Thesis and internship proposals

theses proposal

The determinants of the V'O2 slow component


Guarantor Carlo Capelli
Keyword Slow component of O2 uptake, surface EMG, muscular force
Suggested date November 7, 2019
Expiry date December 31, 2020
Study courses Master's degree in Sport Science and Physical Performance
Description Background
When we perform exercise above Gas Exchange Threshold (GET), we attain steady state oxygen uptake (V ̇O2ss) with a delay due to the appearance of a slow component of the V ̇O2 kinetics (V ̇O2sc) that starts 150-200 s after exercise onset (1). From a performance standpoint, V ̇O2sc is important, as it is related to the loss of homeostasis and to the increased susceptibility to fatigue (2). Results support the notion that the progressive recruitment of Type-II muscle fibres during heavy exercise may be the main determinant of V ̇O2sc, as these fibres are characterized by high ATP cost of force production and by high O2 consumption for ATP synthesis (1). Yet, recent findings have suggested that V ̇O2sc may arise from metabolic processes occurring inside the already recruited fibres leading to a progressive drop of the mechanical efficiency of contraction and to the progressive increase of the ATP cost of force production (3). In addition, it has been shown that V ̇O2sc may be modulated by perturbations of local blood flow and by manipulations of O2 delivery to exercising muscles (4).
 
Aims
The proposed investigation aims to understand whether the progressive: i) decrease of mechanical efficiency and/or; increase of ATP cost of force production of the recruited motor units are the main phenomena underpinning the onset of V ̇O2sc. To this aim, a group of young adults will be investigated in two different conditions: 1) during intermittent isometric bilateral contractions of the knee extensors @ a fixed per cent of MVC; 2) during the same type of muscular contraction, but maintaining a neuromuscular activation of the recruited muscles identical to the one observed at the beginning of the exercise described @ the point 1). In the first case, the intermittent, prolonged isometric exercise will elicit a clear V ̇O2sc. Should the hypothesis of an increasing ATP cost of force production hold true, in the second exercise modality we should observe constant oxygen uptake and neuromuscular activation paralleled by a progressive decay of strength.
 
Methods
10-12 young adult volunteers with similar physiological and strength characteristics will be recruited. After preliminary examination, the subjects will perform on an isometric dynamometer three 5-second isometric contractions of the knee extensors interspersed with 90 second of recovery. During the trials, force and surface EMS (sEMG, 100 Hz) will be recorded. After band pass filtering and rectification, integrated iEMG, RMS and mean peak frequency (MPF) during contractions will be calculated. In two other occasions, the subjects will perform intermittent isometric contractions (contraction/relaxation = 10s/5s, duty cycle 67%) @ 50 % of MVC for 20 minutes or until exhaustion. During all the experiments, V ̇O2 will be measured every 15 seconds; HHb-HbO2 with NIRS and sEMG will be recorded continuously. In the second experiment, carried out in a different day, the subjects will be asked to perform the same type of contraction maintaining, throughout all the test, the level of neuromuscular activation recorded during the first minute experiment #1. The level of neuromuscular activation will be evaluated by calculating every 5 seconds iEMG/RMS that will be shown on a display in front of the subject. The subject, with the aid of this visual feedback, will modulate muscle recruitment so that a constant and pre-set level of iEMG/RMS will be maintained during the entire trial. Also in this case, V ̇O2 HHb-HbO2 and sEMG will be recorded as previously described.
 
Perspective
The parallel assessment of V ̇O2, NIRS and sEMG would help understand whether V ̇O2sc is determined by the progressive recruitment of Type-II fibers or, rather, by the decay of efficiency and/or the increase of ATP cpost of force production the already recruited fibers. In addition, the analysis of NIRS signal would contribute to understand the possible role of O2 delivery in modulating V ̇O2sc. As already stated, V ̇O2sc is important from a performance standpoint. The possibility of maintaining the highest possible WR associated with invariant relevant physiological variables is critical for success in endurance events. If this condition is not satisfied, fatigue develops quickly.
 
References
  1. Jones AM et al (2011) Slow Component of V ̇O2 Kinetics: Mechanistic Bases and Practical Applications. Med Sci Sports Exerc 43: 2046 – 2062
  2. Salvadego D et al. (2010) Gas exchange kinetics in obese adolescents. Inferences on exercise tolerance and prescription. Am J Physiol Regul Integr Comp Physiol; 299: R1298 – R1305.
  3. Zoladz JA et al (2012) Isometric strength training lowers the O2 cost of cycling during moderate-intensity exercise. Eur J Appl Physiol 112: 4151 – 4161.
  4. Burnley M et al (2002) Effects of prior heavy exercise, prior sprint exercise and passive warming on oxygen uptake kinetics during heavy exercise in humans. Eur J Appl Physiol 87: 424 – 432.
 





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