Network integration and evolution of organ systems interactions during incremental exercise

Human function emerges from dynamic interactions among physiological systems. While decades of research have provided understanding of individual systems, how musculoskeletal, cardiovascular, and respiratory systems coordinate as an integrated network during exercise remains unclear. We characterized multisystem coordination during cardiopulmonary exercise test (CPET) of incremental cycling.

Twenty-six young adults performed a graded cycling test until exhaustion (25 W/min). Continuous synchronized recordings included electromyography from bilateral vastus lateralis (Leg) and erector spinae (Back), three-lead electrocardiography, and respiratory waveform via chest belt. Multisystem coordination was assessed using Amplitude-Amplitude Cross-frequency Coupling (ACFC), which quantifies the dynamic co-modulation of signal amplitudes. ACFC yielded three network-based markers: inter-muscular, cardio-muscular, and respiratory-muscular coupling. Analyses compared the Beginning (first third) and End (last third) of the test.

At the Beginning, inter-muscular coupling was strongest within the Leg-Leg sub-network. At the End, Leg-Leg coupling decreased by ~30% (p < 0.05), whereas Leg-Back and Back-Back coupling increased by ~100-300% (p < 0.05). Cardio- and respiratory-muscular coupling also increased, by ~30% in Heart-Leg and ~50-75% in Respiration-Leg, with larger and significant increments in the Heart-Back and Respiration-Back sub-networks (p < 0.05).

Incremental cycling reorganizes multisystem coordination, shifting from leg-dominant toward a distributed muscle-heart-lung network with fatigue. Exercise responses thus arise not only from individual systems, but also from their dynamic coupling as an integrated network. These network markers provide a novel, complementary dimension for assessing integrative physiological function during exercise.