Γελαδάς Ν., Αναπληρωτής Καθηγητής, ΤΕΦΑΑ,, ΕΚΠΑ
Δεληγιάννης Α., Καθηγητής, ΤΕΦΑΑ, ΑΠΘ
Καλογερόπουλος Ι., Καθηγητής, ΤΕΦΑΑ, ΑΠΘ
Μπουντόλος Κ., Καθηγητής, ΤΕΦΑΑ, ΕΚΠΑ
Βράμπας Ι., Αναπληρωτής Καθηγητής, ΤΕΦΑΑ, ΑΠΘ
Μαριδάκη Μ., Επίκουρος Καθηγήτρια, ΤΕΦΑΑ, ΕΚΠΑ
Κοσκολού Μ., Λέκτορας, ΤΕΦΑΑ, ΕΚΠΑ
During prolonged steady state exercise there is a progressive rise in heart rate (HR), a drop in stroke volume (SV), accompanied sometimes by a decline in cardiac output ( ) and mean arterial pressure (MAP). This phenomenon has been named cardiovascular drift (CVdrift) and seems to be affected not only by dehydration and hyperthermia, but also by the mode of exercise. However, the effect of muscle mass size and recruitment on CVdrift has not been yet investigated. Moreover, it is unclear whether these alterations may be affecting blood muscle and brain and oxygenation flow, causing fatigue at central or local level. Therefore, the purpose of this study was to explore the effect of exercising muscle mass and motor unit recruitment on CVdrift. A further aim was to investigate the effect of CVdrift on central and local fatigue. It was hypothesized that: a) the greater the muscle mass and the higher cycling revolutions, the worse the CVdrift b) the development of CVdrift will cause fatigue probably of central origin and c) the aggravate of CVdrift will be associated with greater central fatigue.
To test these hypotheses twelve subjects (23.4±0.6 yrs) participated in four experimental conditions in a counterbalanced order: a) cycling for 50min at 80 revolutions per minute with one leg (1L) followed by another 50min of cycling with two legs (2L) at 60% of maximal oxygen uptake ( O2max) and half of this value in 2L and 1L respectively (effect of muscle mass) b) cycling for 90 min at 126±15Watts (58-60% of O2max), with pedaling rate equal to 40 revolutions per minute (40rpm) c) cycling for 90min at the same workload as in the previous condition at 80 revolutions per minute (80rpm) (conditions b and c: effect of muscle recruitment) and d) cycling for 90min at 80 revolutions per minute with the same O2 as in 40rpm (80abs) (effect of muscle pump). Oxygen uptake, (CO2 rebreathing method), HR, rectal temperature (Tre), and mean skin temperature ( sk), were evaluated and recorded at various time points. Integrated electromyographic activity of vastus lateralis (iEMGVL), vastus medialis (iEMGVM) and skin blood flow (SBF; laser Doppler flowmetry) were also measured. Plasma and blood volume changes (PV%, BV%) were calculated from hemoglobin and hematocrit values before exercise and at 30 and 90 min of the cycling protocol. Second derivative near- infrared spectroscopy (NIRS) was used to evaluate the oxygenation (SatO2) and blood flow (HbT) in the working muscle and the brain. Heart rate variability (HRV) and pulse transit time (PTT) were used as indexes of sympathetic response. Electro-egephalographic activity also measured at rest and during exercise. Before and after 40rpm and 80rpm conditions, EMG, joint position sense (JPS), maximal isometric force and rate of force development (RFD) of knee extensors and handgrip were tested.
Cardiac output, HR, iEMGVL, were significantly higher (p<0.01) during 2L than 1L exercise (20th min onwards). Throughout the 50th min bout, the rate of decline in SV, SatO2 and the respective rate of rise in HR were greater in 2L than with 1L condition without significant changes in . At the 50th min of exercise ΔSV was -20.8±0.6 ml•beat-1 in the 2L and -13.7±1.2 ml•beat-1 in the 1L p<0.05); similarly, ΔHR was +16.1±0.4 beats•min-1 and +10.8±1.0 beats•min-1, in 2L and 1L, respectively (p<0.01). SatO2 changes in 2L and 1L were -9.1±1.0 and +6.5±1.2%, respectively (p<0.01). Final values of Tre and Tsk were higher in 2L than in 1L condition (p<0.01). Changes in these variables couldn’t satisfactorily explain the rise in HR and fall in SV. On the contrary, BV drop, iEMGVL and rise in sympathetic response (HRV, PTT) seem to have a significant influence on CVdrift (p<0.05). Decline in SV and the reciprocal rate of rise in HR were significantly greater in 80rpm than in 40rpm condition (ΔSV=-16.21±0.85 ml•beat-1 vs. -25±0.79 ml•beat-1 in 40rpm and 80rpm, respectively and ΔHR=+18.33±0.88 beats•min-1 vs.+24.5±0.74 beats•min-1, in 40rpm and 80rpm, respectively, p<0.01). Overall compared to the steady state values, at the end of exercise decreased by -1±0.26 L•min-1 in 80rpm (p<0.01) but no changes were observed in 40 rpm (Δ = -0.23±0.1 L•min-1, p=ns). Sympathetic activity was higher in 80rpm (HRV, PTT) due mainly to muscle mass recruitment and central command (increased iEMG and EEG activity; p<0.05). The SatO2 and HbT in the brain and HbT in the muscle were higher in 40rpm compared with the other two conditions (80rpm, 80abs). Moreover, there were no significant differences between 40rpm and 80abs in all cardiovascular and thermoregulatory variables despite the different HbT. Thermal variables (Tre, sk, SBF) did not significantly differ in the three experimental conditions. Arm and leg force and EMG declined, and RFD increased after exercise in both conditions (80rpm vs. 40rpm, p<0.05) while JPS worsened only in the leg. In addition, leg RFD, RPE and JPS were significantly different between conditions (p<0.05).
In conclusion, during 50 min of cycling with 2 legs, CVdrift is exaggerated compared to 1 leg exercise, despite the fact that VO2 in 1L was half that in 2L trial. Thermal status of participating subjects cannot fully explain the observed differences in these conditions. Moreover, during steady state prolonged cycling with 80rpm, CVdrift is more pronounced compared to 40rpm, despite equal mechanical work being performed in both conditions. Also thermal and hydration status of participating subjects and the effect of muscle pump cannot explain the observed differences. Along with hydration status, muscle input to central nervous system and central command may play a role on CVdrift when exercise is performed with higher speed and larger muscle mass mainly via enhanced sympathetic activity. Fatigue occurs after the 90min cycling in 40 and 80rpm and there is strong indication it is mainly of central origin and accentuated in the second condition probably by the participation of local factor as well.