O'Bryan, Steven (2017) Effect of Changes in the Force-generating Capacity of the Knee Extensors on Lower-limb Power Production during Cycling Exercises. PhD thesis, Victoria University.

Abstract

Overview. Neuromuscular fatigue is defined as a reversible exercise-induced reduction in the ability to produce maximal voluntary force or power, originating from central (upper and lower motoneurons) and peripheral (skeletal muscle) sources. It is a symptom that can reduce exercise performance, diminish quality of life, and impair activities of daily living across a wide range of populations. In the lower-limb, the knee extensor muscles are of particular importance as they largely contribute to the execution of functional motor tasks (e.g. locomotion, sit to stand, climbing stairs). Therefore, fatigue of this muscle group is likely to have a large negative impact on the ability of the lower-limbs to generate power. General methodology. Within this thesis, a series of three experiments were conducted to investigate the effect of neuromuscular fatigue in the knee extensor muscles on lower-limb power production. Neuromuscular fatigue of the knee extensor muscles was assessed via changes in isometric maximal voluntary force (IMVF), voluntary activation (VA), maximal evoked resting twitch forces (RT), maximal muscle compound action potentials (M-wave) and voluntary surface electromyography (EMG) amplitude normalized to M-wave (EMG.M-wave-1). Lower-limb power production was measured on a stationary cycle ergometer. Motor command was investigated during cycling exercises via changes in: EMG activity of individual lower-limb muscles [vastus lateralis and vastus medialis (VAS), rectus femoris (RF), gluteus maximus (GMAX), biceps femoris and semitendinosus (hamstrings: HAM), medial and lateral gastrocnemius (GAS) and soleus (SOL) (ankle plantar flexors: APF) and tibialis anterior (TA)]; co-activation indices (CAI) of various muscle pairs [VAS/APF, VAS/HAM, GMAX,RF, GMAX/APF]; and muscle activation variability via the variance ratio (VR). Chapter 3 - study one. Introduction: Neuromuscular fatigue of the knee extensors develops during prolonged high-intensity submaximal exercise and is thought to limit lower-limb power production during subsequent maximal exercise. Aims: The first aim was to investigate the association between changes in EMG activity of VAS muscles during prolonged high intensity cycling exercise and the development of knee extensor fatigue. The second aim was to investigate the effect of knee extensor fatigue developed during prolonged high intensity cycling exercise on lower-limb power production and movement control during a subsequent maximal 30-s cycling exercise. Method: Seven physically active participants volunteered for the study. On one day, a maximal 30-s cycling exercise was completed. On the second day, a maximal 30-s cycling exercise was completed after 10-min of high-intensity cycling. Results: Over the course of high-intensity cycling, VAS EMG and GMAX/RF co-activation increased (P ≤ 0.05). The increase in VAS EMG (range from 6% to 14%) was negatively correlated with the reduction in IMVF following high-intensity exercise (from -2% to -36%; r = 0.791, P ≤ 0.05). During the 30-s maximal effort completed following high-intensity cycling, a positive correlation (r = 0.757, P ≤ 0.05) was seen between changes in IMVF and the changes in maximal lower-limb power production (from 0% to -27%). EMG reduced for all muscles, especially GMAX (-21 ± 8%) and VAS (-16 ± 13%) (P ≤ 0.05). Co-activation reduced for GMAX/RF and VAS/HAM (both P ≤ 0.05), but did not change for VAS/GAS (P > 0.05). Discussion: Larger increases in VAS EMG during prolonged high-intensity cycling exercise were associated with greater levels of knee extensor fatigue, which subsequently decreased maximal power generated by the lower-limbs. The increase in co-activation for GMAX/RF during high intensity exercise and maintained co-activation for VAS/GAS during maximal exercise, suggests that movement control was adjusted to limit fatigue occurrence in the knee extensor muscles and to maximise lower-limb power production. Conclusion: Knee extensor fatigue developed during prolonged high-intensity exercise decreases maximal lower-limb power production during subsequent maximal exercise. Chapter 4 - study two. Introduction: Fatigue is likely to accumulate in the lower-limb muscles during cycling exercises, making it difficult to isolate the effect of knee extensor fatigue on power production. Knee extensor fatigue may also induce movement variability and effect maximal activation of other lower-limb muscles. Aims: The first aim was to investigate if the level of fatigue induced by a pre-fatiguing knee extension exercise determines the level of reduction in power output during the extension and flexion phases of maximal cycling exercise. The second aim was to investigate how motor command during maximal cycling is affected by pre-fatigue of the knee extensors. Method: Ten physically active participants volunteered for this study. On one day, participants completed a 30-s maximal cycling exercise. On the second day, the same participants completed a 30-s maximal cycling exercise following a pre-fatiguing bilateral knee extension exercise. Results: Pre-fatiguing knee extension exercise decreased IMVF by -52 ± 23% (P ≤ 0.05). No association was reported between reductions in knee extensor IMVF following pre-fatiguing exercise (range = -18% to -82%, P ≤ 0.05) and reductions in leg extension power during maximal cycling (range = -8% to -31%, P ≤ 0.05) (r = 0.19). Large reductions were observed for VAS EMG (-15%) GMAX EMG (-12%) and VAS/APF co-activation (-15%) (all P ≤ 0.05). For the primary flexion phase muscles, large reductions were observed for HAM EMG (-15%), TA EMG (-15%) and RF EMG (-11%) (all P ≤ 0.05). Inter-individual variability increased for all crank forces and EMG activity for VAS, RF, HAM and TA (all P ≤ 0.05). Discussion: Overall, the results indicate that knee extensor fatigue developed during pre-fatiguing exercise does not determine reductions in power output during maximal cycling exercise. Alterations in motor command likely explain this result, evidenced via large reductions in EMG of local and non-local muscles, and increased inter-individual variability in crank forces and muscle activation patterns. Conclusion: The level of isolated fatigue observed in the knee extensors does not determine the level of reduction in leg extension power during maximal cycling exercises, presumably due to increased movement variability. Chapter 5 - study three. Introduction: Reducing the complexity of the cycling movement to a unilateral leg extension exercise would potentially reduce the degree of freedom and decrease variability in motor command. In this way, it is possible that knee extensor fatigue would determine the reduction in power output during the extension phase of maximal cycling. Knee extensor fatigue measurements are typically obtained post-exercise and within a time delay of 40-s to 5-min, although it remains unknown if such assessments provide an accurate measure of knee extensor fatigue developed during cycling exercise. Aims: The first aim was to compare the rate of fatigue occurrence in the knee extensors between an isolated knee extension exercise and a modified cycling exercise consisting of the leg extension phase only. The second aim was to investigate any differences in knee extensor fatigue measured post-exercise and the maximum time delays for which fatigue responses could be accurately assessed. Method: On separate days, 16 physically active participants completed 60 maximal knee extensions on an isokinetic dynamometer or 60 maximal leg extensions on an isokinetic cycle ergometer. A mechanical goniometer verified identical knee joint range of motion (~ 120° - 30° flexion, 0° = full extension) and angular velocity (~80°.s-1). Electrical stimulation of the femoral nerve was automated during exercise at a consistent knee joint angle of 90°. Average measures of maximal torque, M-wave and VAS EMG.M-wave-1 were calculated at the iv start (contraction 2 - 4), middle (contraction 29 - 31) and end (contraction 58-60). IMVF, VA, RT100 HZ and RT10:100 HZ were measured pre-exercise, and again at 5-s, 20-s, 40-s, 1-min, 1.5-min, 2-min, 3-min, 4-min and 5-min post-exercise Results: Intra-individual reductions in maximal torque during knee extension were positively correlated to torque reduction during leg extension exercise at the middle (-45 ± 11% vs. -23 ± 12%, r = 0.79, P ≤ 0.05) and end (-59 ± 10% vs. -37 ± 13%, r = 0.86, P ≤ 0.05). Greater reductions during knee extension were shown for RF EMG (middle: -17 ± 15% vs -2 ± 20%; end: -34 ± 16% vs. -4 ± 22%) and VAS EMG.M-wave-1 (end: -21 ± 16% vs. -14 ± 17%) (all P ≤ 0.05). IMVF reduction measured 5-s post knee extension (-32 ± 12%) partially recovered within 2-min (P > 0.05), whereas the reduction in RT100 (-28 ± 18%) and RT10:100 (-17 ± 15%) partially recovered within 20-s and 40-s, respectively (P > 0.05). Discussion: The level of fatigue developed in the knee extensors determines the reduction in maximal torque during leg extension exercise. This is likely due to the removal of the leg flexion phase and contribution of the contralateral leg during exercise. The longest time delay for accurate assessment of isometric maximal voluntary force of the knee extensors post-exercise was 1.5-min. To avoid underestimating high and low frequency peripheral fatigue, assessment must be conducted within less than 20-s and 40-s post-exercise, whereas reductions in central fatigue measurements were greatest between 20-s and 2-min post-exercise. Conclusion: Fatigue resistance in the knee extensors may predict the ability to maintain high levels of power during the extension phase of maximal cycling. Summary: Study one revealed that VAS EMG increase during high-intensity cycling exercise (6% to 14%) is associated to IMVF decrease (-2% to -36%), and that IMVF decrease is associated to reductions in maximal lower-limb power (0% to -27%) (r = 0.76). However, fatigue development was likely in other lower-limb muscles during high-intensity cycling, making it difficult to isolate the effects of knee extensor fatigue on lower-limb power production. Therefore, study two sought to isolate fatigue in the knee extensors prior to a maximal cycling effort, with the results from this study revealing no clear association between an isolated reduction in knee extensor IMVF (-18% to -82%) and reductions in maximal leg extension power (-8% to -31%) (r = 0.1), presumably due to alterations in movement control. Study three aimed compare the rate of fatigue occurrence in the knee extensors between an isolated knee extension exercise and a modified cycling exercise consisting of the leg extension phase only. The novel finding from this study was that torque reduction during knee extension was strongly associated to torque reduction v during leg extension from the start to middle (r = 0.79), middle to end (r = 0.67) and start to end periods of the exercise (r = 0.86). The results also highlighted the need to assess peripheral muscle fatigue within 20-s post-exercise, and isometric force within 1.5-min post-exercise, if accurate measures of the exercise-induced changes are to be obtained. Practical implications and importance: Collectively, the results from all studies suggest that individuals with a greater capacity to resist fatigue development in their knee extensors may have a greater capacity to maintain high levels of power during maximal cycling exercises. However, severe levels of knee extensor fatigue may lead some individuals to adopt a wider range of movement strategies to generate crank power. This information may be used by sport scientists and coaches to improve exercise prescription and performance outcomes.

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