Effects of old age and contraction mode on knee extensor muscle ATP flux and metabolic economy in vivo

Walking Economy and Preferred Speed in Old and Very Old Men

PURPOSE

With aging, the decline in preferred walking speed (PWS), influenced by the increased energy cost of walking (CoW), is a key predictor of morbidity. However, the determinants associated with PWS and CoW remain poorly understood, especially after 80 yr old. The aim of the study was to characterize the amplitude and mechanisms of age-related decline in CoW and PWS in old (OM) and very old (VOM) men.

METHODS

Thirty-nine young men (YM; 22.1 ± 3.4 yr), 34 OM (71.7 ± 4.1 yr), and 23 VOM (85.8 ± 2.7 yr) performed aerobic, neuromuscular, and gait assessments. Net CoW was measured on a treadmill. Physical activity (PA) was evaluated by questionnaire and accelerometry.

RESULTS

Net CoW was 32% ( P < 0.001), 19% ( P < 0.01), and 26% ( P < 0.001) higher in VOM compared with OM for 1.11 m·s -1 , 1.67 m·s -1 , and PWS. Net CoW was also 27% ( P < 0.001), 26% ( P < 0.01), and 29% ( P < 0.001) higher in OM compared with YM at these speeds. Linear regression stratified by age showed that net CoW at PWS was associated with step frequency ( r = 0.79; P < 0.001) for OM and with both coefficient of variation of stride mean time ( r = 0.48; P < 0.05) and maximal strength of knee extensors ( r = -0.54; P < 0.05) for VOM. The same analysis revealed that PWS was correlated with net CoW ( r = -0.56; P < 0.05) and PA ( r = 0.47; P < 0.05) in VOM.

CONCLUSIONS

The progressive increase in net CoW with age was associated with gait and neuromuscular impairments, particularly after the age of 80 yr. This increase in net CoW was related to a decrease in PWS in VOM, suggesting an adaptation of PWS to compensate for the increase in energy demand. Maintaining a high level of PA may potentially delay the age-related decline in PWS despite an age-related increase in net CoW.

Skeletal muscle inosine monophosphate formation preserves ΔGATP during incremental step contractions in vivo

The cause and consequences of inosine monophosphate (IMP) formation when adenosine triphosphate (ATP) declines during muscular contractions in vivo are not fully understood. The purpose of this study was to examine the role of IMP formation in the maintenance of the Gibbs free energy for ATP hydrolysis (ΔGATP) during dynamic contractions of increasing workload and the implications of ATP loss in vivo. Eight males (median 27.5, 25-35 yr range) completed an 8-min incremental protocol [2-min stages of isotonic knee extensions (0.5 Hz)] in a 3-T magnetic resonance (MR) system. Phosphorus MR spectra were obtained from the knee extensor muscles at rest and during contractions and recovery. Although the ATP demand during contractions was met primarily by oxidative phosphorylation, [ATP] decreased from 8.2 mM to 7.5 (range 6.4-8.0) mM and [IMP] increased from 0 mM to 0.6 (0.1-1.7) mM. Modeling showed that, in the absence of IMP formation, excess adenosine diphosphate (ADP) would result in a less favorable ΔGATP (P < 0.001). Neither [ATP] nor [IMP] had returned to baseline following 10 min of recovery (P < 0.001). Notably, Δ[ATP] was linearly related to the post-contraction reduction in muscle oxidative capacity (r = 0.74, P = 0.037). Our results highlight the importance of IMP formation in preserving cellular energy status by avoiding increases in ADP above that necessary to stimulate energy production pathways. However, the consequence of IMP formation was an incomplete recovery of [ATP], which in turn was related to decreased muscle oxidative capacity following contractions. These results likely have implications for the capacity to generate adequate energy during repeated bouts of muscular work.NEW & NOTEWORTHY An ∼9% decline in [ATP] led to the formation of inosine monophosphate (IMP) during submaximal muscular contractions. Modeling revealed IMP formed to preserve a favorable energy state (ΔGATP) by minimizing large increases in [ADP], whereas the loss of [ATP] did not alter ΔGATP. [ATP] did not recover by 10 min, and the loss of [ATP] was associated with a reduced oxidative capacity, providing a new link between [ATP] loss and an impaired energetic capacity in vivo.

Effects of older age on contraction-induced intramyocellular acidosis and inorganic phosphate accumulation in vivo: A systematic review and meta-analysis

Although it is clear that the bioenergetic basis of skeletal muscle fatigue (transient decrease in peak torque or power in response to contraction) involves intramyocellular acidosis (decreased pH) and accumulation of inorganic phosphate (Pi) in response to the increased energy demand of contractions, the effects of old age on the build-up of these metabolites has not been evaluated systematically. The purpose of this study was to compare pH and [Pi] in young (18-45 yr) and older (55+ yr) human skeletal muscle in vivo at the end of standardized contraction protocols. Full study details were prospectively registered on PROSPERO (CRD42022348972). PubMed, Web of Science, and SPORTDiscus databases were systematically searched and returned 12 articles that fit the inclusion criteria for the meta-analysis. Participant characteristics, contraction mode (isometric, dynamic), and final pH and [Pi] were extracted. A random-effects model was used to calculate the mean difference (MD) and 95% confidence interval (CI) for pH and [Pi] across age groups. A subgroup analysis for contraction mode was also performed. Young muscle acidified more than older muscle (MD = -0.12 pH; 95%CI = -0.18,-0.06; p<0.01). There was no overall difference by age in final [Pi] (MD = 2.14 mM; 95%CI = -0.29,4.57; p = 0.08), although sensitivity analysis revealed that removing one study resulted in greater [Pi] in young than older muscle (MD = 3.24 mM; 95%CI = 1.72,4.76; p<0.01). Contraction mode moderated these effects (p = 0.02) such that young muscle acidified (MD = -0.19 pH; 95%CI = -0.27,-0.11; p<0.01) and accumulated Pi (MD = 4.69 mM; 95%CI = 2.79,6.59; p<0.01) more than older muscle during isometric, but not dynamic, contractions. The smaller energetic perturbation in older muscle indicated by these analyses is consistent with its relatively greater use of oxidative energy production. During dynamic contractions, elimination of this greater reliance on oxidative energy production and consequently lower metabolite accumulations in older muscle may be important for understanding task-specific, age-related differences in fatigue.

Measurements of in vivo skeletal muscle oxidative capacity are lower following sustained isometric compared with dynamic contractions

Human skeletal muscle oxidative capacity can be quantified non-invasively using 31-phosphorus magnetic resonance spectroscopy (31P-MRS) to measure the rate constant of phosphocreatine (PCr) recovery (kPCr) following contractions. In the quadricep muscles, several studies have quantified kPCr following 24-30 s of sustained maximal voluntary isometric contraction (MVIC). This approach has the advantage of simplicity but is potentially problematic because sustained MVICs inhibit perfusion, which may limit muscle oxygen availability or increase the intracellular metabolic perturbation, and thus affect kPCr. Alternatively, dynamic contractions allow reperfusion between contractions, which may avoid limitations in oxygen delivery. To determine whether dynamic contraction protocols elicit greater kPCr than sustained MVIC protocols, we used a cross-sectional design to compare quadriceps kPCr in 22 young and 11 older healthy adults following 24 s of maximal voluntary: (1) sustained MVIC and (2) dynamic (MVDC; 120°·s-1, 1 every 2 s) contractions. Muscle kPCr was ∼20% lower following the MVIC protocol compared with the MVDC protocol (p ≤ 0.001), though this was less evident in older adults (p = 0.073). Changes in skeletal muscle pH (p ≤ 0.001) and PME accumulation (p ≤ 0.001) were greater following the sustained MVIC protocol, and pH (p ≤ 0.001) and PME (p ≤ 0.001) recovery were slower. These results demonstrate that (i) a brief, sustained MVIC yields a lower value for skeletal muscle oxidative capacity than an MVDC protocol of similar duration and (ii) this difference may not be consistent across populations (e.g., young vs. old). Thus, the potential effect of contraction protocol on comparisons of kPCr in different study groups requires careful consideration in the future.

Postcontraction [acetylcarnitine] reflects interindividual variation in skeletal muscle ATP production patterns in vivo

In addition to its role in substrate selection (carbohydrate vs. fat) for oxidative metabolism in muscle, acetylcarnitine production may be an important modulator of the energetic pathway by which ATP is produced. A combination of noninvasive magnetic resonance spectroscopy measures of cytosolic acetylcarnitine and ATP production pathways was used to investigate the link between [acetylcarnitine] and energy production in vivo. Intracellular metabolites were measured in the vastus lateralis muscle of eight males (mean: 28.4 yr, range: 25-35) during 8 min of incremental, dynamic contractions (0.5 Hz, 2-min stages at 6%, 9%, 12%, and 15% maximal torque) that increased [acetylcarnitine] approximately fivefold from resting levels. ATP production via oxidative phosphorylation, glycolysis, and the creatine kinase reaction was calculated based on phosphorus metabolites and pH. Spearman rank correlations indicated that postcontraction [acetylcarnitine] was positively associated with both absolute (mM) and relative (% total ATP) glycolytic ATP production (rs = 0.95, P = 0.001; rs = 0.93, P = 0.002), and negatively associated with relative (rs = -0.81, P = 0.02) but not absolute (rs = -0.14, P = 0.75) oxidative ATP production. Thus, acetylcarnitine accumulated more when there was a greater reliance on "nonoxidative" glycolysis and a relatively lower contribution from oxidative phosphorylation, reflecting the fate of pyruvate in working skeletal muscle. Furthermore, these data indicate striking interindividual variation in responses to the energy demand of submaximal contractions. Overall, the results of this preliminary study provide novel evidence of the coupling in vivo between ATP production pathways and the carnitine system.NEW & NOTEWORTHY Production of acetylcarnitine from acetyl-CoA and free carnitine may be important for energy pathway regulation in contracting skeletal muscle. Noninvasive magnetic resonance spectroscopy was used to investigate the link between acetylcarnitine and energy production in the vastus lateralis muscle during dynamic contractions (n = 8 individuals). A positive correlation between acetylcarnitine accumulation and "nonoxidative" glycolysis and an inverse relationship with oxidative phosphorylation, provides novel evidence of the coupling between ATP production and the carnitine system in vivo.

Muscle fatigue, bioenergetic responses and metabolic economy during load- and velocity-based maximal dynamic contractions in young and older adults

We evaluated whether task-dependent, age-related differences in muscle fatigue (contraction-induced decline in normalized power) develop from differences in bioenergetics or metabolic economy (ME; mass-normalized work/mM ATP). We used magnetic resonance spectroscopy to quantify intracellular metabolites in vastus lateralis muscle of 10 young and 10 older adults during two maximal-effort, 4-min isotonic (20% maximal torque) and isokinetic (120°s-1 ) contraction protocols. Fatigue, inorganic phosphate (Pi), and pH (p ≥ 0.213) differed by age during isotonic contractions. However, older had less fatigue (p ≤ 0.011) and metabolic perturbation (lower [Pi], greater pH; p ≤ 0.031) than young during isokinetic contractions. ME was lower in older than young during isotonic contractions (p ≤ 0.003), but not associated with fatigue in either protocol or group. Rather, fatigue during both tasks was linearly related to changes in [H+ ], in both groups. The slope of fatigue versus [H+ ] was 50% lower in older than young during isokinetic contractions (p ≤ 0.023), consistent with less fatigue in older during this protocol. Overall, regardless of age or task type, acidosis, but not ME, was the primary mechanism for fatigue in vivo. The source of the age-related differences in contraction-induced acidosis in vivo remains to be determined, as does the apparent task-dependent difference in the sensitivity of muscle to [H+ ].

Age-related changes in gait biomechanics and their impact on the metabolic cost of walking: Report from a National Institute on Aging workshop

Changes in old age that contribute to the complex issue of an increased metabolic cost of walking (mass-specific energy cost per unit distance traveled) in older adults appear to center at least in part on changes in gait biomechanics. However, age-related changes in energy metabolism, neuromuscular function and connective tissue properties also likely contribute to this problem, of which the consequences are poor mobility and increased risk of inactivity-related disease and disability. The U.S. National Institute on Aging convened a workshop in September 2021 with an interdisciplinary group of scientists to address the gaps in research related to the mechanisms and consequences of changes in mobility in old age. The goal of the workshop was to identify promising ways to move the field forward toward improving gait performance, decreasing energy cost, and enhancing mobility for older adults. This report summarizes the workshop and brings multidisciplinary insight into the known and potential causes and consequences of age-related changes in gait biomechanics. We highlight how gait mechanics and energy cost change with aging, the potential neuromuscular mechanisms and role of connective tissue in these changes, and cutting-edge interventions and technologies that may be used to measure and improve gait and mobility in older adults. Key gaps in the literature that warrant targeted research in the future are identified and discussed.

Age-related performance fatigability: a comprehensive review of dynamic tasks

Adult ageing is associated with a myriad of changes within the neuromuscular system, leading to reductions in contractile function of old adults. One of the consequences of these age-related neuromuscular adaptations is altered performance fatigability, which can limit the ability of old adults to perform activities of daily living. Whereas age-related fatigability of isometric tasks has been well characterized, considerably less is known about fatigability of old adults during dynamic tasks involving movement about a joint, which provides a more functionally relevant task compared to static contractions. This review provides a comprehensive summary of age-related fatigability in dynamic contractions, where the importance of task specificity is highlighted with a brief discussion of the potential mechanisms responsible for differences in fatigability between young and old adults. The angular velocity of the task is critical for evaluating age-related fatigability, as tasks which constrain angular velocity (i.e., isokinetic) produce equivocal age-related differences in fatigability, whereas tasks involving unconstrained velocity (i.e., isotonic-like) consistently induce greater fatigability of old compared to young adults. These unconstrained velocity tasks, that are more closely associated with natural movements, offer an excellent model to uncover the underlying age-related mechanisms of increased fatigability. Future work evaluating the mechanisms of increased age-related fatigability of dynamic tasks should be evaluated using task-specific contractions (i.e., dynamic), particularly for assessment of spinal and supra-spinal components. Advancing our understanding of age-related fatigability is likely to yield novel insights and approaches for improving mobility limitations in old adults.