Introduction and Aim: Blood flow restriction (BFR) is a method that is used in training and rehabilitation settings to improve functional outcomes. Although widely and successfully applied, the underlying neuromuscular mechanisms are not yet fully understood. By tracking the same motor units (MU) active before and after BFR (trapezoid ramp contractions), we showed an increased MU discharge rate after BFR. Persistent inward currents (PICs) are known to impact synaptic inputs to motoneurons by providing additional excitatory current. Thus, this study aimed at investigating the acute passive BFR effects on the central neuromodulation of PICs (Gorassini et al. 2002).

Methods: Thirteen uninjured male handball players (age: 24 [SD 3] years, body mass index: 24 [2] kg/m²) participated in the study after providing written informed consent (IRB approval number: 2024-167). High-Density surface EMG (HDsEMG) grids (HD08MM1305) were placed on participants’ dominant lower limbs over the vastus medialis (VM) and lateralis (VL) muscles. Signals were recorded before and after a 5-minute passive BFR intervention at 80% of maximal arterial occlusion pressure (AOP: 157 [14] mmHg). Maximal AOP was defined using a mobile Doppler ultrasound in the popliteal fossa. Isometric force-matching triangular contractions (duration 20 s) at 20% maximal voluntary contraction (4.06 [0.84] Nm/kg) were performed before and immediately after BFR. HDsEMG signals recorded before BFR were decomposed (Negro et al. 2016) into firing instances of MU action potentials. To compare the properties of the same MU, the MU identified before BFR were tracked after BFR using their MU filters (Francic & Holobar 2021). Analyses were performed in Python with the openhdemg V0.1.1 library (Valli et al. 2024) by applying the paired MU analysis technique (Gorassini et al. 2002). Thereby, low recruitment threshold MU (control units) were paired with higher recruitment threshold MU (test units). Delta frequency (F) values of each test-control unit pair were estimated and served as the outcome. Effects were assessed utilizing the linear mixed effects model.

Results: In total, 97 (96%) out of 101 identified MUs were successfully tracked after BFR for VL and 56 (71%) out of 79 for VM. The linear mixed effects model revealed a significant main effect of Time (p < 0.001) on the estimated Delta F values. Specifically, the Delta F values increased by 20.3% after BFR (from 2.63 [0.24] to 3.17 [0.24] pps). No relevant effects for the main effect of Muscle (p = 0.65) or the Time*Muscle interaction (p = 0.49) were observed.

Conclusions: This study analyzed the effects of a passive BFR intervention on central neuromodulation indirectly by estimating the PICs of thigh muscles. The absence of an effect of Muscle underscores the underlying central mechanisms caused by local ischaemic interventions at the extremities (McNulty et al. 2002). Since individuals with knee joint injuries suffer from altered motor system excitability and reduced volitional muscle activation BFR, particularly the passive mode, may be used to counteract these neuromuscular alterations by enhancing excitatory plasticity.