May the Force be with You: the Role of Strength Training in Exercise Tolerance and Health Promotion

Traditionally, exercise intensity is expressed as a percentage of maximal oxygen uptake (VO2max). It is known that as exercise intensity increases in any individual, unique thresholds exist that demarcate boundaries associated with specific physiological and metabolic profiles. The concept of threshold intensities is complex and has generated a large amount of debate within the scientific community. Much of the controversy extends from differences in ideas in determining a specific intensity of exercise below which exercise is sustainable for long durations and above which is unsustainable for prolonged periods. This thesis is organized into five main chapters. The 1st study aimed at clarifying the physiological rational behind the concept of threshold-based exercise intensity. We focused on comparing the most common threshold indexes used to partitions intensity of exercise into ranges (i.e. moderate, heavy, very heavy and severe domains)). Those are: Maximal Lactate Steady State (MLSS), Critical Power (CP), Respiratory Compensation Point (RCP) and the deoxyhemoglobin breakpoint ([HHb]BP). Whether commonalities between these “thresholds” exist, the notion of “critical metabolic rate” is presented as the highest metabolic rate at which exercise is well tolerated for long durations. The 2nd and 3rd studies focused on the determination of alternative methods for exercise intensity threshold identification using novel statistical approaches and/or different measurement techniques. Identification of exercise intensity domains has important implications/applications for research interventions, however the most common indexes of threshold intensity that are nowadays used are often cumbersome, invasive, time consuming and pose a burden to participants and researchers involved. For these reasons, alternative methods that are not only valid but also time effective and non-invasive are very attractive. Finally, in the 4th and the 5th study the body response to exercise above this threshold intensity phenomenon that demarks sustainable versus unsustainable physical activity is investigated. It is nowadays recognised that the exercise intensity at which this progressive loss of metabolic stability become manifest (called “excess VO2” and “VO2 slow component” in the two exercise paradigms respectively) occurs at intensity of exercise above the “critical metabolic rate” (study #1), a landmark that demarcates the lower boundary of the “heavy” intensity domain. Attention is therefore given to the possible role of an intervention affecting this loss of efficiency in the heavy-intensity domain of exercise and therefore affects the genesis of the “excess” VO2 and/or the VO2 slow component. Collectively, the studies contained within this thesis have contributed to better understand the “threshold” phenomenon and the physiological bases behind it. Additionally, new methods of threshold determination have been developed giving practical alternative tests for its determination in different contexts and populations. Finally, this thesis has also provided useful information regarding the body metabolic response in the heavy intensity domain (i.e. above “threshold”) and the role of strength training as a possible determinant in affect such a body response to high intensity training.