Heart rate variability response of intensity-matched strength training dependent on body position in females: a pilot randomized crossover study

Heart rate variability (HRV), as an indicator of autonomic control, has been rarely studied during strength training application. This study investigates the acute HRV responses to intensity-matched resistance exercises, targeting similar muscle groups but performed in different body positions. Fourteen healthy females (21.6 ± 2.0 years) performed a 3-repetition maximum test (3-RM) for the squat movement in the Smith machine (SM, upright) and the leg press (LP, seated). During two subsequent visits, they randomly completed two exercise sessions in SM and LP (two sets of 10 repetitions at 50% 3-RM). HRV was assessed continuously (via ECG) throughout the interventions. At pre-exercise, the Root Mean Square of Successive Differences (RMSSD) and RR intervals (RRI) were significantly higher for the LP condition. Alpha1 of Detrended Fluctuation Analysis (DFAa1) was significantly higher with SM at rest. During exercise sessions, the LP condition revealed significantly reduced RMSSD (∆exercise: SM -1.19 ± 14.57 ms vs. LP -22.23 ± 22.46 ms; p = .013) and higher RRI (p < .001). No differences were observed for DFAa1. The changes between pre- and post-exercise for RMSSD and DFAa1 showed no differences between LP and SM conditions; however, RRI was significantly reduced for SM (p = .008). Within the conditions, RMSSD decreased significantly from pre- to post with SM (p 0.008) but not with LP (p = 0.271). Squats within the SM condition led to increased vagal withdrawal both at rest and after exercise, probably due to orthostatic stress. While exercising, the differences in autonomic regulation are less noticeable. Future analyses should examine the body position-dependent HRV responses to post-exercise hypotension.