πForce—Repeatability and Reliability of Peak Force and Rate of Force Development in a Portable Multi-Exercise Device

Isometric strength is widely used to monitor training adaptations, assess neuromuscular fatigue, and play a critical role in the maintenance of muscle health. This study assessed repeatability (intra-session) and reliability (inter-session) of a force production machine in different exercises: Isometric Mid-Thigh Pull (IMTP), Knee Flexion (KF) at 30°, and Seated Calf Raise (SCR). Parameters measured included Peak Force (PF), RFD at 0–50, 0–100, 0–150, 0–200, 50–100, 100–150 and 150–200 ms. Thirty male individuals (IMTP = 30; KF = 11 and SCR = 30) participated (age: 20.6 ± 3.6 years, mass: 75.3 ± 7.5 kg, height: 1.80 ± 0.64 m). Repeatability and reliability were calculated for bilateral PF and RFD for IMTP and SCR, and unilateral for KF. PF demonstrated good to excellent repeatability in all exercises: IMTP (ICC = 0.93), KF (left: ICC = 0.98; right: ICC = 0.97), SCR (ICC = 0.84). RFD displayed poor to good repeatability in IMTP (ICC = 0.45–0.87) and SCR (ICC = 0.40–0.85), moderate to excellent repeatability in KF (left: ICC = 0.53–0.96; right: ICC = 0.61–0.92). PF reliability was excellent in IMTP (ICC = 0.93) and KF (left: ICC = 0.99; right: ICC = 0.97), and moderate in SCR (ICC = 0.64). RFD reliability was moderate to excellent in IMTP (ICC = 0.58–0.94), poor to good in SCR (ICC = 0.13–0.64), and poor to excellent in KF (left: ICC = 0.33–0.96; right: ICC = −0.19–0.95). This study shows that portable dynamometry can measure maximal and explosive strength in different exercises, demonstrating good reliability for most parameters in IMTP and KF.