Neuromuscular Fatigability Associated With Different Pacing Strategies During an Ultra-Endurance Pull-Up Task: A Case Study

International Journal of Exercise Science 15(3): 1514-1527, 2022. While neuromuscular fatigability has been previously characterized after running and cycling, no study has investigated an ultra-endurance upper body task. In preparation for a world record attempt, three pacing strategies to perform 1980 pull-ups in 6 hrs were compared during independent sessions: fast pace, long recovery (FL), fast pace, multiple short recoveries (FMS), and slow pace, no recovery (SN). Elbow flexion maximal voluntary contraction (MVC) force, grip strength, peripheral fatigue, and biceps brachii electromyography were quantified every 330 pull-ups and during recovery, alongside heart rate, perceived effort, and arm muscle pain. In all conditions, MVC force decreased rapidly within the first set of 330 pull-ups, with the greatest depression observed in FL (-29.1%) and more gradual declines in FMS (-18.6%) and SN (-8.6%). Similarly, FL displayed the greatest decline in potentiated single twitch (FL: -75.0%; FMS: -53.9%; SN: -41.8%) and high-frequency doublet forces (FL: -63.3%; FMS: -29.2%; SN: -41.8%) following the first set, as well as higher heart rate, effort, and pain throughout the task. Following 24 hrs, MVC force recovered slowest in FL and grip strength recovered fastest in SN. Therefore, for the world record attempt, a strategy with a continuous workload at slower pace should be used

Tibial Strains are Sensitive to Speed, but not Grade, Perturbations During Running

Tibial stress fractures are thought to result from a fatigue-failure process where bone failure is highly dependent on peak strain magnitude. Little is known regarding the mechanical loading environment of the tibia during graded running despite the prevalence of this terrain. To probe the sensitivity of the mechanical loading environment of the tibia to running grade, tibial strains were quantified using a combined musculoskeletal-finite element modeling routine during graded and level running. Seventeen participants ran on a treadmill at $\pm$10{\deg}, $\pm$5{\deg}, and 0{\deg} while force and motion data were captured. At each grade, participants ran at 3.33 m/s and a grade-adjusted speed, that was 2.20 m/s and 4.17 m/s for uphill and downhill conditions, respectively. Muscle and joint contact forces were estimated using inverse-dynamics-based static optimization. These forces were applied to a participant-informed finite element model of the tibia. 50th percentile pressure-modified von Mises strain was lower ($\leq$-130 $\mu\varepsilon$) during downhill running compared to level and uphill running at 3.33 m/s. However, neither 95th percentile strain (peak strain) nor the volume of bone experiencing strains $\geq$4000 $\mu\varepsilon$ (strained volume) were different between grades (F(4)$\leq$3.28, p$\geq$0.01). In contrast, peak strain and strained volume were highly sensitive to running speed (F(1)$\geq$10.61, p$\leq$0.001), where a 1 m/s increase in speed resulting in a 9 % and 155 % increase in peak strain and strained volume, respectively. Overall, these findings suggest that faster running speeds, but not changes in running grade, may increase the risk of developing a tibial stress fracture

Use of transcranial magnetic stimulation to assess relaxation rates in unfatigued and fatigued knee-extensor muscles

We examined whether transcranial magnetic stimulation (TMS) delivered to the motor cortex 13 allows assessment of muscle relaxation rates in unfatigued and fatigued knee extensors (KE). 14 We assessed the ability of this technique to measure time course of fatigue-induced changes 15 in muscle relaxation rate and compared relaxation rate from resting twitches evoked by 16 femoral nerve stimulation. Twelve healthy men performed maximal voluntary isometric 17 contractions (MVC) twice before (PRE) and once at the end of a 2-min KE MVC and five 18 more times within 8 min during recovery. Relative (intraclass correlation coefficient; ICC2,1) 19 and absolute (repeatability coefficient) reliability and variability (coefficient of variation) 20 were assessed. Time course of fatigue-induced changes in muscle relaxation rate was tested 21 with generalized estimating equations. In unfatigued KE, peak relaxation rate coefficient of 22 variation and repeatability coefficient were similar for both techniques. Mean (95% CI) 23 ICC2,1 for peak relaxation rates were [0.933 (0.724-0.982)] and [0.889 (0.603-0.968)] for 24 TMS and femoral nerve stimulation, respectively. TMS-induced normalized muscle 25 relaxation rate was -11.5 ± 2.5 s-1 at PRE, decreased to -6.9 ± 1.2 s-1 (-37 ± 17%, P < 0.001), 26 and recovered by 2 min post-exercise. Normalized peak relaxation rate for resting twitch did 27 not show a fatigue-induced change. During fatiguing KE exercise, the change in muscle 28 relaxation rate as determined by the two techniques was different. TMS provides reliable 29 values of muscle relaxation rates. Furthermore, it is sufficiently sensitive and more 30 appropriate than the resting twitch evoked by femoral nerve stimulation to reveal fatigue-31 induced changes in KE

Repeated bout effect and musculoskeletal loading during prolonged downhill running

Running is one of the most common forms of exercise to maintain physical activity and health. Despite decades of research in the field of running biomechanics, the rate of running-related injuries remains high. A vast majority of studies investigating running biomechanics have focused on level running. However, recreational running on urban and rural terrains frequently consists of uphill and downhill running. Owing to high eccentric muscular contraction, downhill running is known to induce muscle damage and symptoms of delayed onset muscle soreness that is generally attenuated during and after a subsequent downhill running bout; a phenomenon known as the repeated bout effect. The primary objective of this thesis was to understand the physiological and biomechanical consequences of an unaccustomed eccentric-biased downhill running bout as well as how the repeated bout effect mediates these consequences. A series of studies were conducted using a model of two prolonged downhill running bouts separated by three weeks. We observed that an unaccustomed downhill run caused substantial neuromuscular fatigue (i.e., central and peripheral fatigue) that persisted up to 48 hours after the initial bout. A repeated bout effect manifested as less sever neuromuscular fatigue following the second downhill run, which was likely due to neural adaptation (i.e., less central fatigue). A repeated bout effect was also observed for downhill running biomechanics, where changes in duty factor and knee quasi-stiffness were attenuated over the course of the second bout compared to the first bout. Changes to bone strain at the lower-extremity over the course of the downhill run were then estimated using combined musculoskeletal-finite element modeling. We observed that the neuromuscular fatigue associated with prolonged downhill running did not impact tibial-fibular strains. The findings from this thesis provide new and important insight to our current understanding of the repeated bout effect in unaccustomed eccentric-biased downhill running as well as the influence of neuromuscular fatigue on bone strain during a prolonged downhill run

Tibial-fibular geometry and density variations associated with elevated bone strain and sex disparities in young active adults

Tibial stress fracture is a common injury in runners and military personnel. Elevated bone strain is believed to be associated with the development of stress fractures and is influenced by bone geometry and density. The purpose of this study was to characterize tibial-fibular geometry and density variations in young active adults, and to quantify the influence of these variations on finite element-predicted bone strain. A statistical appearance model characterising tibial-fibular geometry and density was developed from computed tomography scans of 48 young physically active adults. The model was perturbed ±1 and 2 standard deviations along each of the first five principal components to create finite element models. Average male and female finite element models, controlled for scale, were also generated. Muscle and joint forces in running, calculated using inverse dynamics-based static optimization, were applied to the finite element models. The resulting 95th percentile pressure-modified von Mises strain (peak strain) and strained volume (volume of elements above 4000 με) were quantified. Geometry and density variations described by principal components resulted in up to 12.0% differences in peak strain and 95.4% differences in strained volume when compared to the average tibia-fibula model. The average female illustrated 5.5% and 41.3% larger peak strain and strained volume, respectively, when compared to the average male, suggesting that sexual dimorphism in bone geometry may indeed contribute to greater stress fracture risk in females. Our findings identified important features in subject-specific geometry and density associated with elevated bone strain that may have implications for stress fracture risk.Natural Sciences and Engineering Research Council (NSERC