Although skeletal muscle work efficiency probably plays a key role in limiting mobility of the elderly, the physiological mechanisms responsible for this diminished function remain incompletely understood. Thus, in the quadriceps of young (n=9) and old (n=10) subjects, we measured the cost of muscle contraction (ATP cost) with 31P-magnetic resonance spectroscopy (31P-MRS) during (i) maximal intermittent contractions to elicit a metabolic demand from both cross-bridge cycling and ion pumping and (ii) a continuous maximal contraction to predominantly tax cross-bridge cycling. The ATP cost of the intermittent contractions was significantly greater in the old (0.30±0.22 mM·min−1·N·m−1) compared with the young (0.13±0.03 mM·min−1·N·m−1, P<0.05). In contrast, at the end of the continuous contraction protocol, the ATP cost in the old (0.10±0.07 mM·min−1·N·m−1) was not different from the young (0.06±0.02 mM·min−1·N·m−1, P=0.2). In addition, the ATP cost of the intermittent contractions correlated significantly with the single leg peak power of the knee-extensors assessed during incremental dynamic exercise (r=−0.55; P<0.05). Overall, this study reveals an age-related increase in the ATP cost of contraction, probably mediated by an excessive energy demand from ion pumping, which probably contributes to both the decline in muscle efficiency and functional capacity associated with aging.

CLINICAL PERSPECTIVES

  • For elderly individuals, preserved function of the knee-extensors is critical for everyday mobility and independent living. There is significant evidence which suggests that typical sedentary aging is characterized by a progressive decline in skeletal muscle work efficiency assessed during cycling and knee-extension exercise. However, the physiological mechanisms for this age-related decline in work efficiency remain incompletely understood.

  • Our results illustrate that a higher ATP cost of contraction contributes significantly to the decline in muscle efficiency observed with age and appears to be mediated by an excessive energy demand from non-contractile processes. Our findings suggest that the excessive cost associated with ion transport, potentially mediated by chronic oxidative stress, is an important mechanism contributing to the decline in muscle efficiency with age and probably compromises functional capacity in older adults.

  • Together with the previous demonstration of lower mitochondrial efficiency in the skeletal muscle of older individuals, these findings highlight the need for development of therapies, perhaps exercise-based, designed to maintain muscle efficiency in older individuals.

INTRODUCTION

Work efficiency, defined as the ratio of the mechanical work performed and the energy expended [1], is a key determinant of exercise capacity, and, thus, mobility for the elderly. Although not without challenge [2,3], there is significant evidence which suggests that typical sedentary aging is characterized by a progressive decline in skeletal muscle work efficiency assessed during cycling [4] and knee-extension exercise [5,6]. However, the physiological mechanisms for this age-related decline in work efficiency remain incompletely understood.

Consistent with an impaired muscle efficiency in the quadriceps with age, several studies have demonstrated lower mitochondrial efficiency (conversion of chemical energy into ATP) at rest [7] and during exercise in older individuals compared with their younger counterparts [5,8,9]. In light of these studies, it is therefore rather surprising that, despite being a major determinant of muscle efficiency [1], studies investigating the influence of aging on the cost of muscle contraction (conversion of ATP into mechanical work) in the quadriceps are scarce [5]. It is also worth noting that in, what appears to be, the only study to date on this topic [5], the level of physical activity was not controlled, which probably has some bearing on the interpretation of the results as exercise training has previously been documented to affect the cost of muscle contraction [10]. Therefore, additional studies examining potential age-related alterations in the cost of muscle contraction in the quadriceps are warranted.

Aging is also associated with alterations in the intrinsic contractile properties of skeletal muscle. Specifically, although relatively modest and again potentially related to physical activity [11,12], slower rates of muscle force development [13] have been documented in the quadriceps with advancing age. In contrast, there appears to be a rather substantial age-related decline in the rate of fibre [12,13] and whole muscle relaxation [11,14]. This lengthening of the muscle fibre relaxation time predominantly stems from impaired Ca2+ sequestration by the sarcoplasmic reticulum as evidenced in vitro by lower rates of Ca2+ uptake and Ca2+-ATPase activity with age [15,16]. Therefore, these differential age-related impairments in muscle contractile properties could conceptually lead to an exaggerated cost of muscle contraction during conventional exercise involving both muscle shortening and lengthening. Here ATP is also hydrolysed by myosin ATPase and there is a significant metabolic demand from the non-contractile processes of ion transport (Ca2+ ATPase and to a lesser extent Na+–K+ ATPase). In contrast, the effect of aging on exercise requiring predominantly muscle shortening, such as an isometric exercise with constant force generation, may be far less apparent. However, such a hypothesis has yet to be tested.

Therefore, in the quadriceps, a crucial muscle group for locomotion, using 31P-magnetic resonance spectroscopy (31P-MRS) and two differing modes of contraction (maximal intermittent and continuous contractions) this study sought to determine whether the ATP cost of contraction increases with age as well as the relative contribution from cross-bridge cycling and ion pumping to this potentially augmented metabolic demand. With the overall hypothesis that the ATP cost of the non-contractile processes account for the attenuated muscle efficiency with age, we hypothesized that the old would exhibit a greater ATP cost of contraction during exercise involving both muscle shortening and relaxation, which depends heavily upon the non-contractile processes of ion transport. In contrast, we hypothesized that, in comparison with the young, the older subjects would exhibit a similar ATP cost of contraction during an exercise modality requiring predominantly muscle shortening.

MATERIALS AND METHODS

Subjects

Following informed consent procedures, nine (seven male, two female) young and ten (eight male, two female) old activity matched subjects participated in this study (Table 1). The subjects were recruited based upon no evidence of regular physical activity above that required for activities of daily living (documented after recruitment by both questionnaire and accelerometry) and aged between 18 and 32 years for the young and greater than 60 years of age for the old. All subjects were non-smokers, free from diabetes and known cardiovascular, peripheral vascular, neuromuscular or pulmonary disease. Additionally, none of the subjects were taking statins or other medications recognized to affect muscle function. Pre-menopausal women were studied during days 1–7 of their menstrual cycle to standardize the influence of female hormones. Women taking hormone replacement therapy were excluded from the study. The study was approved by the Human Research Protection Program of the University of Utah and the Salt Lake City VAMC.

Table 1
Subject characteristics

Data expressed as means±S.D. BMI, body mass index; HDL, high-density lipoprotein; LDL, low-density lipoprotein; WBC, white blood cells; RBC, red blood cells. *P<0.05, significantly different from the old.

CharacteristicYoungOld
Sample size 10 
Age and anthropometrics   
 Age (years) 25±3 71±7* 
 Height (cm) 175±9 172±9 
 Weight (kg) 72±12 75±12 
 BMI (kg/m223±4 25±3 
 Thigh volume (dl) 62±18 58±14 
Function   
 Steps per day (n7288±2355 5907±2323 
 Moderate to vigorous activity (min/day) 44±21 27±13 
 Dynamic knee-extensor peak power (Watts) 46±9 27±13* 
Blood and plasma   
 Glucose (mg/dl) 78±13 84±13 
 Cholesterol (mg/dl) 176±21 193±36 
 Triacylglycerols (mg/dl) 106±28 126±61 
 HDL (mg/dl) 54±13 49±12 
 LDL (mg/dl) 110±21 126±32 
 WBC (×103/μl) 5.5±1.2 5.4±1.1 
 RBC (×106/μl) 5.4±0.3 5.0±0.5 
 Haemoglobin (g/dl) 16.1±0.9 15.2±1.6 
 Haematocrit (%) 46±3 45±4 
 Neutrophil (×103/μl) 17±24 3±1 
 Lymphocyte (×103/μl) 2.2±0.6 1.6±0.5 
 Monocyte (×103/μl) 0.4±0.1 0.5±0.1 
CharacteristicYoungOld
Sample size 10 
Age and anthropometrics   
 Age (years) 25±3 71±7* 
 Height (cm) 175±9 172±9 
 Weight (kg) 72±12 75±12 
 BMI (kg/m223±4 25±3 
 Thigh volume (dl) 62±18 58±14 
Function   
 Steps per day (n7288±2355 5907±2323 
 Moderate to vigorous activity (min/day) 44±21 27±13 
 Dynamic knee-extensor peak power (Watts) 46±9 27±13* 
Blood and plasma   
 Glucose (mg/dl) 78±13 84±13 
 Cholesterol (mg/dl) 176±21 193±36 
 Triacylglycerols (mg/dl) 106±28 126±61 
 HDL (mg/dl) 54±13 49±12 
 LDL (mg/dl) 110±21 126±32 
 WBC (×103/μl) 5.5±1.2 5.4±1.1 
 RBC (×106/μl) 5.4±0.3 5.0±0.5 
 Haemoglobin (g/dl) 16.1±0.9 15.2±1.6 
 Haematocrit (%) 46±3 45±4 
 Neutrophil (×103/μl) 17±24 3±1 
 Lymphocyte (×103/μl) 2.2±0.6 1.6±0.5 
 Monocyte (×103/μl) 0.4±0.1 0.5±0.1 

Exercise protocols

On an initial day, participants were familiarized with all testing procedures and performed preliminary assessments. On another day, blood samples were collected to perform a complete blood cell count and dynamic single leg knee-extensor peak power was determined on a knee-extensor ergometer. Subject-specific knee-extensor protocols, designed to reach exhaustion within 8–12 min, consisting of 2–5 W/min increments (60 rpm), were used, as previously described [17].

On the final day, subjects performed the two modes of isometric knee-extension exercise in the whole body MRI system (TimTrio 3T Siemens Medical Systems). While supine, the knee of the dominant leg was positioned at a ~45° knee joint angle over a custom-built knee support and the foot attached to a strain gauge (SSM-AJ-250, Interface). To minimize hip movement and back extension during the contraction, participants were secured to the bed with a non-elastic strap placed over the hips and the thigh. The force signal was converted from analogue-to-digital (MP150, Biopac Syst) and collected with a sample frequency of 100 Hz on a personal computer (Acknowledge, Biopac Syst). Utilizing this equipment, initially each subject performed two baseline maximum voluntary contractions (MVC) of ~5–10 s duration, separated by 1 min of recovery. After ~10 min of additional rest and 4 min of baseline data collection, subjects performed two exercise bouts of 24 s, each followed by 5 min of recovery. The exercise bouts consisted of either a maximal continuous contraction throughout the 24 s or a maximal intermittent contraction (2 s contraction, 2 s relaxation) for the same duration. The torque-time integral (TTI, N·m) was averaged over 6 s to match the time resolution of the 31P-MRS data. To minimize any potential ordering effects, the protocols were performed in a balanced design. All experimental trials were performed in a thermoneutral environment, with the subjects fasted overnight.

31P-MRS

31P-MRS was performed using a clinical 3T MRI system (Tim-Trio, Siemens Medical Solutions) operating at 49.9 MHz for 31P resonance. 31P-MRS data were acquired with a 31P–1H dual surface coil with linear polarization (Rapid Biomedical) positioned above the quadriceps at the mid-thigh level in order to sample all major muscles of the quadriceps. The 31P single-loop coil diameter was 125 mm surrounding a 110 mm 1H coil loop. After a three-plane scout proton imaging, advanced localized volume shimming was performed. Before each experiment, two fully relaxed spectra were acquired at rest with three averages per spectrum and a repetition time of 30 s. Then, MRS data acquisition was performed throughout the rest-exercise-recovery protocol using a similar free-induction-decay (FID) pulse sequence with a 2.56 ms adiabatic-half-passage excitation RF pulse and the following parameters (repetition time=2 s, receiver bandwidth =5 kHz, 1024 data points, and three averages per spectrum). Saturation factors were quantified by the comparison between fully relaxed (TR=30 s) and partially relaxed spectra (TR=2 s).

As previously described [18], relative concentrations of phosphocreatine [PCr], inorganic phosphate [Pi], phosphodiester (PDE) and [ATP] were obtained by a time-domain fitting routine using the AMARES algorithm [19] incorporated into the CSIAPO software [20]. Intracellular pH was calculated from the chemical shift difference between the Pi and PCr signals. The free cytosolic [ADP] was calculated from [PCr] and pH using the creatine kinase equilibrium constant (KCK=1.66×109 M−1) and the assumption that PCr represents 85% of the total creatine content [21]. The resting concentrations were calculated from the average peak areas of the two relaxed spectra (TR=30 s; n=3) recorded at rest and assuming an 8.2 mM [ATP] under these conditions. Changes in both pH and the concentrations of phosphorus metabolites during contraction were used to calculate the ATP cost of contraction, as previously described by members of our group [22] and others groups [23,24]. Due to the isometric nature of the continuous contraction protocol and the potential for vascular occlusion induced by high intramuscular pressure, proton (H+) efflux was assumed to be zero [23] in this condition, and was estimated during the intermittent protocol [22]. To ensure that our findings were not confounded by the model of metabolic control utilized, two differing methods were employed to quantify the rate of oxidative ATP production. The Indirect model did not rely on any assumptions related to models of respiratory control, with the rate of oxidative ATP production at the end of exercise estimated from the initial rate of [PCr] recovery, as previously described [23] and validated (method #1 in [22]). The ADP control model assumed a sigmoid relationship between the rate of oxidative ATP synthesis and free cytosolic ADP concentration [25]. Then, total ATP synthesis generated from aerobic and anaerobic pathways were scaled to the torque produced (N·cm), an approach that has previously been documented to provide a valid and reliable estimate of the total cost of contraction (method #2 in [22], and [24,26]).

Thigh volume

Thigh volume was calculated based on lower leg circumference (three sites: distal, middle and proximal), lower leg length and skinfold measurements [27], which has demonstrated excellent agreement with 1H-MR imaging across a wide spectrum of adults [28].

Physical activity level

All subjects completed a modified physical activity questionnaire included items regarding the average type, frequency, intensity and duration of physical activity in any given week. Additionally, after receiving standardized operating instructions, subjects wore an accelerometer (GT1M; Actigraph) for seven consecutive days, with adherence automatically assessed by the data collected. Average daily physical activity was expressed as steps per day and the time spent performing moderate to vigorous physical activity.

Data analysis

The PCr recovery kinetic was determined by fitting the PCr time-dependent changes during the recovery period to a single exponential curve described by the following equation:

 
formula
(1)

where [PCr]end is the concentration of [PCr] measured at end-of-exercise and [PCr]cons refers to the amount of PCr consumed at the end of the exercise session. The initial rate of PCr resynthesis (ViPCr) was calculated as follows:

 
formula
(2)

in which [PCr]cons indicates the amount of PCr consumed at end of exercise and the rate constant, k=1/τ.

Model variables were determined with an iterative process by minimizing the sum of squared residuals (RSS) between the fitted function and the observed values. Goodness of fit was assessed by visual inspection of the residual plot and the frequency plot distribution of the residuals, χ2 values and the coefficient of determination (r2) calculated as follows [29]:

 
formula
(3)

where SSreg is the sum of squares of the residuals from the fit and SStot is the sum of squares of the residuals from the mean.

Statistical analysis

The assessment of differences between young and old was performed with either independent Student's t tests or non-parametric Mann–Whitney tests, where appropriate (Statsoft, version 5.5; Statistica). A two-way repeated ANOVA was used to identify changes in measured variables within (time) and between conditions (mode of contraction) and groups. Following a significant interaction effect pairwise comparisons were made using Tukey HSD. Potential relationships between variables were analysed using the Pearson test or the non-parametric Spearman rank-order correlation. Statistical significance was accepted at P<0.05. Results are presented as means±S.D. in the Tables and means±S.E.M. in the Figures for clarity.

RESULTS

Subject characteristics

As documented in Table 1, the young and the old were well matched anthropometrically and in general functional measures, with the exception of dynamic knee-extensor peak power where the old exhibited a significantly attenuated capacity. Blood and plasma analyses also revealed very similar values between the young and old, emphasizing the relative healthy status of the old subjects.

Mechanical measurements

Representative examples of the force produced by the continuous and intermittent protocols are presented in Figure 1. The TTI over the entire exercise bout was significantly greater in both groups during the continuous contraction compared with intermittent contractions (P<0.01 for both young and old). Additionally, the TTI at the end of exercise was significantly higher in the young compared with the old during both the continuous and intermittent protocols (see Figure 4).

Representative examples in young and old of the force produced by the knee-extensors over time during the continuous and intermittent contraction protocols

Figure 1
Representative examples in young and old of the force produced by the knee-extensors over time during the continuous and intermittent contraction protocols
Figure 1
Representative examples in young and old of the force produced by the knee-extensors over time during the continuous and intermittent contraction protocols

High-energy phosphate compounds and intracellular pH

An example of the MR spectra acquired from the knee-extensor muscles during the continuous and intermittent contraction protocols is illustrated in Figure 2. Table 2 summarizes intracellular metabolite concentrations and pH at rest and at the end of the continuous and intermittent contraction protocols in the young and old. The group mean changes in PCr and pH during these two exercise paradigms in the young and old are illustrated in Figure 3. Specifically, there was no significant interaction between time and age during exercise for all the variables.

Table 2
Metabolic indices at rest and during both continuous and intermittent contractions of the knee-extensors in the young and old subjects

Data expressed as means±S.D. *P<0.05, significantly different from OLD.

ParameterYoungOld
Resting concentrations   
 PCr (mM) 31±4 35±5 
 Pi (mM) 1.2±0.5 1.6±0.3 
 ADP (μM) 8.3±0.7 8.3±0.7 
 pH 6.98±0.04 6.98±0.03 
 PDE (mM) 0.7±0.7 1.7±1.0* 
End-exercise concentrations   
 Continuous contraction   
  PCr (mM) 10±5 14±4 
  Pi (mM) 11±3 10±3 
  ADP (μM) 129±57 114±41 
  pH 6.94±0.09 7.02±0.05* 
 Intermittent contractions   
  PCr (mM) 16±5 17±4 
  Pi (mM) 9±2 8±3 
  ADP (μM) 73±24 84±49 
  pH 7.01±0.05 7.03±0.04 
ParameterYoungOld
Resting concentrations   
 PCr (mM) 31±4 35±5 
 Pi (mM) 1.2±0.5 1.6±0.3 
 ADP (μM) 8.3±0.7 8.3±0.7 
 pH 6.98±0.04 6.98±0.03 
 PDE (mM) 0.7±0.7 1.7±1.0* 
End-exercise concentrations   
 Continuous contraction   
  PCr (mM) 10±5 14±4 
  Pi (mM) 11±3 10±3 
  ADP (μM) 129±57 114±41 
  pH 6.94±0.09 7.02±0.05* 
 Intermittent contractions   
  PCr (mM) 16±5 17±4 
  Pi (mM) 9±2 8±3 
  ADP (μM) 73±24 84±49 
  pH 7.01±0.05 7.03±0.04 

Representative examples of a rapid 1H imaging transverse slice, illustrating the spatial sensitivity of the coil, and a stack plot of 31P spectra acquired from the knee-extensor muscles of a single subject during rest (1 min) and two exercise (Ex.) bouts (24 s) consisting of continuous and intermittent contractions each followed by 5 min of recovery

Figure 2
Representative examples of a rapid 1H imaging transverse slice, illustrating the spatial sensitivity of the coil, and a stack plot of 31P spectra acquired from the knee-extensor muscles of a single subject during rest (1 min) and two exercise (Ex.) bouts (24 s) consisting of continuous and intermittent contractions each followed by 5 min of recovery
Figure 2
Representative examples of a rapid 1H imaging transverse slice, illustrating the spatial sensitivity of the coil, and a stack plot of 31P spectra acquired from the knee-extensor muscles of a single subject during rest (1 min) and two exercise (Ex.) bouts (24 s) consisting of continuous and intermittent contractions each followed by 5 min of recovery

PCr (top panels) and pH (bottom panels) over time during the continuous and intermittent contraction protocols in young and old

Figure 3
PCr (top panels) and pH (bottom panels) over time during the continuous and intermittent contraction protocols in young and old

Data are presented as means±S.E.M. There was no interaction effect between time and age.

Figure 3
PCr (top panels) and pH (bottom panels) over time during the continuous and intermittent contraction protocols in young and old

Data are presented as means±S.E.M. There was no interaction effect between time and age.

ATP synthesis during exercise

For the sake of clarity and given that the two approaches (Indirect and ADP models) employed to quantify the rates of oxidative ATP production and the ATP cost of contraction yielded similar results and conclusions, only the findings from the indirect method are presented here. The rates of ATP synthesis from glycolysis, the creatine kinase reaction, and oxidative phosphorylation are presented in Table 3 and were not significantly different between young and old or the continuous and intermittent contraction protocols (P>0.05, Table 3). Consequently, total ATP synthesis rate was not significantly different between the young and old during the continuous and intermittent contraction protocols (Figure 4). Given the similar values between groups (62±18 dl in the young and 58±14 dl in the old, Table 1) and to preserve statistical power, the ATP cost of contraction was not normalized for muscle volume. At the end of the continuous contraction protocol, the ATP cost of contraction in the young (0.06±0.02 mM·min−1·N·m−1) was not significantly different from the old (0.10±0.07 mM·min−1·N·m−1, P=0.2). The ATP cost of contraction during the intermittent contractions was significantly increased compared with the continuous contraction protocol in the young (P<0.05), and this was true, but to an even greater extent, in the old, such that the ATP cost was significantly greater in the old compared with the young during the intermittent contractions (P<0.05, Figure 4). In addition, when the data from young and old subjects were pooled, the ATP cost of contraction during the intermittent protocol was significantly inversely correlated with dynamic single leg knee-extensor peak power (P<0.05, Figure 5).

Table 3
ATP synthesis rates calculated by the Indirect model at the end of both the continuous and intermittent contractions of the knee-extensors in the young and old

Data expressed as means±S.D.

ProtocolYoungOld
Continuous contraction protocol   
 ATP synthesis (mM·min−1  
  Glycolysis 22±9 15±16 
  Creatine kinase 13±7 17±16 
  Oxidative phosphorylation 19±5 17±5 
Intermittent contractions protocol   
 ATP synthesis (mM·min−1  
  Glycolysis 14±9 17±15 
  Creatine kinase 7±8 10±10 
  Oxidative phosphorylation 16±7 16±7 
ProtocolYoungOld
Continuous contraction protocol   
 ATP synthesis (mM·min−1  
  Glycolysis 22±9 15±16 
  Creatine kinase 13±7 17±16 
  Oxidative phosphorylation 19±5 17±5 
Intermittent contractions protocol   
 ATP synthesis (mM·min−1  
  Glycolysis 14±9 17±15 
  Creatine kinase 7±8 10±10 
  Oxidative phosphorylation 16±7 16±7 

TTI, total ATP synthesis rates and ATP cost of contraction at the end of the continuous and intermittent contraction protocols in young and old subjects using the Indirect model to calculate the rate of oxidative ATP production

Figure 4
TTI, total ATP synthesis rates and ATP cost of contraction at the end of the continuous and intermittent contraction protocols in young and old subjects using the Indirect model to calculate the rate of oxidative ATP production

Data are presented as means±S.E.M. *P<0.05 and **P<0.01, significantly different from the old.

Figure 4
TTI, total ATP synthesis rates and ATP cost of contraction at the end of the continuous and intermittent contraction protocols in young and old subjects using the Indirect model to calculate the rate of oxidative ATP production

Data are presented as means±S.E.M. *P<0.05 and **P<0.01, significantly different from the old.

The significant inverse relationship between dynamic single leg knee-extensor peak power and the ATP cost of contraction during the intermittent contraction protocol

Figure 5
The significant inverse relationship between dynamic single leg knee-extensor peak power and the ATP cost of contraction during the intermittent contraction protocol

The dotted line represents the 95% confidence interval.

Figure 5
The significant inverse relationship between dynamic single leg knee-extensor peak power and the ATP cost of contraction during the intermittent contraction protocol

The dotted line represents the 95% confidence interval.

DISCUSSION

Utilizing 31P-MRS and two different modes of contraction with the quadriceps, this study sought to examine whether (i) the ATP cost of muscle contraction is modulated by age, and (ii) the relative contribution from cross-bridge cycling and ion pumping to this potential age-related alteration in ATP cost. Consistent with our hypotheses, the ATP cost of intermittent contractions, requiring energy for both contractile and non-contractile processes, was exaggerated in healthy older individuals. On the other hand, the ATP cost of a continuous contraction, predominantly requiring energy production for cross-bridge cycling, was similar between the young and old. In addition, the ATP cost of intermittent contractions was significantly correlated with peak power of the knee-extensors. Together, these findings reveal an age-related increase in the ATP cost of contraction, which is probably mediated by an excessive energy demand from ion pumping, and probably contributes to the decline in muscle efficiency and functional capacity with age.

Evidence of an increased metabolic cost of contraction with age

A key and novel finding of this study is the documentation of an excessive ATP cost of intermittent contractions in the old compared with the young (Figure 4). As intermittent exercise requires energy for both contractile (i.e. myosin ATPase) and non-contractile processes (i.e. Na+–K+ and Ca2+ ATPases), this finding alone, although of importance, does not partition the relative importance of these two components in terms of the greater ATP cost of contraction with age. Interestingly, however, this finding of an excessive ATP cost of contraction contrasts with a recent study reporting a similar cost of contraction in the quadriceps of young and old subjects [5]. Nevertheless, although a very interesting study, there are some methodological concerns with that work. Specifically, the cost of muscle contraction was estimated based on elaborate calculations which combined peak power output attained during incremental cycling exercise and the level of free energy of ATP hydrolysis (∆GATP) elicited during electrical stimulation of the quadriceps. Such an approach does not take into account the differences in exercise intensity and muscle recruitment between these two very different exercise modalities [30]. Furthermore, the assumption of complete and constant recruitment during exercise in both young and old (fraction of thigh volume recruited of 0.49 in the young and 0.44 in the old) is not supported by previous studies indicating substantial inter-individual heterogeneity in muscle recruitment during cycling [31]. It is also well recognized that during cycle exercise only a small fraction (less than 30%) of the muscle fibres are actually recruited at maximal aerobic power [32,33]. Additionally, it is important to note that, unlike the present study, subjects were not matched for physical activity.

Evidence of a preserved metabolic cost of cross-bridge cycling with age

One of the experimental paradigms employed here, isometric exercise with constant force generation, was designed to minimize the activity of the ionic pump such that the metabolic demand predominantly originated from the contractile apparatus (i.e. myosin ATPase). Accordingly, the finding that ATP cost in the older individuals was similar to that of the young subjects during this protocol (Figure 4) suggests that contractile efficiency or the energetic cost associated with static muscle force production is unaffected by age. Importantly, this conclusion was confirmed by applying two differing methods (ADP and Indirect model) to quantify the cost of muscle contraction in vivo. Consistent with this conclusion, a review of the literature indicated a preserved and even improved fatigue resistance during isometric contractions in older individuals [34]. Indeed, in previous years, there has been growing evidence suggesting that the force-generating capacity and kinetic properties assessed during muscle shortening are preserved with age in the knee-extensors [12,13,3539]. For instance, a comparable rate of force development and specific strength (force normalized by cross-sectional area) evoked by electrical stimulation has been documented in the quadriceps of older individuals in comparison with their younger counterparts [12,38]. Likewise, elderly individuals (~80 years) exhibited only a modest (~11%) increase in the time to peak tension induced by electrical stimulation and this was associated with only a trivial leftward shift in the force–frequency relationship [13]. In combination, these observations are consistent with relatively well preserved cross-bridge function and cost of contraction with age.

Consistent with the concept of preserved intrinsic cross-bridge kinetics and force-generating capacity in the skeletal muscle of older individuals is the documentation of an unaltered myofilament function in a skinned fibre preparation, an experimental model which is not confounded by neural influences and fibre architecture [40]. Additional in vitro studies have documented unaltered specific force, shortening velocity and cross-bridge cycling kinetic measurements in muscle from the knee-extensors of older individuals [35,3739]. Furthermore, it has been reported that aging itself does not modulate protein expression of the myofilament (myosin heavy chain), the sarcoplasmic reticulum (receptor ryanodine) and the sensitivity to Ca2+ release (force–Ca2+ relationship) [16,35,41], which is governed by the expression of troponin, tropomyosin and myosin light chains [42]. It is, however, important at this point to acknowledge that conflicting results from similar in vitro studies have been reported on this topic [14,4345]. Nevertheless, these discrepancies can most probably be explained by the limited sample size (multiple fibres from two to three subjects) and the failure to control for physical activity of the subjects. Thus, intrinsic skeletal muscle contractile function and efficiency appear not to be impaired with advancing age, and are therefore unlikely to account for the previously reported decline in muscle efficiency of the knee-extensors [46].

Evidence of an increased metabolic cost from non-contractile processes with age

One of the experimental paradigms employed here, intermittent isometric exercise, was designed to require energy consumption by both contractile (i.e. myosin ATPase) and non-contractile processes (i.e. Na+–K+ and Ca2+ ATPases). The novel observation of a greater ATP cost of contraction in the old compared with the young during this form of exercise, but not during continuous isometric exercise (Figure 4), implies an excessive energy demand from non-contractile processes (i.e. Na+–K+ and Ca2+ ATPases) with age. The sarcoplasmic reticulum Ca2+ ATPase (SERCA) is primarily responsible for muscle relaxation by transporting cytosolic Ca2+ into the lumen of the sarcoplasmic reticulum coupled to ATP hydrolysis. Therefore, impairment in Ca2+ sequestration by the sarcoplasmic reticulum could be responsible for this exaggerated metabolic demand during an exercise requiring repeated contraction–relaxation cycle. In line with this concept, slower rates of relaxation after contractions evoked by electrical stimulation have been consistently documented in the knee-extensors of older subjects [12,13,15,36] and associated with lower Ca2+ uptake and Ca2+-ATPase activity measured in vitro [15,16]. Furthermore, ATP hydrolysis and Ca2+ transport has been suggested to become uncoupled with age [46], playing into this age-associated decrement in Ca2+ handling.

The mechanisms responsible for this deleterious effect of age on Ca2+ sequestration are unclear, but chronic oxidative stress commonly associated with aging may play a role. Indeed, SERCA proteins are sensitive to muscle redox state as these proteins are typically characterized by a relatively long half-life, with turnover being even more prolonged with age [47]. Indeed, nitrotyrosine, a marker of oxidative stress, has previously been reported to accumulate on SERCA in an age-dependent manner [48]. In light of these observations, it is interesting to note that the old subjects in the present study exhibited a higher resting concentration of PDE than their younger counterparts (Table 2). Glycerophosphocholine, the primary PDE in human skeletal muscle [49], is an important metabolite of cell membrane turnover arising from the breakdown of membrane phospholipids [50]. Therefore, the increased level of PDE is suggestive of higher membrane breakdown and is in agreement with the concept that the accumulation of oxidative damage on cell membranes and SERCA would then compromise Ca2+ pumping and increase ATP demand. Although, the Na+–K+ pump is another ATP-dependent process activated during muscle contraction, it has been consistently demonstrated that total Na+–K+ protein content and function are not affected by age [5153]. Additionally, as Na+–K+ pumps account for a negligible proportion (less than ~7%) of total ATP turnover [54] this is an unlikely explanation for the large increase in metabolic cost from non-contractile processes with age, identified in the current study.

The relative ATP use from contractile (myosin ATPase) and non-contractile processes (Ca2+ and Na+–K+ ATPases) has been extensively studied in vitro [54]. It has thus been estimated that the energy demand from non-contractile processes represents 30–50% of the total energy consumed by contracting muscle in human [55] and rodents [54,56]. Consistent with this in vitro estimate, using two approaches to quantify total ATP turnover, the young subjects in the current study exhibited a ~50% increase in the ATP cost during intermittent contractions (ATP demand from both contractile and non-contractile processes) in comparison with the continuous and constant contraction (ATP demand predominantly from the contractile process). Although, some minimal contribution from Ca2+ and Na+–K+ pumping to ATP demand cannot be completely ruled out during the continuous contraction protocol, the good agreement between previous in vitro results and the current in vivo estimate supports the validity of the present approach.

Perspectives and significance

For elderly individuals, preserved function of the knee-extensors is critical for everyday mobility and independent living. In this respect, our findings that the exaggerated increase in the ATP cost of intermittent contractions was correlated with the peak knee-extensor power (Figure 5) have important physiological and clinical implications. Indeed, these results reveal that the excessive cost associated with ion transport, potentially mediated by chronic oxidative stress, is an important mechanism contributing to the decline in muscle efficiency with age and probably compromises functional capacity in healthy older adults. Also, according to Whipp and Wasserman [1], muscle efficiency [oxygen consumption (W/V̇O2)] is determined to a similar extent by both contractile (Watt/ATP) and mitochondrial efficiency (ATP/V̇O2, conceptually similar to phosphorylative coupling). Interestingly, contractile and mitochondrial efficiencies can sometimes vary independently of each other, as recently demonstrated in the quadriceps muscle of older individuals [5], such that the decline in muscle efficiency commonly reported with aging can stem from a change in mitochondrial [5,79] and/or contractile efficiency. Thus, it is likely that these two explanations are not mutually exclusive and both may, to some extent, explain a greater cost of locomotion with age. Together, these findings highlight the need for development of therapies, perhaps exercise-based, designed to maintain muscle efficiency in older individuals.

Conclusion

In summary, this study provides novel mechanistic insight into the age-related decline in quadriceps muscle efficiency. Specifically, our results illustrate that a higher ATP cost of contraction contributes significantly to the decline in muscle efficiency observed with age and appears to be mediated by an excessive energy demand from non-contractile processes which is associated with compromised functional capacity.

Abbreviations

     
  • MRS

    magnetic resonance spectroscopy

  •  
  • PCr

    phosphocreatine

  •  
  • PDE

    phosphodiester

  •  
  • Pi

    inorganic phosphate

  •  
  • SERCA

    sarcoplasmic reticulum Ca2+ ATPase

  •  
  • TTI

    torque-time integral

AUTHOR CONTRIBUTION

Gwenael Layec, Eun-Kee Jeong and Russ Richardson conceived and designed the study; Gwenael Layec, Joel Trinity and Corey Hart collected the data; Gwenael Layec, Corey Hart and Yann Le Fur analysed the data; Gwenael Layec performed the statistical analysis, interpreted results, prepared the Figures and drafted the paper; Gwenael Layec, Joel Trinity, Corey Hart, Yann Le Fur, Eun-Kee Jeong and Russ Richardson edited and revised the paper. All authors approved the final version of the paper.

We thank all the subjects in this study for their committed participation in this research.

FUNDING

This work was funded in part by the National Institutes of Health National Heart, Lung, and Blood Institute [grant number PO1 HL 091830] and a VA Merit grant [grant number E6910R].

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