High anaerobic energy release during submaximal arm exercise

Wheelchair propulsion fatigue thresholds in electromyographic and ventilatory testing

STUDY DESIGN

Qualitative study.

OBJECTIVE

The objective of the present study are physiological processes occurring when the intensity of manual wheelchair propulsion approaches levels causing muscular fatigue. In particular, we set out to (1) detect the electromyographic (EMG) and ventilatory fatigue threshold during a single wheelchair incremental test, (2) examine the relationship between EMG threshold (EMGT) and ventilatory threshold (VT), and (3) detect the EMG threshold differences between the propulsive and recovery muscle synergies.

SETTING

Biomechanics laboratory at the University of Alberta, Canada.

METHODS

Oxygen uptake and EMG signals from ten wheelchair users (seven males and three females) were recorded as they were each performing an incremental propulsion bout in their own wheelchairs on a wheelchair ergometer. The V-slope method was used to identify the VT, and the EMGT of each of the eight muscles (anterior deltoid, middle deltoid, posterior deltoid, infraspinatus, upper trapezius, sternal head of pectoralis major, biceps brachii, and triceps brachii) was determined using the bisegmental linear regression method.

RESULTS

For each participant, we were able to determine the EMGT and VT from a single incremental wheelchair propulsion bout. EMGT stands in good agreement with VT, and there was a high similarity in EMGT between push and recovery muscles (intraclass correlation coefficient = 0.91).

CONCLUSION

The EMG fatigue threshold method can serve as a valid and reliable tool for identifying the onset of muscular fatigue during wheelchair propulsion, thus providing a foundation for automated muscle fatigue detection/prediction in wearable technology.

Kinetics of oxygen uptake and cardiac output at onset of arm exercise

Pulmonary oxygen uptake (V O2) kinetics at onset of exercise is reported to be slower for arm than for leg exercise. This could be attributed to reduced cardiac output (Q) or reduced arteriovenous O2 content difference or both. To test this, V O2 mean tissue oxygen consumption (V O2T) and Q kinetics in arm cranking were compared with corresponding values found in leg cycling. The increase in V O2 during phase 1 (abrupt increase after onset of exercise) was less in arm than in leg exercise, suggesting that immediate Q adjustments to arm exercise were less pronounced. Mean response times (MRT, the relative rates at which a steady state was attained) for V O2, V O2T, and Q were prolonged during arm exercise. The MRT of VO 2 in arm exercise at a given blood lactate increase was higher than in leg exercise. The delayed V O2 kinetics in arm exercise might be due to delayed Q kinetics and higher anaerobic glycolysis occurring early during arm exercise.

Effect of hypoxia on muscle oxygenation and metabolism during arm exercise in humans

The influence of hypoxaemia on anaerobic energy production during arm exercise (AE) has been investigated. Six men were studied during maximal AE and during 10 min of sitting submaximal AE under both normoxic (AEN) and hypoxic (AEH, respiratory hypoxia, 12% O2) conditions. Peak pulmonary oxygen uptake (VO2) during maximal AE in normoxia and hypoxia was 2.25 +/- 0.15 and 2.18 +/- 0.14 l min-1, respectively (P < 0.05). The absolute workload was the same during submaximal AEN and AEH and corresponded to 54% of peak VO2 during normoxic maximal AE. To eliminate the potential influence of differences in catecholamine levels on the metabolic response, the submaximal experiments were performed during beta-adrenoceptor blockade. Oxygen deficit was 1.45 +/- 0.26 and 1.67 +/- 0.191 during AEN and AEH, respectively (n.s.). Oxygen extraction at steady state was lower during AEH than during AEN, and assuming a similar O2 demand this corresponds to a 14% higher muscle blood flow during AEH. At the onset of both AEN and AEH, O2 extraction (a-v O2) across the arm increased transiently above that at steady state, the increase being more pronounced during AEN than during AEH (P < 0.05). Muscle oxygenation, measured by near-infrared spectroscopy, demonstrated an initial decrease which was partially reversed as exercise proceeded. The reversal of muscle O2 desaturation was slower in all subjects during AEH (t1/2 = 2.4 +/- 0.2 min) than during AEN (t1/2 = 1.2 +/- 0.2 min; P < 0.01). After 10 min of exercise, arterial blood lactate was higher (P < 0.05) during AEH (5.5 +/- 0.2 mmol l-1) than during AEN (4.9 +/- 0.6 mmol l-1), whereas arterial plasma ammonia (NH3) was similar. The arteriovenous difference for both lactate and ammonia was similar during AEN and AEH. It is concluded that the high anaerobic energy production at the onset of AE is associated with a transient increase in O2 extraction and a transient decrease in muscle oxygenation. The effects of hypoxaemia on peak VO2, oxygen deficit and blood metabolites are less pronounced than previously described during submaximal leg exercise (LE).