Author(s): SOHEI, O., MAKO, F., YOSHIYUKI, F., Institution: DOSHISHA UNIVERSITY, Country: JAPAN, Abstract-ID: 977

INTRODUCTION:
Although synchronization of the cardiac and locomotor rhythms (i.e., cardio-locomotor coupling, CLC) has been observed in walking humans, its effects on energy efficiency during walking remain unclear. In particular, whether the phase-dependent coupling between cardiac and locomotor rhythms, where foot strikes occur at different times within the cardiac cycle, modulates the energetic cost of walking has not been examined. Therefore, this study examined the influence of cardiac phase-dependent CLC on energetic responses during human walking.
METHODS:
Nineteen healthy young adults (12 males and 7 females) participated in this study. Following a warm-up walk on a treadmill, participants completed a 12-min walking bout divided into three foot-strike timing conditions for 4 min each. Foot-strike timing was determined based on each participant’s R-R interval (RRI), with strikes occurring at 30% of the RRI (systolic phase, RRI30), 80% (diastolic phase, RRI80), or freely without cardiac-cycle modulation (RRIfree). We determined each participant’s treadmill speed at which their heart rate (HR) was maintained 100 beats•min-1. The RRI was measured from a surface electrocardiogram (ECG) using standard bipolar leads. Foot-strike timing measured using an accelerometer was regarded as the locomotor rhythm. A synchronization index (λ) was calculated to quantify the strength of phase locking between the heartbeats and the locomotor rhythm. Breath-by-breath carbon dioxide output (VCO2) and oxygen uptake (VO2) were continuously measured during walking, and the energetic cost of transport per unit distance (CoT; J•kg-1•km-1) was calculated.
RESULTS:
The mean λ values for the final 2 min of each condition were significantly greater during the RRI30 (λ=0.76) and RRI80 (λ=0.80) conditions compared to the RRIfree (λ=0.15) condition. The VO2 under the RRI80 conditions were significantly decreased compared to the other two conditions (p<0.01). In addition, the CoT was significantly decreased under the RRI80 conditions compared to the RRI30 conditions (p<0.01). In contrast, HR remained relatively constant across the conditions (99.5 ± 1.7 beats•min-1), despite a statistically significant difference between conditions (p<0.01).
CONCLUSION:
The significantly greater λ values in the RRI30 and RRI80 conditions compared to the RRIfree indicate the presence of CLC during cardiac phase-dependent foot-strike modulation in walking. Moreover, the reduced VO2 and CoT observed in the RRI80 condition suggest that CLC occurring during the diastolic phase may improve circulatory efficiency and thereby enhance the energetic efficiency of walking.