ECSS Paris 2023: OP-BM07
INTRODUCTION: Aging is accompanied by a progressive decline in skeletal muscle mass and function that leads to impaired mobility of older adults. Importantly, the ability to produce power is one of the strongest predictors of physical function with aging, and power declines at a greater rate than the loss in mass (~2-4%/yr vs. ~0.5-1%/yr) (1-2). These observations suggest factors such as impaired neural activation, altered intrinsic contractile properties, and/or the selective atrophy of the myosin heavy chain (MyHC) II muscle fibers may contribute to the more rapid decline in power relative to muscle mass. Thus, the aim of this study was to determine the primary factors responsible for the age-related decline in power. METHODS: 10 young (23±3 yrs, 5 women) and 13 older adults (72±5 yrs, 7 women) performed maximal velocity contractions lifting a load that was 20% of the maximal voluntary isometric contraction to assess power of the knee extensors. Voluntary activation and contractile properties were measured with electrical stimulation. Biopsies were obtained from the vastus lateralis to assess fiber type distribution and proportional area with immunohistochemistry. Knee extensor muscle volume was quantified with MRI, and the fiber type specific volume was estimated as the product of fiber type proportional area and volume. RESULTS: Power was ~46% lower in older (120±37W) compared to young (223±73W; p=0.001), while muscle volume was ~34% lower with age (1692±625 vs. 1123±302cm3; p=0.021). When power was normalized to muscle volume, specific power in older adults remained ~23% lower than young (p<0.001). Voluntary activation was near maximal in young (92±7%) and older adults (93±6%) and did not differ with age (p=0.95). In contrast, the involuntary electrically-evoked twitch (Qtw) was ~36% lower in older (49±14Nm) compared to young (76±24Nm) and closely associated with power (R2=0.91; p<0.001). MyHC I proportional area was greater in older adults (62±15% vs. 40±12%, p=.001), and MyHC II proportional area was lower with age (39±16% vs. 60±12%; p=0.002). The estimated volume of muscle composed of MyHC I did not differ with age (p=0.840), but MyHC II muscle volume was ~56% lower in older (458±242cm3) than young (1033±500cm3; p=0.006). Accordingly, there were strong associations between the estimated MyHC II muscle volume and both power (R2=0.77; p<0.001) and Qtw (R2=0.78; p<0.001). In contrast, there were only weak associations with MyHC I muscle volume and both power (R2=0.21; p=0.029) and Qtw (R2=0.24; p=0.019). CONCLUSION: These preliminary data suggest that the greater age-related loss in power relative to muscle mass of the knee extensors is determined, at least in part, by the selective atrophy and/or loss of MyHC II fibers and that interventions should target fast fiber atrophy to help attenuate the loss of power with age. References 1. Alcazar J et al. 2020 2. Petrella J et al. 2004 Funding: Supported by a NIH fellowship (2TL1 TR001437) to JJJ and RO1 grant (AG048262) to CWS and SKH.
Read CV Jessica JamesECSS Paris 2023: OP-BM07
INTRODUCTION: Aging is associated with a progressive decline in neuromuscular function, partly due to neural alterations, particularly affecting motor unit (MU) function and properties. In contrast, resistance training can enhance muscle force production and control by partially reversing age-related neural alterations at the MU level. Previous evidences in young and older adults have shown that short-term resistance training (< 6 weeks) increases MU discharge rate (MU DR), in turn mediating the early force gains. However, the time course of MU adaptations to longer resistance training interventions remains elusive. This study aimed to investigate and compare the effects of an eight-week resistance training intervention on MU properties in young (YA) and older adults (OA). METHODS: Eleven OA (age: 71.4±5.2 yr) and twelve YA (age: 23±2.4 yr) participated in an 8-week dynamic and progressive resistance training protocol. Measurements were taken at baseline (T0), after 4 weeks (T4), and at the end of the intervention (T8). During these sessions, participants performed maximal (MVF) isometric knee extensions and submaximal trapezoidal ramps (35, 50, 70% MVF). Simultaneously, high-density surface EMG signals muscle were recorded from vastus lateralis muscle and decomposed offline into individual MU spike trains. Training-induced changes in MU properties, including recruitment (MU RT), derecruitment (MU DERT) thresholds and average discharge rates (MU DR), were analyzed with generalized mixed-effects models. Repeated measure correlations were performed to assess the association between changes in MU DR and MVF. RESULTS: Preliminary results revealed that, after the training protocol, MVF increased similarly in both YA and OA (time effect, p < 0.001). Specifically, MVF increased by ~10% between T0-T4 (p < 0.001), by ~9% between T4-T8 (p < 0.001), and by ~20% between T0 -T8 (p < 0.001). Similarly, MU DR increased after the training protocol (time effect, p < 0.001), but with a different time-course between OA and YA (time by group effect, p = 0.008). Specifically, MU DR increased between T0-T4 in both OA (p = 0.039) and YA (p = 0.002), but between T0-T8 in YA (p < 0.001) only. Interestingly, the initial changes (T0-T4) in MU DR were moderately associated with changes in MVF (rrm = 0.51, 95% CI [0.126, 0.763], p = 0.013). Conversely, no changes were observed in normalized MU RT (% MVF) and MU DERT after the training. CONCLUSION: Eight weeks of resistance training increased MVF in both OA and YA, with comparable MU adaptations particularly during the first four weeks of training. Notably, the observed association between changes in MU DR and MVF emphasizes the similar early (< 4 weeks) adaptability of the nervous system to resistance training in OA and YA. However, the relative contribution of neural adaptations to muscle force production may diverge between OA and YA in the subsequent weeks of training.
Read CV Andrea CasoloECSS Paris 2023: OP-BM07
INTRODUCTION: Bed rest represents a well-accepted model to study the effects of prolonged periods of muscle disuse due to injury, surgery, illness, hospitalization or spaceflight. Muscle disuse is primarily associated with a reduction in muscle mass and strength and, in addition to functional alterations of the peripheral nervous system, changes in central activation have been suggested to play an important role in the loss of muscle strength. We recently showed that persistent inward current (PIC), an intrinsic motoneuron property proportional to monoaminergic neuromodulatory input, is reduced after 10 days of lower limb suspension (1). However, our previous study did not consider that the decreased maximal voluntary contraction (MVC) after disuse leads to changes in the assessed torque profiles across time points, introducing a confounding factor in estimating changes in PICs. This study aims to assess PIC changes after prolonged bed rest followed by supervised recovery in young and older adults at matched torque profiles across time points. METHODS: Nine young (YA, age 22±4) and 10 older (OA, 69±3) healthy males underwent 21 (BR21) and 10 days (BR10) of bed rest, respectively, followed by 21 days of endurance training (ET). High density surface electromyography (HDsEMG) was recorded from vastus lateralis muscle during isometric triangular contractions at 20% of pre bed rest (BR0) MVC. After decomposing motor unit (MU) firing rates from HDsEMG signals, PICs were estimated via paired motor unit (MU) technique (2) as the delta frequency (∆F) of a lower threshold MU (control unit) at the time of recruitment and derecruitment of a higher threshold MU (test unit). Individual ΔF differences across time points were assessed using linear mixed models. RESULTS: A total of 234 (58 at BR0, 74 at BR21 and 102 at RT) and 373 (111, 126 and 136) MUs were successfully decomposed from HDEMG signals for YA and OA groups, respectively. From the total MU pool, we identified 251 (37 at BR0, 51 at BR21 and 70 at RT) test units in YA and 284 (82, 98 and 104) test units in OA. In YA, ΔF values decreased at BR21 compared to BR0 (p<0.001) and were restored after RT compared to BR21 (p<0.001). Similarly, in OA ΔF values were lower at BR10 compared to BR0 (p<0.002) and RT (p<0.03). CONCLUSION: This is the first study assessing the neuromodulatory contribution to force generation after prolonged bed rest in young and older individuals. Remarkably, even adopting the same torque profiles for the assessment, we found changes in PIC estimates across time points. These results further highlight the important role of neuromodulation in muscle force generating capacity following muscle disuse and its recovery through physical activity. This study was cofinanced by the ARIS (project J5-4593 and programme P2-0041), the ASI (n.2024-5-E.0 - CUP n.I53D24000060005) and the MUR (PRIN 2022 PNRR, ‘ReActiveAge’, project P2022FNCPR). REFERENCES: 1. Martino et al. MSSE 2024 2. Gorassini et al. J Neurophysiol 2002
Read CV Giovanni MartinoECSS Paris 2023: OP-BM07