Dissertation committee:
Νίκος Γελαδάς, Καθηγητής Εργοφυσιολογίας, της Σχολής Επιστήμης Φυσικής Αγωγής και Αθλητισμού του Πανεπιστήμιου Αθηνών (επιβλέπων)
Ιωάννης Βράμπας, Καθηγητής Εργοφυσιολογίας, της Σχολής Επιστήμης Φυσικής Αγωγής και Αθλητισμού του Αριστοτέλειου Πανεπιστημίου Θεσσαλονίκης (Σερρών).
Μαρία Κοσκολού, Αναπληρώτρια Καθηγήτρια Εργοφυσιολογίας, της Σχολής Επιστήμης Φυσικής Αγωγής και Αθλητισμού του Πανεπιστήμιου Αθηνών.
Κωνσταντίνα Δίπλα, Επίκουρη Καθηγήτρια Κυτταρικές Λειτουργίες και Άσκησης της Σχολής Επιστήμης Φυσικής Αγωγής και Αθλητισμού του Αριστοτέλειου Πανεπιστημίου Θεσσαλονίκης (Σερρών).
Κωνσταντίνος Καρτερολιώτη, Καθηγητής Μεθοδολογίας Έρευνας της Σχολής Επιστήμης Φυσικής Αγωγής και Αθλητισμού του Πανεπιστήμιου Αθηνών.
Αργύρης Τουμπέκης, Επίκουρος Καθηγητής Κολύμβησης της Σχολής Επιστήμης Φυσικής Αγωγής και Αθλητισμού του Πανεπιστήμιου Αθηνών
Γρηγόρης Μπογδάνης, Επίκουρος Καθηγητής Αλμάτων της Σχολής Επιστήμης Φυσικής Αγωγής και Αθλητισμού του Πανεπιστήμιου Αθηνών
Summary:
Since 1924, exercise physiologists have investigated rigorously the limiting factors of maximal oxygen uptake (VO2max). The classical approach proposed the notion that oxygen transfer system (cardiovascular system and blood oxygen carrying capacity) is the primary limiting factor of VO2max. However, a number of recent investigations have challenged that notion, supporting the aspect that cerebral activation is a determinant factor of VO2max. In the present study, we hypothesized that oxygen deficiency in skeletal muscle may be a determinant factor in inhibition of cardiovascular function either via suddenly recruitment of vagal tone or via excessive increase in blood pressure and excessive activation of the arterial baroreflex. This, in turn, was assumed to affect cerebral blood flow and neuro-motor alertness. Based on the above, the aim of the study was: i) to in-vestigate the role of muscle oxygenation as a limiting factor of O2max and ii) to explore, among skeletal muscle, heart and cerebral activity, the functional sequence of events that leads to voluntary termination of a maximal effort. Additionally, we wanted to investigate whether the possible varia-tion of baroreflex sensitivity during increasing exercise intensity may contribute to the termination of exercise through reflexive bradycardia.
Twenty six healthy well-trained males (age 33±2 yrs; VO2max 52±1 ml/kg/min), participated in the present investigation and they were divided into two equal sized groups. One group (n=13) performed an incremental exercise test to volitional exhaustion on a cycle ergometer during which muscle blood flow and muscle oxygenation was modified by applying cuffs on the thigh, which were inflated to external pressure of 120 mmHg either at the begging of the exercise or abruptly during cycling. The second group (n=13) performed 4 min constant load exercise at 30%, 60%, 80% and 100% of peak power output (PPO), under two different conditions, with and without muscle blood flow restriction via thigh cuffs, during which arterial baroreflex sensitivity (BRS) was evaluated. Cardiovascular and ventilatory responses, muscle and cerebral oxygenation, muscle (iEMG) and cerebral activation (EEG) were recorded continuously throughout tests.
Thigh cuffs application effectively reduced muscle blood flow, muscle oxygenation, induced venous occlusion and restricted venous return at rest and during submaximal and maximal exercise. Exercise tolerance as defined by VO2max (-17±2%) and PPO was significantly reduced during exer-cise with thigh cuffs application. This limitation was accompanied by lower maximal cardiovascular response (Qmax: -14±7%, SVmax: -18±2%, HRmax: -9±2%), reduction in local cerebral blood volume (-4±1 μΜ), but higher blood pressure (15±5%) and higher rate of leg perceived exertion (RPEleg), whereas, at the point of exhaustion the magnitude of changes in muscle oxygenation, peripheral muscle fatigue and RPEleg were no different among experimental conditions. In addition, VO2max and ΡΡΟ were restored to control levels only after the acute release of venous occlusion. Furthermore, despite the fact that the activity of iEMG and EEG was increased in a similar way with the increases in workload, no difference was found among the experimental conditions at the point of exhaustion. At rest, BRS was significantly reduced (-8±3%) with thigh cuffs application. Additionally, muscle blood flow restriction during dynamic submaximal exercise caused significant increases in blood pressure response (SBP: 59±13%), reduced the increase in heart rate (-6±1%) and reduced BRS (-22±5%) compared to absence of muscle blood flow restriction. The reduction of BRS during low and moderate intensity exercise (30-60% PPO) was significantly restricted during high intensity exercise (80-100% ΡΡΟ). Heart rate variability was similar between conditions.
In conclusion, muscle blood flow restriction via thigh cuffs application significantly reduced VO2max and aerobic power output that was accompanied by lower maximal cardiovascular response. At the exhaustion with or without occlusion were observed similar reduction of muscle oxygenation, increase of leg perceived exertion and increase of cerebral activation. The limitation of maximal cardiovascular response was mainly caused by the restriction of venous return and the reactivation of arterial baroreflex that inhibits exercise tachycardia. Furthermore, the increased perceived exertion is directly related to the change in muscle oxygenation. We suggest that during maximal effort, in the presences of a mismatch between skeletal muscle blood oxygenation and metabolism along with maximal level of perceived exertion, may lead to a point where the exercise inevitably is ter-minated due to lower cardiovascular response.
