with their hands placed on their hips; then a vocal three-second countdown was given, after which the participants were instructed to self-select their countermovement depth and accelerate upwards as quickly as possible to achieve maximal concentric velocity during each jump, ensuring that their hands were placed on their hips throughout the entirety of the jump, otherwise the jump would be invalidated.