INTRODUCTION: In addition to baseline muscle weakness, stroke survivors often demonstrate increased neuromuscular fatigability (an acute exercise-induced reduction in power, hereafter referred to as fatigability), which limits endurance during functional tasks. Ischemic conditioning (IC) is a non-invasive and easy-to-administer intervention that can reduce fatigability in stroke survivors during a sustained isometric contraction of their paretic knee extensors. However, IC’s effects on fatigability during dynamic contractions are unknown, despite the greater relevance to everyday activities. Additionally, fatigability is known to be task specific. Thus, the purpose of this study was to quantify the effects of IC on fatigability (percent reduction in power) during dynamic contractions of the paretic knee extensors in stroke survivors. We hypothesized that fatigability would be less after a single session of IC versus IC-sham treatment in stroke survivors. Methods: Eight stroke survivors (66 ± 6 years of age, 6 male) performed 60 maximal voluntary concentric contractions (1 every 4 seconds) with their paretic knee extensors against a resistance of 30% of their maximal voluntary isometric contraction torque through a 60-degree range of motion after receiving a single session of IC and IC-sham treatment on two separate days. IC consisted of 5 sets of 5-minute upper thigh blood flow occlusion at 225 mmHg (25 mmHg for IC-sham) with 5 minutes of no occlusion between each set. The order of the 2 sessions was randomized. All participants visited the laboratory on a different day for functional measurements (Fugl-Meyer Assessment, 10-meter walk test, and 6-minute walk test). Reduction in power was calculated as the difference between the highest average power output from 5 consecutive contractions within the first 15 contractions and the average power output from the last 5 contractions. A paired t-test was used to compare if fatigability during dynamic exercise would differ between the 2 sessions after IC and IC-sham treatment. Pearson correlation was performed to determine the relationship between the difference in fatigability (after IC versus IC-sham treatment) and functional measurements. Results: After a single session of IC treatment, stroke survivors were less fatigable compared to IC-sham treatment (21.9 ± 19.2% power reduction vs. 29.4 ± 19.0%, p = 0.016). Improvement in fatigability after IC versus IC-sham treatment was positively correlated with Fugl-Meyer Assessment lower extremity motor score ( r = 0.82, p = 0.013). CONCLUSION: These preliminary results indicate that IC could serve as a potential intervention to reduce fatigability during dynamic exercise in stroke survivors, especially those who are mildly affected by stroke. Funding Source: American Heart Association Predoctoral Fellowship (903373, ZZ) and Eunice Kennedy Shriver National Institute of Child Health and Human Development R01 Grant (HD099340, AH & MD). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

ABSTRACT The mechanisms for the loss in limb muscle power in old (60-79 years) and very old (≥80 years) adults and whether the mechanisms differ between men and women are not well-understood. We compared maximal power of the knee extensor muscles between young, old, and very old men and women and identified the neural and muscular factors contributing to the age-related loss of power. 31 young (22.9±3.0 years, 15 women), 83 old (70.4±4.9 years, 39 women), and 16 very old adults (85.8±4.2 years, 9 women) performed maximal isokinetic contractions at 14 different velocities (30- 450°/s) to identify peak power. Voluntary activation (VA) and contractile properties were assessed with transcranial magnetic stimulation to the motor cortex and electrical stimulation of the femoral nerve. The age-related loss in power was ∼6.5 W·year -1 for men ( R 2 =0.62, p <0.001), which was a greater rate of decline ( p =0.002) than the ∼4.2 W·year -1 for women ( R 2 =0.77, p <0.001). Contractile properties were the most closely associated variables with power output for both sexes, such as the rate of torque development of the potentiated twitch (men: R 2 =0.69, p <0.001; women: R 2 =0.57, p <0.001). VA was weakly associated with power in women ( R 2 =0.13, p =0.012) but not men ( p =0.191), whereas neuromuscular activation (EMG amplitude) during the maximal power contraction was not associated with power in men ( p =0.347) or women ( p =0.106). These data suggest that the age-related loss in power of the knee extensor muscles is due primarily to factors within the muscle for both sexes, although neural factors may play a minor role in older women. NEW & NOTEWORTHY The accelerated age-related loss in power relative to the loss in muscle mass of the knee extensors was primarily due to factors altering the contractile properties of the muscle for both old and very old (≥80 yr) adults. The mechanisms for the decrements in power with aging appear largely similar for men and women, although neural factors may play more of a role in older women.

Identifying measures which accurately quantify reactive balance adaptation during walking is essential to understand how emerging perturbation-based gait paradigms impact stability over the course of an intervention. These perturbation paradigms have shown promise in reducing falls for numerous clinical populations, however tracking progress in objective terms throughout an intervention remains challenging. Whole body angular momentum (H) may be particularly suited to detect subtle adaptations in the reactive balance response and is applicable within numerous perturbation environments. We assessed the ability of young healthy adults to adapt to varying intensities of discrete, unexpected, treadmill-based perturbations directed mediolaterally, anteriorly, and posteriorly during a single session while ambulating at their comfortable walking speed. We assessed corrective step length and width, trunk deviation and flexion, peak H over a stride, peak-to-peak differences in whole-body angular momentum over a stride (HR), and the participants ability to maintain their H trajectory within two standard deviations of their normal (PNT). Measures derived from H, particularly HR and PNT, demonstrated significant changes with increasing intensity and repetition. Corrective step length and width, trunk deviation and flexion, and peak H also demonstrated significant, but weaker, differences with increasing intensity and repetition. Derivatives of H are sensitive to changes in intensity and repetition, particularly when assessed as peak-to-peak differences and ability to maintain a normal trajectory over a stride. These measures may be utilized to detect changes in reactive balance during perturbation-based gait paradigms

The mechanisms for the loss in limb muscle power output in old (60-79 years) and very old adults (≥80 years) and whether these mechanisms differ between older men and women are not well-understood. PURPOSE: Compare peak power output of the knee extensor muscles between young, old, and very old adults and determine the physiological mechanisms for the age-related loss of power in men compared with women. METHODS: 31 young (22.9 ± 3.0 years, 16 men), 83 old (70.4 ± 4.9 years, 44 men), and 16 very old adults (85.8 ± 4.2 years, 7 men) performed maximal isokinetic contractions at 15 different velocities (0 - 450°/s) to identify peak power output across the torque-velocity relationship. Voluntary activation (VA) and contractile properties were assessed with transcranial magnetic stimulation and electrical stimulation of the femoral nerve. RESULTS: Old and very old men and women generated less power than young across all velocities (p < 0.01). Compared with young men (673 ± 41 W), peak power output was ~42% lower in old (399 ± 18 W, p < 0.01) and ~ 67% lower in very old (225 ± 19 W, p < 0.01). For women, peak power output was ~49% lower in old (223 ± 9 W, p < 0.01) and ~ 60% lower in very old (170 ± 15 W) compared with young (428 ± 20 W, p < 0.01). This corresponded with an age-related loss in peak power output of ~6.5 W/year for men (R2 = 0.62, p < 0.01) and ~ 4.2 W/year for women (R2 = 0.77, p < 0.01). Factors within the muscle were the most closely associated variables with peak power for both sexes including; thigh lean tissue mass (men: R2 = 0.53, p < 0.01; women: R2 = 0.34, p < 0.01), potentiated twitch amplitude (men: R2 = 0.60, p < 0.01; women: R2 = 0.55, p < 0.01), and rate of torque development of the potentiated twitch (men: R2 = 0.60, p < 0.01; women: R2 = 0.48, p < 0.01). In contrast, VA was weakly associated with peak power output for women (R2 = 0.13, p = 0.01) but not for men (R2 = 0.03, p = 0.18), and muscle activation (relative EMG amplitude) of the vastus lateralis during the maximal power contraction was not associated with peak power output for either men (R2 = 0.01, p = 0.35) or women (R2 = 0.04, p = 0.11). CONCLUSION: These data suggest that the age-related loss in power of the knee extensor muscles is due primarily to factors within the muscle for both men and women, and that impaired neural activation may play a minor role in the loss in power output for women.

Aging is associated with a reduced ability of limb muscles to generate force and power that can be exacerbated during dynamic contractions by the age-related increase in fatigability (acute activity induced reduction in torque or power). The mechanisms for the increased age-related decrease in power and fatigability in upper extremity muscles are not well studied. PURPOSE: Compare between young and old males and females 1) torque and power through a range of isokinetic velocities, and 2) mechanisms of fatigability induced by high-velocity dynamic contractions of the elbow flexor muscles. METHODS: 28 young (23.2 ± 2.6 years, 14 men) and 33 old adults (72.6 ± 5.6 years, 18 men) performed: 1) maximal isokinetic contractions at 15 velocities (0 - 450°/s) to assess power output across the torque-velocity curve, and 2) a dynamic fatiguing task involving 80 maximal-effort isotonic contractions with a load equivalent to 20% of maximal voluntary isometric torque. Voluntary activation (VA) and contractile properties were assessed with transcranial magnetic stimulation and electrical stimulation before and immediately following the fatiguing exercise. RESULTS: Elbow flexor muscles of old adults were weaker and less powerful than young adults across all velocities (P < 0.01). There were no age differences in VA before the fatiguing task. The reductions in power during the dynamic fatiguing exercise were greater in old compared with young adults (15% difference, P < 0.001), with no sex differences (P > 0.05). Old adults had larger reductions in VA (7.5% age difference, P = 0.036) and greater increases in twitch relaxation time (55%) than young (36%, P = 0.021) following the fatiguing task. CONCLUSIONS: Elbow flexor muscles of old men and women were weaker and less powerful than young, but was not due to differences in VA. In contrast to previous findings on the lower limb, the age-related increase in fatigability of the elbow flexors was due to both neural and muscular mechanisms.