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.

Hand grip strength, standing balance, and rapid foot tapping in relation to the menopausal transition in Campeche, Mexico

OBJECTIVES

This cross-sectional study investigated menopause status in relation to hand grip strength, standing balance, and rapid foot tapping. A secondary aim was to examine the relationship between physical performance and urban/rural residence with a focus on habitual daily tasks.

METHODS

Maya and non-Maya women (40-60 years) were drawn from urban and rural sites in Campeche, Mexico (n = 543). Demographic, reproductive, and lifestyle information was collected in face-to-face interviews along with anthropometric and physical function measures. Linear regression was used to evaluate menopause status in relation to strength, balance, and foot tapping speed while adjusting for residence, ethnicity, and other variables.

RESULTS

Hand grip strength was 22.5, 21.6, and 20.0 kg in pre-, peri-, and postmenopausal women, respectively, but menopause status was not significantly related to grip strength in models adjusted for age. Grip strength was negatively associated with age and socioeconomic index, and positively associated with height and weight, self-reported health, and hours/week spent grinding corn/making tortillas. Postural stability was 9.4, 6.9, and 5.6 s across menopause categories; and menopause status remained significant in adjusted models. The number of foot taps in 10 s was 35.7, 33.4, and 33.9 taps in pre-, peri-, and postmenopausal women. Parity was negatively associated with foot tapping in adjusted models.

CONCLUSIONS

While age is a key predictor of physical function in women aged 40-60 years, menopausal status appears to have additional influences on postural control beyond age alone. Hours spent grinding corn/making tortillas were significantly associated with grip strength among rural women.

Exercise Physiology From 1980 to 2020: Application of the Natural Sciences

The field of exercise physiology has enjoyed tremendous growth in the past 40 years. With its foundations in the natural sciences, it is an interdisciplinary field that is highly relevant to human performance and health. The focus of this review is on highlighting new approaches, knowledge, and opportunities that have emerged in exercise physiology over the last four decades. Key among these is the adoption of advanced technologies by exercise physiologists to address fundamental research questions, and the expansion of research topics to range from molecular to organismal, and population scales in order to clarify the underlying mechanisms and impact of physiological responses to exercise in health and disease. Collectively, these advances have ensured the position of the field as a partner in generating new knowledge across many scientific and health disciplines.

Non-ambulatory measures of lower extremity sensorimotor function are associated with walking function in Multiple Sclerosis

BACKGROUND

Disease progression of multiple sclerosis (MS) is often monitored by ambulatory measures, but how non-ambulatory sensorimotor measures differentially associate to walking measures in MS subtypes is unknown. We determined whether there are characteristic differences between relapsing-remitting MS (RRMS), progressive MS (PMS), and non-MS controls in lower extremity sensorimotor function and clinical walking tasks and the sensorimotor associations with walking function in each group.

METHODS

18 RRMS, 13 PMS and 28 non-MS control participants were evaluated in their plantar cutaneous sensitivity (vibration perception threshold, Volts), proprioception during ankle joint position-matching (|∆°| in dorsiflexion), motor coordination (rapid foot-tap count/10 s), and walking function with three tests: Timed 25-foot walk (T25FW) at preferred and fast speeds (s), and timed-up-and-go (TUG, s).

RESULTS

Foot-tapping (p = 0.039, Mean difference (MD)= 5.65 taps) and plantar cutaneous sensation (p = 0.026, MD= -10.30 V) differed between the MS subtypes. For the RRMS group faster walking was related to better proprioceptive function (preferred T25FW: p = 0.019, Root mean square error (RMSE)=1.94; fast T25FW: p = 0.004, RMSE=1.65; TUG: p = 0.001, RMSE=2.12) and foot-tap performance (preferred T25FW: p = 0.033, RMSE = 2.74; fast T25FW: p = 0.010, RMSE=2.02). These associations were not observed in the PMS group.

CONCLUSIONS

Foot-tap performance and plantar cutaneous sensitivity but not ankle proprioception differed between MS subtypes. Lower walking performance was associated with lower foot-tapping and plantar cutaneous sensitivity in the RRMS but not the PMS group. This result suggests a change in the relationship of lower extremity sensorimotor function to walking performance in the PMS subtype.

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

KEY POINTS

We used 31-phosphorus magnetic resonance spectroscopy to quantify in vivo skeletal muscle metabolic economy (ME; mass-normalized torque or power produced per ATP consumed) during three 24 s maximal-effort contraction protocols: (1) sustained isometric (MVIC), (2) intermittent isokinetic (MVDC_IsoK ), and (3) intermittent isotonic (MVDC_IsoT ) in the knee extensor muscles of young and older adults. ME was not different between groups during the MVIC but was lower in older than young adults during both dynamic contraction protocols. These results are consistent with an increased energy cost of locomotion, but not postural support, with age. The effects of old age on ME were not due to age-related changes in muscle oxidative capacity or ATP flux. Specific power was lower in older than young adults, despite similar total ATP synthesis between groups. Together, this suggests a dissociation between cross-bridge activity and ATP utilization with age.

ABSTRACT

Muscle metabolic economy (ME; mass-normalized torque or power produced per ATP consumed) is similar in young and older adults during some isometric contractions, but less is known about potential age-related differences in ME during dynamic contractions. We hypothesized that age-related differences in ME would exist only during dynamic contractions, due to the increased energetic demand of dynamic versus isometric contractions. Ten young (Y; 27.5 ± 3.9 years, 6 men) and 10 older (O; 71 ± 5 years, 5 men) healthy adults performed three 24 s bouts of maximal contractions: (1) sustained isometric (MVIC), (2) isokinetic (120°·s^-1 , MVDC_IsoK ; 0.5 Hz), and (3) isotonic (load = 20% MVIC, MVDC_IsoT ; 0.5 Hz). Phosphorus magnetic resonance spectroscopy of the vastus lateralis muscle was used to calculate ATP flux (mM ATP·s^-1 ) through the creatine kinase reaction, glycolysis and oxidative phosphorylation. Quadriceps contractile volume (cm³ ) was measured by MRI. ME was calculated using the torque-time integral (MVIC) or power-time integral (MVDC_IsoK and MVDC_IsoT ), total ATP synthesis and contractile volume. As hypothesized, ME was not different between Y and O during the MVIC (0.12 ± 0.03 vs. 0.12 ± 0.02 Nm^. s^. cm^{-3 .} mM ATP^-1 , mean ± SD, respectively; P = 0.847). However, during both MVDC_IsoK and MVDC_IsoT , ME was lower in O than Y adults (MVDC_IsoK : 0.011 ± 0.003 vs. 0.007 ± 0.002 J^. cm^{-3 .} mM ATP^-1 ; P < 0.001; MVDC_IsoT : 0.011 ± 0.002 vs. 0.008 ± 0.002; P = 0.037, respectively), despite similar muscle oxidative capacity, oxidative and total ATP flux in both groups. The lower specific power in older than young adults, despite similar total ATP synthesis between groups, suggests there is a dissociation between cross-bridge activity and ATP utilization with age.

Rates of oxidative ATP synthesis are not augmented beyond the pH threshold in human vastus lateralis muscles during a stepwise contraction protocol

KEY POINTS

The oxygen cost of high-intensity exercise at power outputs above an individual's lactate threshold (LT) is greater than would be predicted by the linear oxygen consumption-power relationship observed below the LT. However, whether these augmentations are caused by an increased ATP cost of force generation (ATP_COST ) or an increased oxygen cost of ATP synthesis is unclear. We used ³¹ P-MRS to measure changes in cytosolic [ADP] (intramyocellular marker of oxidative metabolism), oxidative ATP synthesis (ATP_OX ) and ATP_COST during a 6-stage, stepwise knee extension protocol. ATP_COST was unchanged across stages. The relationship between [ADP] and muscle power output was augmented at workloads above the pH threshold (pH_T ; proxy for LT), whereas increases in ATP_OX were attenuated. These results suggest the greater oxygen cost of contractions at workloads beyond the pH_T is not caused by mechanisms that increase ATP_COST , but rather mechanisms that alter intrinsic mitochondrial function or capacity.

ABSTRACT

Increases in skeletal muscle metabolism and oxygen consumption are linearly related to muscle power output for workloads below the lactate threshold (LT), but are augmented (i.e. greater rate of increase relative to workload) thereafter. Presently, it is unclear whether these metabolic augmentations are caused by increases in the ATP cost of force generation (ATP_COST ) or changes in the efficiency of mitochondrial oxygen consumption and oxidative ATP synthesis (ATP_OX ). To partition these two hypotheses in vivo, we used ³¹ P-MRS to calculate slopes relating step-changes in muscle work to concurrent changes in cytosolic phosphates and ATP_OX before and after the pH threshold (pH_T ; used here as a proxy for LT) within the vastus lateralis muscle of eight young adults during a stepwise knee extension test. Changes in muscle phosphates and ATP_OX were linearly related to workload below the pH_T . However, slopes above the pH_T were greater for muscle phosphates (P < 0.05) and lower for ATP_OX (P < 0.05) than were the slopes observed below the pH_T . The maximal capacity for ATP_OX ( V ̇ max ) and ADP-specific ATP_OX also declined beyond the pH_T (P < 0.05), whereas ATP_COST was unchanged (P = 0.10). These results oppose the hypothesis that high-intensity contractions increase ATP_COST and suggest that greater oxidative metabolism at workloads beyond the pH_T is caused by mechanisms that affect intrinsic mitochondrial function or capacity, such as alterations in substrate selection or electron entry into the electron transport chain, temperature-mediated changes in mitochondrial permeability to protons, or stimulation of mitochondrial uncoupling by reactive oxygen species generation.

Muscle architecture, voluntary activation, and low-frequency fatigue do not explain the greater fatigue of older compared with young women during high-velocity contractions

Although high-velocity contractions elicit greater muscle fatigue in older than young adults, the cause of this difference is unclear. We examined the potential roles of resting muscle architecture and baseline contractile properties, as well as changes in voluntary activation and low-frequency fatigue in response to high-velocity knee extensor work. Vastus lateralis muscle architecture was determined in quiescent muscle by ultrasonography in 8 young (23.4±1.8 yrs) and 8 older women (69.6±1.1). Maximal voluntary dynamic (MVDC) and isometric (MVIC), and stimulated (80Hz and 10Hz, 500ms) isometric contractions were performed before and immediately after 120 MVDCs (240°^.s^-1, one every 2s). Architecture variables did not differ between groups (p≥0.209), but the half-time of torque relaxation (T_1/2) was longer in older than young women at baseline (151.9±6.0 vs. 118.8±4.4 ms, respectively, p = 0.001). Older women fatigued more than young (to 33.6±4.7% vs. 55.2±4.2% initial torque, respectively; p = 0.004), with no evidence of voluntary activation failure (ΔMVIC:80Hz torque) in either group (p≥0.317). Low-frequency fatigue (Δ10:80Hz torque) occurred in both groups (p<0.001), as did slowing of T_1/2 (p = 0.001), with no differences between groups. Baseline T_1/2 was inversely associated with fatigue in older (r² = 0.584, p = 0.045), but not young women (r² = 0.147, p = 0.348). These results indicate that differences in muscle architecture, voluntary activation, and low-frequency fatigue do not explain the greater fatigue of older compared with young women during high-velocity contractions. The inverse association between baseline T_1/2 and fatigue in older women suggests that factors related to slower muscle contractile properties may be protective against fatigue during fast, repetitive contractions in aging.

Validity and accuracy of calculating oxidative ATP synthesis in vivo during high-intensity skeletal muscle contractions

Several methods have been developed for using ³¹ P-MRS to calculate rates of oxidative ATP synthesis (ATP_OX ) during muscular contractions based on assumptions that (1) the ATP cost of force generation (ATP_COST ) remains constant or (2) Michaelis-Menten coupling between cytosolic ADP and ATP_OX does not change. However, growing evidence suggests that one, or both, of these assumptions are invalid during high-intensity fatigue protocols. Consequently, there is a need to examine the validity and accuracy of traditional ATP_OX calculation methods under these conditions. To address this gap, we measured phosphate concentrations and pH in the vastus lateralis muscle of nine young adults during four rest-contraction-recovery trials lasting 24, 60, 120, and 240 s. The initial velocity of phosphocreatine resynthesis (V_iPCr ) following each trial served as the criterion measure of ATP_OX because this method makes no assumptions of constant ATP_COST or Michaelis-Menten coupling between changes in cytosolic ADP and ATP_OX . Subsequently, we calculated ATP_OX throughout the 240 s trial using several traditional calculation methods and compared estimations of ATP_OX from each method with time-matched measurements of V_iPCr . Method 1, which assumes that ATP_COST does not change, was able to model changes in V_iPCr over time, but showed poor accuracy for predicting V_iPCr across a wide range of ATP_OX values. In contrast, Michaelis-Menten methods, which assume that the relationship between changes in cytosolic ADP and ATP_OX remains constant, were invalid because they could not model the decline in V_iPCr . However, adjusting these Michaelis-Menten methods for observed changes in maximal ATP_OX capacity (i.e., V_max ) permitted modeling of the decline in V_iPCr and markedly improved accuracy. The results of these comprehensive analyses demonstrate that valid, accurate measurements of ATP_OX can be obtained during high-intensity contractions by adjusting Michaelis-Menten ATP_OX calculations for changes in V_max observed from baseline to post-fatigue.

Sensorimotor function in progressive multiple sclerosis

BACKGROUND

A sensitive test reflecting subtle sensorimotor changes throughout disease progression independent of mobility impairment is currently lacking in progressive multiple sclerosis.

OBJECTIVES

We examined non-ambulatory measures of upper and lower extremity sensorimotor function that may reveal differences between relapsing-remitting and progressive forms of multiple sclerosis.

METHODS

Cutaneous sensitivity, proprioception, central motor function and mobility were assessed in 32 relapsing-remitting and 31 progressive multiple sclerosis patients and 30 non-multiple sclerosis controls.

RESULTS

Cutaneous sensation differed between relapsing-remitting and progressive multiple sclerosis at the foot and to a lesser extent the hand. Proprioception function in the upper but not the lower extremity differed between relapsing-remitting and progressive multiple sclerosis, but was different for both upper and lower extremities between multiple sclerosis patients and non-multiple sclerosis controls. Foot-tap but not hand-tap speed was slower in progressive compared to relapsing-remitting multiple sclerosis, suggestive of greater central motor function impairment in the lower extremity in progressive multiple sclerosis. In addition, the non-ambulatory sensorimotor measures were more sensitive in detecting differences between relapsing-remitting and progressive multiple sclerosis than mobility assessed with the 25-foot walk test.

CONCLUSION

This study provides novel information about changes in sensorimotor function in progressive compared with relapsing-remitting forms of multiple sclerosis, and in particular the importance of assessing both upper and lower extremity function. Importantly, our findings showed loss of proprioceptive function in multiple sclerosis but also in progressive compared to relapsing-remitting multiple sclerosis.

Magnetic Resonance Compatible Knee Extension Ergometer

A magnetic resonance (MR) compatible ergometer has been developed to study contracting lower limb muscles during acquisition of MR spectroscopy data, a technique to noninvasively measure metabolic energy in muscle tissue. Current active and passive MR-compatible ergometer designs lack torque or velocity control to allow precise mechanical measurements during isotonic and isokinetic contractions; incorporating load and velocity controllers while maintaining MR-compatibility is the main challenge. Presented in this paper is the design and evaluation of an MR-compatible ergometer designed to control knee torque or velocity up to 420 N·m and 270 deg/s and is able to operate in a 3 Tesla magnetic field. The ergometer comprising of a passive component with no electronics or ferrous materials is located inside the bore of the scanner. The active component with the electronics and actuator located outside of the magnetic field in an adjacent room. The active components connect to the passive components via a cable that passes through the waveguide, a hole in the wall of the scanner room. System evaluations were performed and human subject evaluations were performed that measured the mechanical performance and show the mean percent errors below 9% in isotonic and 2% in isokinetic conditions.

Oxidative ATP synthesis in human quadriceps declines during 4 minutes of maximal contractions

KEY POINTS

During maximal exercise, skeletal muscle metabolism and oxygen consumption remain elevated despite precipitous declines in power. Presently, it is unclear whether these responses are caused by an increased ATP cost of force generation (ATP_COST ) or mitochondrial uncoupling; a process that reduces the efficiency of oxidative ATP synthesis (ATP_OX ). To address this gap, we used 31-phosphorus magnetic resonance spectroscopy to measure changes in ATP_COST and ATP_OX in human quadriceps during repeated trials of maximal intensity knee extensions lasting up to 4 min. ATP_COST remained unchanged. In contrast, ATP_OX plateaued by ∼2 min and then declined (∼15%) over the final 2 min. The maximal capacity for ATP_OX (V_max ), as well as ADP-specific rates of ATP_OX , were also significantly diminished. Collectively, these results suggest that mitochondrial uncoupling, and not increased ATP_COST , is responsible for altering the regulation of skeletal muscle metabolism and oxygen consumption during maximal exercise.

ABSTRACT

The relationship between skeletal muscle oxygen consumption and power output is augmented during exercise at workloads above the lactate threshold. Potential mechanisms for this response have been hypothesized, including increased ATP cost of force generation (ATP_COST ) and mitochondrial uncoupling, a process that reduces the efficiency of oxidative ATP synthesis (ATP_OX ). To test these hypotheses, we used phosphorus magnetic resonance spectroscopy to non-invasively measure changes in phosphate concentrations and pH in the vastus lateralis muscle of nine young adults during repeated trials of maximal, all-out dynamic knee extensions (120°s^-1 , 1 every 2 s) lasting 24, 60, 120, and 240 s. ATP_OX was measured at each time point from the initial velocity of PCr resynthesis, and ATP_COST was calculated as the sum of ATP synthesized by the creatine and adenylate kinase reactions, non-oxidative glycolysis, ATP_OX and net changes in [ATP]. Power output declined in a reproducible manner for all four trials. ATP_COST did not change over time (main effect P = 0.45). ATP_OX plateaued from 60 to 120 s and then decreased over the final 120 s (main effect P = 0.001). The maximal capacity for oxidative ATP synthesis (V_max ), as well as ADP-specific rates of ATP_OX , also decreased over time (main effect P = 0.001, both). Collectively, these results demonstrate that prolonged maximal contraction protocols impair oxidative energetics and implicate mitochondrial uncoupling as the mechanism for this response. The causes of mitochondrial uncoupling are presently unknown but may offer a potential explanation for the dissociation between skeletal muscle power output and oxygen consumption during maximal, all-out exercise protocols.

Comparable Stride Time Fractal Dynamics and Gait Adaptability in Active Young and Older Adults Under Normal and Asymmetric Walking

Previous research indicates the correlation structure of gait parameters (i.e., fractal dynamics) decreases with age. This decrease is suggested to reflect a reduced capacity for locomotor adaptation in older adults. The purpose of this study was to investigate potential differences between physical activity-matched young and older adults’ fractal dynamics and gait adaptability during unperturbed and asymmetric walking, and to determine if fractal dynamics predict adaptive capacity. Fifteen young (28.9 ± 5.6 years, nine women) and 15 older (64.7 ± 2.7, nine women) adults with similar habitual physical activity levels walked at preferred speed, half of preferred speed, and asymmetrically whereby their dominant and non-dominant legs moved at preferred and half-preferred speed, respectively. Fractal correlations (scaling exponent α) of stride times were assessed through detrended fluctuation analysis, and gait adaptation to asymmetric walking on the basis of lower limb relative phase. Both cohorts displayed similar fractal dynamics at preferred speed and asymmetric walking, while older adults exhibited greater α during slow walking. Both groups exhibited comparable gait adaptation to split-belt walking based on analysis of lower limb relative phase. Fractal dynamics during preferred speed and asymmetric walking was moderately associated with gait adaptation in the young and older adult cohorts, respectively. In these activity-matched groups, there were no age-based reductions in fractal dynamics or gait adaptation, and fractal scaling α was moderately associated with gait adaptation. These findings suggest that stride time fractal dynamics and gait adaptation may be preserved in older adults who habitually perform moderate intensity physical activity.

Physical activity and age-related biomechanical risk factors for knee osteoarthritis

BACKGROUND

Knee osteoarthritis (OA) is a highly prevalent disease leading to mobility disability in the aged that could, in part, be initiated by age-related alterations in knee mechanics. However, if and how knee mechanics change with age remains unclear.

RESEARCH QUESTION

What are the impacts of age and physical activity (PA) on biomechanical characteristics that can affect the loading environment in the knee during gait?

METHODS

Three groups (n = 20 each, 10 male and 10 female) of healthy adults were recruited: young (Y, 21-35 years), mid-life highly active (MHi, 55-70 years, runners), and mid-life less active (MLo, 55-70 years, low PA). Outcome measures included knee kinematics and kinetics and co-activation during gait, and knee extensor muscle torque and power collected at baseline and after a 30-minute treadmill trial to determine the impact of prolonged walking on knee function.

RESULTS

At baseline, high-velocity concentric knee extensor power was lower for MLo and MHi compared with Y, and MLo displayed greater early (6.0 ± 5.8 mm) and peak during stance (11.3 ± 7.8 mm) femoral anterior displacement relative to the tibia compared with Y (0.2 ± 5.6 and 4.4 ± 6.8 mm). Also at baseline, MLo showed equal quadriceps:hamstrings activation, while Y showed greater relative hamstrings activation during midstance. The walking bout induced substantial knee extensor fatigue (decrease in maximal torque and power) in Y and MLo, while MHi were fatigue-resistant.

SIGNIFICANCE

These results indicate that maintenance of PA in mid-life may impart small but measurable effects on knee function and biomechanics that may translate to a more stable loading environment in the knee through mid-life and thus could reduce knee OA risk long-term.

Magnetic resonance images and measurements of the volume, proportion, and longitudinal distribution of contractile and non-contractile tissue in the dorsi- and plantar flexor muscles of healthy young and older adults

Objective

This paper presents magnetic resonance images of the dorsi- and plantar flexor muscles for individual young and older healthy adults. Also included are measurements of the volume, proportion, and longitudinal distribution of contractile and non-contractile tissue. This dataset was previously used to quantify age-related differences in these measures, constrain subject- and muscle-specific estimates of dorsi- and plantar flexor maximal isometric force capability, and quantify the degree to which maximal isometric force capability explains the age-related variance in postural control.

Data description

The data include contiguous axial magnetic resonance images of the lower leg for 12 young (21–31 years) and 12 older (66–79 years) healthy adults. The data are in the form of MATLAB binary files with a freely distributable custom MATLAB analysis program that allows image viewing and navigation in two and three dimensions, muscle outlining, tissue segmentation, and cross-sectional area calculation. The latter measurements are contained in a set of companion MATLAB binary files, which are included with the image data files. If desired, the magnetic resonance images could be used to identify other anatomical structures, or the MATLAB programs could be used to analyze other image sets.

Post-fatigue recovery of power, postural control and physical function in older women

Low muscle power, particularly at high velocities, has been linked to poor physical function in older adults. Any loss in muscle power following fatiguing exercise or daily activities could impact physical function and postural control until power has fully recovered. To test the overall hypothesis that a common task such as walking can result in prolonged power loss and decreased physical function and balance, 17 healthy older (66–81 years) women completed a 32-min walking test (32MWT) designed to induce neuromuscular fatigue, followed by 60min of recovery (60R). Fatigue and recovery of knee extensor muscle power (3 velocities) were quantified by dynamometry. Function was quantified by chair rise time and postural control by measures of center of pressure (COP) range (mm) and velocity (mm·s^-1) during quiet stance. Power declined at all velocities by 8–13% 2min following the 32MWT (p≤0.02) and remained depressed by 8–26% at 60R (p≤0.04). Postural control decreased following the 32MWT, indicated by increased COP range in the anterior-posterior (AP, p<0.01) direction and a trend in the medial-lateral (ML) direction (p = 0.09), and returned to baseline by 60R (p≥0.10). COP velocity was unchanged immediately following the 32MWT, but at 60R was lower in ML (p = 0.03) and tended to be reduced in AP (p = 0.07). Changes in high-velocity power (270°·s^-1) were associated with altered postural control (p = 0.02) and chair rise performance (p≤0.03). These results provide evidence of long-duration neuromuscular changes following fatigue in healthy older women that may place them at increased risk for functional deficits during everyday mobility tasks.

No Muscle Is an Island: Integrative Perspectives on Muscle Fatigue

Muscle fatigue has been studied with a variety approaches, tools and technologies. The foci of these studies have ranged tremendously, from molecules to the entire organism. Single cell and animal models have been used to gain mechanistic insight into the fatigue process. The theme of this review is the concept that the mechanisms of muscle fatigue do not occur in isolation in vivo: muscular work is supported by many complex physiological systems, any of which could fail during exercise and thus contribute to fatigue. To advance our overall understanding of fatigue, a combination of models and approaches is necessary. In this review, we examine the roles that neuromuscular properties, intracellular glycogen, oxygen metabolism, and blood flow play in the fatigue process during exercise and pathological conditions.

Performance of Activity Classification Algorithms in Free-Living Older Adults

PURPOSE

The objective of this study is to compare activity type classification rates of machine learning algorithms trained on laboratory versus free-living accelerometer data in older adults.

METHODS

Thirty-five older adults (21 females and 14 males, 70.8 ± 4.9 yr) performed selected activities in the laboratory while wearing three ActiGraph GT3X+ activity monitors (in the dominant hip, wrist, and ankle; ActiGraph, LLC, Pensacola, FL). Monitors were initialized to collect raw acceleration data at a sampling rate of 80 Hz. Fifteen of the participants also wore GT3X+ in free-living settings and were directly observed for 2-3 h. Time- and frequency-domain features from acceleration signals of each monitor were used to train random forest (RF) and support vector machine (SVM) models to classify five activity types: sedentary, standing, household, locomotion, and recreational activities. All algorithms were trained on laboratory data (RFLab and SVMLab) and free-living data (RFFL and SVMFL) using 20-s signal sampling windows. Classification accuracy rates of both types of algorithms were tested on free-living data using a leave-one-out technique.

RESULTS

Overall classification accuracy rates for the algorithms developed from laboratory data were between 49% (wrist) and 55% (ankle) for the SVMLab algorithms and 49% (wrist) to 54% (ankle) for the RFLab algorithms. The classification accuracy rates for SVMFL and RFFL algorithms ranged from 58% (wrist) to 69% (ankle) and from 61% (wrist) to 67% (ankle), respectively.

CONCLUSIONS

Our algorithms developed on free-living accelerometer data were more accurate in classifying the activity type in free-living older adults than those on our algorithms developed on laboratory accelerometer data. Future studies should consider using free-living accelerometer data to train machine learning algorithms in older adults.

Heterogeneous effects of old age on human muscle oxidative capacity in vivo: a systematic review and meta-analysis

Despite intensive efforts to understand the extent to which skeletal muscle mitochondrial capacity changes in older humans, the answer to this important question remains unclear. To determine what the preponderance of evidence from in vivo studies suggests, we conducted a systematic review and meta-analysis of the effects of age on muscle oxidative capacity as measured noninvasively by magnetic resonance spectroscopy. A secondary aim was to examine potential moderators contributing to differences in results across studies, including muscle group, physical activity status, and sex. Candidate papers were identified from PubMed searches (n = 3561 papers) and the reference lists of relevant papers. Standardized effects (Hedges’ g) were calculated for age and each moderator using data from the 22 studies that met the inclusion criteria (n = 28 effects). Effects were coded as positive when older (age, ≥55 years) adults had higher muscle oxidative capacity than younger (age, 20–45 years) adults. The overall effect of age on oxidative capacity was positive (g = 0.171, p < 0.001), indicating modestly greater oxidative capacity in old. Notably, there was significant heterogeneity in this result (Q = 245.8, p < 0.001; I2 = ∼70%–90%). Muscle group, physical activity, and sex were all significant moderators of oxidative capacity (p ≤ 0.029). This analysis indicates that the current body of literature does not support a de facto decrease of in vivo muscle oxidative capacity in old age. The heterogeneity of study results and identification of significant moderators provide clarity regarding apparent discrepancies in the literature, and indicate the importance of accounting for these variables when examining purported age-related differences in muscle oxidative capacity.

Closing out a large clinical trial: lessons from the Prospective Study of Pravastatin in the Elderly at Risk (PROSPER)

Relatively little has been written about the practicalities of the closeout of large, multi-centre clinical trials, but this aspect of trial conduct and design is both important and requires careful planning in order to be accomplished in a timely and orderly fashion. Here, we document our recent experiences of closing down the Prospective Study of Pravastatin in the Elderly at Risk (PROSPER). This five-year study with over 5800 subjects was closed down and published in 2002. We describe the methods used to ensure the speedy progression from the start of the closeout period through data lock to publication and presentation of the results. We discuss the strategic planning of all aspects of the closeout process, the training of staff for final visits, the methods used to follow-up all study participants including those used in dealing with “difficult to contact” and defaulted patients, and the strategies employed to ensure that study participants were left with positive feelings about the study. We also detail the methods employed to expedite the throughput of study paperwork and endpoints and the cleaning of data in preparation for data lock and subsequent publication and presentation of the results. Based on our experiences we summarize the most important aspects of closeout design and make recommendations for future studies, the most important of which is that a well-planned and well-managed closeout is a key feature of any large scale clinical trial and a coherent and practicable closeout strategy should be an integral part of the design.

In vivo mitochondrial function in aging skeletal muscle: capacity, flux, and patterns of use

Because of the fundamental dependence of mammalian life on adequate mitochondrial function, the question of how and why mitochondria change in old age is the target of intense study. Given the importance of skeletal muscle for the support of mobility and health, this question extends to the need to understand mitochondrial changes in the muscle of older adults, as well. We and others have focused on clarifying the age-related changes in human skeletal muscle mitochondrial function in vivo. These changes include both the maximal capacity for oxidative production of energy (ATP), as well as the relative use of mitochondrial ATP production for powering muscular activity. It has been known for nearly 50 yr that muscle mitochondrial content is highly plastic; exercise training can induce an ∼2-fold increase in mitochondrial content, while disuse has the opposite effect. Here, we suggest that a portion of the age-related changes in mitochondrial function that have been reported are likely the result of behavioral effects, as physical activity influences have not always been accounted for. Further, there is emerging evidence that various muscles may be affected differently by age-related changes in physical activity and movement patterns. In this review, we will focus on age-related changes in oxidative capacity and flux measured in vivo in human skeletal muscle.

Energy cost of walking, symptomatic fatigue and perceived exertion in persons with multiple sclerosis

A higher energy cost of walking (Cw) is sometimes observed in MS, and could contribute to fatigue. The purpose of this study was to compare Cw at three speeds in MS and controls, and determine the effects of walking speed on fatigue and perceived exertion. We hypothesized that MS would have higher Cw, fatigue and exertion during walking than controls. Ten persons with MS and 14 controls of similar age and physical activity levels were studied. Oxygen consumption (VO2) was obtained at rest and during treadmill walking at 0.6 and 1.4ms(-1), and preferred speed. Cw was calculated as net VO2:velocity. Fatigue and exertion were assessed using the visual analog fatigue and modified Borg scales, respectively. Preferred treadmill speed was not different between groups. Cw was higher in MS than controls across walking speeds (p=0.003), with a group-by-speed interaction indicating higher Cw in MS at 0.6ms(-1) (p=0.001), but not at preferred speed or 1.4ms(-1). MS reported greater fatigue (p=0.001) and exertion (p=0.004) at all speeds. Despite similar preferred speeds, and Cw at preferred and fast speeds, MS exhibited higher fatigue and exertion at all walking speeds. These results suggest that increased energy demands in MS are most notable at low speeds such as those used in everyday activities, which may contribute to fatigue over the day.

Mechanisms of in vivo muscle fatigue in humans: investigating age-related fatigue resistance with a computational model

KEY POINTS

Muscle fatigue can be defined as the transient decrease in maximal force that occurs in response to muscle use. Fatigue develops because of a complex set of changes within the neuromuscular system that are difficult to evaluate simultaneously in humans. The skeletal muscle of older adults fatigues less than that of young adults during static contractions. The potential sources of this difference are multiple and intertwined. To evaluate the individual mechanisms of fatigue, we developed an integrative computational model based on neural, biochemical, morphological and physiological properties of human skeletal muscle. Our results indicate first that the model provides accurate predictions of fatigue and second that the age-related resistance to fatigue is due largely to a lower reliance on glycolytic metabolism during contraction. This model should prove useful for generating hypotheses for future experimental studies into the mechanisms of muscle fatigue.

ABSTRACT

During repeated or sustained muscle activation, force-generating capacity becomes limited in a process referred to as fatigue. Multiple factors, including motor unit activation patterns, muscle fibre contractile properties and bioenergetic function, can impact force-generating capacity and thus the potential to resist fatigue. Given that neuromuscular fatigue depends on interrelated factors, quantifying their independent effects on force-generating capacity is not possible in vivo. Computational models can provide insight into complex systems in which multiple inputs determine discrete outputs. However, few computational models to date have investigated neuromuscular fatigue by incorporating the multiple levels of neuromuscular function known to impact human in vivo function. To address this limitation, we present a computational model that predicts neural activation, biomechanical forces, intracellular metabolic perturbations and, ultimately, fatigue during repeated isometric contractions. This model was compared with metabolic and contractile responses to repeated activation using values reported in the literature. Once validated in this way, the model was modified to reflect age-related changes in neuromuscular function. Comparisons between initial and age-modified simulations indicated that the age-modified model predicted less fatigue during repeated isometric contractions, consistent with reports in the literature. Together, our simulations suggest that reduced glycolytic flux is the greatest contributor to the phenomenon of age-related fatigue resistance. In contrast, oxidative resynthesis of phosphocreatine between intermittent contractions and inherent buffering capacity had minimal impact on predicted fatigue during isometric contractions. The insights gained from these simulations cannot be achieved through traditional in vivo or in vitro experimentation alone.

Physical activity, sleep quality, and self-reported fatigue across the adult lifespan

Deteriorating sleep quality and increased fatigue are common complaints of old age, and poor sleep is associated with decreased quality of life and increased mortality rates. To date, little attention has been given to the potential effects of physical activity on sleep quality and fatigue in aging. The purpose of this study was to examine the relationships between activity, sleep and fatigue across the adult lifespan. Sixty community-dwelling adults were studied; 22 younger (21-29 years), 16 middle-aged (36-64 years), and 22 older (65-81 years). Physical activity was measured by accelerometer. Sleep quality was assessed using the Pittsburg Sleep Quality Index. Self-reported fatigue was evaluated with the Patient-Reported Outcomes Measurement Information System (PROMIS). Regression analysis revealed a positive relationship between activity and sleep quality in the older (r(2)=0.18, p=0.05), but not the younger (r(2) = 0.041, p = 0.35) or middle-aged (r(2) = 0.001, p = 0.93) groups. This association was mainly established by the relationship between moderate-vigorous activity and sleep quality (r(2)=0.37, p=0.003) in older adults. No association was observed between physical activity and self-reported fatigue in any of the groups (r(2) ≤ 0.14, p ≥ 0.15). However, an inverse relationship was found between sleep quality and fatigue in the older (r(2) = 0.29, p = 0.05), but not the younger or middle-aged (r(2) ≤ 0.13, p ≥ 0.10) groups. These results support the hypothesis that physical activity may be associated with sleep quality in older adults, and suggest that improved sleep may mitigate self-reported fatigue in older adults in a manner that is independent of activity.

Effect of age on in vivo oxidative capacity in two locomotory muscles of the leg

To determine the effects of age and sex on in vivo mitochondrial function of distinct locomotory muscles, the tibialis anterior (TA) and medial gastrocnemius (MG), of young (Y; 24 ± 3 years) and older (O; 69 ± 4) men (M) and women (W) of similar overall physical activity (PA) was compared. In vivo mitochondrial function was measured using phosphorus magnetic resonance spectroscopy, and PA and physical function were measured in all subjects. Overall PA was similar among the groups, although O (n = 17) had fewer daily minutes of moderate-to-vigorous PA (p = 0.001), and slowed physical function (p < 0.05 for all variables), compared with Y (n = 17). In TA, oxidative capacity (V max; mM s(-1)) was higher in O than Y (p < 0.001; Y = 0.90 ± 0.12; O = 1.12 ± 0.18). There was no effect of age in MG (p = 0.5; Y = 0.91 ± 0.17; O = 0.96 ± 0.24), but women had higher oxidative capacity than men (p = 0.007; M = 0.84 ± 0.18; W = 1.03 ± 0.18). In vivo mitochondrial function was preserved in healthy O men and women, despite lower intensity PA and physical function in this group. The extent to which compensatory changes in gait may be responsible for this preservation warrants further investigation. Furthermore, women had higher oxidative capacity in the MG, but not the TA.

Human skeletal muscle metabolic economy in vivo: effects of contraction intensity, age, and mobility impairment

We tested the hypothesis that older muscle has greater metabolic economy (ME) in vivo than young, in a manner dependent, in part, on contraction intensity. Twenty young (Y; 24±1 yr, 10 women), 18 older healthy (O; 73±2, 9 women) and 9 older individuals with mild-to-moderate mobility impairment (OI; 74±1, 7 women) received stimulated twitches (2 Hz, 3 min) and performed nonfatiguing voluntary (20, 50, and 100% maximal; 12 s each) isometric dorsiflexion contractions. Torque-time integrals (TTI; Nm·s) were calculated and expressed relative to maximal fat-free muscle cross-sectional area (cm2), and torque variability during voluntary contractions was calculated as the coefficient of variation. Total ATP cost of contraction (mM) was determined from flux through the creatine kinase reaction, nonoxidative glycolysis and oxidative phosphorylation, and used to calculate ME (Nm·s·cm(-2)·mM ATP(-1)). While twitch torque relaxation was slower in O and OI compared with Y (P≤0.001), twitch TTI, ATP cost, and economy were similar across groups (P≥0.15), indicating comparable intrinsic muscle economy during electrically induced isometric contractions in vivo. During voluntary contractions, normalized TTI and total ATP cost did not differ significantly across groups (P≥0.20). However, ME was lower in OI than Y or O at 20% and 50% MVC (P≤0.02), and torque variability was greater in OI than Y or O at 20% MVC (P≤0.05). These results refute the hypothesis of greater muscle ME in old age, and provide support for lower ME in impaired older adults as a potential mechanism or consequence of age-related reductions in functional mobility.

Unblinding of trial participants to their treatment allocation: lessons from the Prospective Study of Pravastatin in the Elderly at Risk (PROSPER)

BACKGROUND

The gold standard clinical trial design is the double-blind, randomized, controlled trial. No standard practice exists for the "unblinding" of trial participants and no legal obligation is placed on investigators to inform participants of their treatment allocation or study results at the end of a trial. Here we document our experiences of unblinding the 2520 Scottish participants in the Prospective Study of Pravastatin in the Elderly at Risk (PROSPER).

METHODS

The objectives of the PROSPER unblinding process were to provide all study participants with their study medication status and on-trial cholesterol levels and to respect the rights of participants not to be unblinded. It was considered imperative by the study executive that the blind was maintained until the presentation and publication of the results. Staff therefore remained "blinded" throughout the unblinding process. Inappropriate contact with the PROSPER participants was avoided by confirming their current vital status and health status.

RESULTS

To coincide with the presentation of the PROSPER results, all participants, for whom it was deemed appropriate, were sent a summary of the results and were offered the opportunity to be advised of their treatment allocation and on-trial lipid profiles. The majority of participants opted for telephone unblinding. All primary care physicians who had patients randomised to the study were also sent a summary of the study results and sealed documents detailing the treatment allocation and lipid profiles for each patient. Relocated patients were traced and the information forwarded to their new primary care physicians.

CONCLUSION

The dissemination of study results and treatment allocation to study participants is an integral part of the research process and should be included in the design of any clinical trial.