Niels Ørtenblad

Effects of aging on human skeletal muscle after immobilization and retraining

Inactivity is a recognized compounding factor in sarcopenia and muscle weakness in old age. However, while the negative effects of unloading on skeletal muscle in young individuals are well elucidated, only little is known about the consequence of immobilization and the regenerative capacity in elderly individuals. Thus the aim of this study was to examine the effect of aging on changes in muscle contractile properties, specific force, and muscle mass characteristics in 9 old (61-74 yr) and 11 young men (21-27 yr) after 2 wk of immobilization and 4 wk of retraining. Both young and old experienced decreases in maximal muscle strength, resting twitch peak torque and twitch rate of force development, quadriceps muscle volume, pennation angle, and specific force after 2 wk of immobilization (P < 0.05). The decline in quadriceps volume and pennation angle was smaller in old compared with young (P < 0.05). In contrast, only old men experienced a decrease in quadriceps activation. After retraining, both young and old regained their initial muscle strength, but old had smaller gains in quadriceps volume compared with young, and pennation angle increased in young only (P < 0.05). The present study is the first to demonstrate that aging alters the neuromuscular response to short-term disuse and recovery in humans. Notably, immobilization had a greater impact on neuronal motor function in old individuals, while young individuals were more affected at the muscle level. In addition, old individuals showed an attenuated response to retraining after immobilization compared with young individuals.

Increased subsarcolemmal lipids in type 2 diabetes : effect of training on localization of lipids, mitochondria, and glycogen in sedentary human skeletal muscle.

The purpose of the study was to investigate the effect of aerobic training and type 2 diabetes on intramyocellular localization of lipids, mitochondria, and glycogen. Obese type 2 diabetic patients (n = 12) and matched obese controls (n = 12) participated in aerobic cycling training for 10 wk. Endurance-trained athletes (n = 15) were included for comparison. Insulin action was determined by euglycemic-hyperinsulinemic clamp. Intramyocellular contents of lipids, mitochondria, and glycogen at different subcellular compartments were assessed by transmission electron microscopy in biopsies obtained from vastus lateralis muscle. Type 2 diabetic patients were more insulin resistant than obese controls and had threefold higher volume of subsarcolemmal (SS) lipids compared with obese controls and endurance-trained subjects. No difference was found in intermyofibrillar lipids. Importantly, following aerobic training, this excess SS lipid volume was lowered by approximately 50%, approaching the levels observed in the nondiabetic subjects. A strong inverse association between insulin sensitivity and SS lipid volume was found (r(2)=0.62, P = 0.002). The volume density and localization of mitochondria and glycogen were the same in type 2 diabetic patients and control subjects, and showed in parallel with improved insulin sensitivity a similar increase in response to training, however, with a more pronounced increase in SS mitochondria and SS glycogen than in other localizations. In conclusion, this study, estimating intramyocellular localization of lipids, mitochondria, and glycogen, indicates that type 2 diabetic patients may be exposed to increased levels of SS lipids. Thus consideration of cell compartmentation may advance the understanding of the role of lipids in muscle function and type 2 diabetes.

Effects of aging on muscle mechanical function and muscle fiber morphology during short-term immobilization and subsequent retraining

Very little attention has been given to the combined effects of aging and disuse as separate factors causing deterioration in muscle mechanical function. Thus the purpose of this study was to investigate the effects of 2 wk of immobilization followed by 4 wk of retraining on knee extensor muscle mechanical function (e.g., maximal strength and rapid force capacity) and muscle fiber morphology in 9 old (OM: 67.3 ± 1.3 yr) and 11 young healthy men (YM: 24.4 ± 0.5 yr) with comparable levels of physical activity. Following immobilization, OM demonstrated markedly larger decreases in rapid force capacity (i.e., rate of force development, impulse) than YM (∼ 20-37 vs. ∼ 13-16%; P < 0.05). In contrast, muscle fiber area decreased in YM for type I, IIA, and IIx fibers (∼ 15-30%; P < 0.05), whereas only type IIa area decreased in OM (13.2%; P < 0.05). Subsequent retraining fully restored muscle mechanical function and muscle fiber area in YM, whereas OM showed an attenuated recovery in muscle fiber area and rapid force capacity (tendency). Changes in maximal isometric and dynamic muscle strength were similar between OM and YM. In conclusion, the present data reveal that OM may be more susceptible to the deleterious effects of short-term muscle disuse on muscle fiber size and rapid force capacity than YM. Furthermore, OM seems to require longer time to recover and regain rapid muscle force capacity, which may lead to a larger risk of falling in aged individuals after periods of short-term disuse.

Role of glycogen availability in sarcoplasmic reticulum Ca2+ kinetics in human skeletal muscle

Glucose is stored as glycogen in skeletal muscle. The importance of glycogen as a fuel during exercise has been recognized since the 1960s; however, little is known about the precise mechanism that relates skeletal muscle glycogen to muscle fatigue. We show that low muscle glycogen is associated with an impairment of muscle ability to release Ca2+, which is an important signal in the muscle activation. Thus, depletion of glycogen during prolonged, exhausting exercise may contribute to muscle fatigue by causing decreased Ca2+ release inside the muscle. These data provide indications of a signal that links energy utilization, i.e. muscle contraction, with the energy content in the muscle, thereby inhibiting a detrimental depletion of the muscle energy store.

Muscle glycogen stores and fatigue

Abstract  Studies performed at the beginning of the last century revealed the importance of carbohydrate as a fuel during exercise, and the importance of muscle glycogen on performance has subsequently been confirmed in numerous studies. However, the link between glycogen depletion and impaired muscle function during fatigue is not well understood and a direct cause‐and‐effect relationship between glycogen and muscle function remains to be established. The use of electron microscopy has revealed that glycogen is not homogeneously distributed in skeletal muscle fibres, but rather localized in distinct pools. Furthermore, each glycogen granule has its own metabolic machinery with glycolytic enzymes and regulating proteins. One pool of such glycogenolytic complexes is localized within the myofibrils in close contact with key proteins involved in the excitation–contraction coupling and Ca2+ release from the sarcoplasmic reticulum (SR). We and others have provided experimental evidence in favour of a direct role of decreased glycogen, localized within the myofibrils, for the reduction in SR Ca2+ release during fatigue. This is consistent with compartmentalized energy turnover and distinctly localized glycogen pools being of key importance for SR Ca2+ release and thereby affecting muscle contractility and fatigability.

Human skeletal muscle glycogen utilization in exhaustive exercise: role of subcellular localization and fibre type

Non‐technical summary  During prolonged high‐intensity exercise the main fuel for muscular work is glycogen, the storage form of glucose in skeletal muscle. The role of muscle glycogen in muscle function is best demonstrated by the inability to sustain prolonged high‐intensity exercise when the glycogen stores are depleted. Despite this knowledge, the reason why muscle function is depressed when glycogen levels are low is still not known. We show that after prolonged exhaustive exercise the depletion of glycogen stores is dependent on its localization within the muscle cells. These results show that consideration of distinct localizations within the muscle cells may advance understanding of how and why low muscle glycogen content impairs muscle function.

Distinct effects of subcellular glycogen localization on tetanic relaxation time and endurance in mechanically skinned rat skeletal muscle fibres

In vitro experiments indicate a non‐metabolic role of muscle glycogen in contracting skeletal muscles. Since the sequence of events in excitation–contraction (E–C) coupling is known to be located close to glycogen granules, at specific sites on the fibre, we hypothesized that the distinct compartments of glycogen have specific effects on muscle fibre contractility and fatigability. Single skeletal muscle fibres (n= 19) from fed and fasted rats were mechanically skinned and divided into two segments. In one segment glycogen localization and volume fraction were estimated by transmission electron microscopy. The other segment was mechanically skinned and, in the presence of high and constant myoplasmic ATP and PCr, electrically stimulated (10 Hz, 0.8 s every 3 s) eliciting repeated tetanic contractions until the force response was decreased by 50% (mean ±s.e.m., 81 ± 16, range 22–252 contractions). Initially the total myofibrillar glycogen volume percentage was 0.46 ± 0.07%, with 72 ± 3% in the intermyofibrillar space and 28 ± 3% in the intramyofibrillar space. The intramyofibrillar glycogen content was positively correlated with the fatigue resistance capacity (r2= 0.32, P= 0.02). Intermyofibrillar glycogen was inversely correlated with the half‐relaxation time in the unfatigued tetanus (r2= 0.25, P= 0.03). These results demonstrate for the first time that two distinct subcellular populations of glycogen have different roles in contracting single muscle fibres under conditions of high myoplasmic ATP.

Excitability of the T‐tubular system in rat skeletal muscle: roles of K+ and Na+ gradients and Na+–K+ pump activity

Strenuous exercise causes an increase in extracellular [K+] and intracellular Na+ ([Na+]i) of working muscles, which may reduce sarcolemma excitability. The excitability of the sarcolemma is, however, to some extent protected by a concomitant increase in the activity of muscle Na+–K+ pumps. The exercise‐induced build‐up of extracellular K+ is most likely larger in the T‐tubules than in the interstitium but the significance of the cation shifts and Na+–K+ pump for the excitability of the T‐tubular membrane and the voltage sensors is largely unknown. Using mechanically skinned fibres, we here study the role of the Na+–K+ pump in maintaining T‐tubular function in fibres with reduced chemical K+ gradient. The Na+–K+ pump activity was manipulated by changing [Na+]i. The responsiveness of the T‐tubules was evaluated from the excitation‐induced force production of the fibres. Compared to control twitch force in fibres with a close to normal intracellular [K+] ([K+]i), a reduction in [K+]i to below 60 mm significantly reduced twitch force. Between 10 and 50 mm Na+, the reduction in force depended on [Na+]i, the twitch force at 40 mm K+ being 22 ± 4 and 54 ± 9% (of control force) at a [Na+]i of 10 and 20 mm, respectively (n= 4). Double pulse stimulation of fibres at low [K+]i showed that although elevated [Na+]i increased the responsiveness to single action potentials, it reduced the capacity of the T‐tubules to respond to high frequency stimulation. It is concluded that a reduction in the chemical gradient for K+, as takes place during intensive exercise, may depress T‐tubular function, but that a concomitant exercise‐induced increase in [Na+]i protects T‐tubular function by stimulating the Na+–K+ pump.

Effects of ageing on single muscle fibre contractile function following short-term immobilisation

Non‐Technical Summary  The contractile function of human single muscle fibres is of particular importance for whole muscle contractile function. Yet, whereas ageing and short‐term disuse (immobilisation) separately have been shown to impair single fibre contractile function, very little attention has been given to their combined effects. We show that 2 weeks of lower limb immobilisation reduces force and specific force (force per cross‐sectional area) of both slow and fast single muscle fibres and that this occurred to a similar extent in young and old individuals. In contrast, disuse led to reduced Ca2+ sensitivity in fast fibres of young and in slow fibres of old, respectively. These results help us to better understand the underlying physiological mechanisms responsible for the deleterious effects of short‐term disuse on whole muscle contractile function in both young and old.

Aging impairs the recovery in mechanical muscle function following 4 days of disuse.

As aged individuals are frequently exposed to short-term disuse caused by disease or musculoskeletal injury, it is important to understand how short-term disuse and subsequent retraining affect lower limb mechanical muscle function. The purpose of the present study was, therefore, to investigate the effect of 4 days of lower limb disuse followed by 7 days of active recovery on mechanical muscle function of the knee extensors in young (24.3±0.9 years, n=11) and old (67.2±1.0 years, n=11) recreationally active healthy males. Slow and moderate dynamic muscle strength were assessed using isokinetic dynamometry (60 and 180° s(-1), respectively) along with isometric muscle strength and rapid muscle force capacity examined as contractile rate of force development (RFD), Impulse, and relative RFD (rRFD) during the initial phase of contraction (100 ms time interval relative to onset of contraction). Prior to disuse, marked age-related differences (p<0.05) were observed in isometric and dynamic muscle strength (~35%) as well as in RFD and Impulse (~39%). Following disuse, young and old individuals experienced comparable decrements (p<0.05) in isometric strength (~9%), slow dynamic strength (~13%), and RFD and Impulse (~19%), whereas old individuals only experienced decrements (p<0.05) in moderate dynamic strength (12%) and rRFD (~17%). Following recovery, all measures of mechanical muscle function were restored in young individuals compared to pre-disuse values, while isometric, slow and moderate dynamic muscle strength remained suppressed (p<0.05) in old individuals (~8%) along with a tendency to suppressed RFD100 (p=0.068). In conclusion, 4 days of lower limb disuse led to marked decrements in knee extensor mechanical muscle function in both young and old individuals, yet with greater decrements observed in moderate dynamic strength and rapid muscle force capacity in old individuals. While 7 days of recovery - including free ambulation, one test session and a single session of strength training - was sufficient to restore mechanical muscle function in young individuals, old individuals appeared to have an impaired ability to fully recover as evidenced by suppressed values of isometric and dynamic muscle strength and rapid muscle force capacity.