1. ASSESSING THE RELATIONSHIP BETWEEN EXERCISE EFFICIENCY AND MITOCHONDRIAL ENERGY TURNOVER IN ATHLETES USING NEAR-INFRARED SPECTROSCOPY
Ciaran O’Grady1 1School of Sport and Exercise Science, University of Kent, Chatham Maritime
Introduction
Cycling efficiency has been shown to decrease during prolonged cycling (Passfield & Doust, 2000). One of the methods of measuring cycling efficiency is by analysing expired gasses, suggested to accurately represent muscular oxygen consumption (Poole et al., 1991). Changes in cycling efficiency that are the result of adaptations to endurance training are assumed to represent underlying changes in cellular energetics of the working muscle. Previous research suggests that these adaptations are mainly orientated around an increase in mitochondrial oxidative capacity rather than from the muscle fibre itself. To investigate this, the purpose of the current study is to assess differences in skeletal muscle mitochondrial function in athletes during a fatiguing bout of endurance exercise.
The aims of this study were to test the hypotheses that
1) Decreases in efficiency can be attributed to decreased mitochondrial oxidative capacity
2) Peak sprint power decreases significantly after a fatiguing exercise bout
Fig. 2 Efficiency values over the time course of the fatiguing exercise bout. Values are means ± SE. * P < 0.05 = significant difference compared to Time = 0. Ŧ P < 0.1 = Displays trend, P values reported. Ŧ
0.09 Ŧ
0.05 ⃰
Methods
Twelve well-trained cyclists (11 male, 1 female) with mean  SD VO2max 73  9 mL.kg-1.min-1 performed a ramp test, one familiarisation trial and one fatiguing bout of endurance exercise (SRM Ergometer). The exercise consisted of 120 minutes of riding at 60% PPO, with cadence controlled throughout the test. The exercise was preceded and followed by a set of 3 sprints (6 s maximal), fixed at different cadences (60, 90 & 120 rpm). Pre and post exercise muscle occlusions and reoxygenation measurements were also taken. During the 120 minute exercise, efficiency measurements were taken every 30 minutes, and a muscular occlusion was performed for 20 s whilst riding (Figure 1).
Muscle oxygenation was estimated by the use of a near-infrared spectroscopy device, placed on the distal end of the Vastus Lateralis muscle.
Fig. 3 Muscle tissue oxygen saturation throughout the time course of the fatiguing exercise bout. Values are means ± SE. * P < 0.05; = significant difference compared with Time = 0.
Fig. 4 Peak Power results from cadence sprints. Solid line representing sprints completed prior to the fatiguing exercise bout, and dashed line representing sprints completed after. Values are means ± SE. $ Significant main effect of time (P < 0.05). # Significant main effect of condition (P < 0.05 ) #
&
NIRS Baseline
Occl.
Warm-up
Occl
Sprints
2hr Test
2min
6min +
re-oxy
10min
3min 48s
120min
Fig. 1 Schematic illustration of the experimental protocol
* < 0.05 = significant difference from warm-up. Values are means ± SE
Results – Table 1: Mean values for endurance test
Conclusions
Decreases in muscle O2 saturation suggest that mitochondrial oxidative capacity can result in the decrease in efficiency observed during endurance exercise
-Peak sprint power reduces significantly after a fatiguing bout of exercise. Sprint power is significantly reduced at lower cadences both before and after fatigue.
0 min
30 min
60 min
90 min
120 min
Mean power (W)
195  10
Mean heart rate (bpm)
151 5
154 5
156 4
1584
160 4
Mean cadence (rpm)
84 1
85 1
Mean blood lactate (mmol/g)
Warm Up
1.18 0.13
2.94 
0.50 *
2.99
0.42
2.55 
0.31
2.46
0.25
2.81 
0.44
References
Poole, D. C. et al. (1991). Contribution of exercising legs to the slow component of oxygen uptake kinetics in humans. Journal of applied physiology, 71(4),
Passfield, L. & Doust, J. H. (2000). Changes in cycling efficiency and performance after endurance exercise. Medicine and science in sports and exercise, 32(11),