Single muscle fiber adaptations to resistance training in old (>80 yr) men: evidence for limited skeletal muscle plasticity

The purpose of this study was to investigate whole muscle and single muscle fiber adaptations in very old men in response to progressive resistance training (PRT). Six healthy independently living old men (82 +/- 1 yr; range 80-86 yr, 74 +/- 4 kg) resistance-trained the knee extensors (3 sets, 10 repetitions) at approximately 70% one repetition maximum 3 days/wk for 12 wk. Whole thigh muscle cross-sectional area (CSA) was assessed before and after PRT using computed tomography (CT). Muscle biopsies were obtained from the vastus lateralis before and after the PRT program. Isolated myosin heavy chain (MHC) I and IIa single muscle fibers (n = 267; 142 pre; 125 post) were studied for diameter, peak tension, shortening velocity, and power. An additional set of isolated single muscle fibers (n = 2,215; 1,202 pre; 1,013 post) was used to identify MHC distribution. One repetition maximum knee extensor strength increased (P < 0.05) 23 +/- 4 kg (56 +/- 4 to 79 +/- 7 kg; 41%). Muscle CSA increased (P < 0.05) 3 +/- 1 cm2 (120 +/- 7 to 123 +/- 7 cm2; 2.5%). Single muscle fiber contractile function and MHC distribution were unaltered with PRT. These data indicate limited muscle plasticity at the single-muscle fiber level with a resistance-training program among the very old. The minor increases in whole muscle CSA coupled with the static nature of the myocellular profile indicate that the strength gains were primarily neurological. These data contrast typical muscle responses to resistance training in young ( approximately 20 yr) and old ( approximately 70 yr) humans and indicate that the physiological regulation of muscle remodeling is adversely modified in the oldest old.     

Low-intensity training produces muscle adaptations in rats with femoral artery stenosis.

The effectiveness of a mild-intensity exercise program to induce adaptations within skeletal muscle of animals with peripheral arterial insufficiency was evaluated using an isolated perfused hindlimb preparation at a muscle blood flow similar to the peak found in vivo. Adult rats were subjected to bilateral femoral artery stenosis sufficient to limit peak blood flow during exercise but not alter resting blood flow. Stenosed-trained (Sten-Trained) rats walked on a treadmill at an easily achieved speed (20 m/min with a 15% grade) 5 days wk. Exercise tolerance improved from 10 min initially to 2 h/day. Non-stenosed-sedentary (Non-Sten-Sed) and stenosed-sedentary (Sten-Sed) animals were limited to cage activity. Oxygen delivery to the contracting muscles was similar among groups (7.0 +/- 0.4, 7.3 +/- 0.6, and 6.6 +/- 0.6 mumol.min-1.g-1 in Non-Sten-Sed, Sten-Sed, and Sten-Trained, respectively; n = 13 each). Force development was better maintained by Sten-Trained muscle (P less than 0.001) during a sequence of tetanic contraction conditions. Peak oxygen consumption was greater (P less than 0.05) in the Sten-Trained (5.23 +/- 0.34 mumol.min-1.g-1) than in Non-Sten-Sed (4.08 +/- 0.35) and Sten-Sed (4.34 +/- 0.37) rats. The increased peak oxygen extraction (P less than 0.05) by the muscle of the Sten-Trained rats (82.5 +/- 7.1% of oxygen inflow vs. 58.7 +/- 4.7 and 57.4 +/- 5.0%, respectively) was probably related to the increased muscle capillarity and mitochondrial enzyme activity.(ABSTRACT TRUNCATED AT 250 WORDS)     

Creatine kinase-MB isoenzyme adaptations in stressed human skeletal muscle of marathon runners

The creatine kinase (CK) isoenzyme composition was determined in serial gastrocnemius muscle biopsies obtained from 12 male marathon runners. The mean muscle CK-MB composition significantly increased after chronic exercise (training) from 5.3% (pretraining) to 7.7% (premarathon) as well as after acute exercise (postmarathon) to 10.5% of the total CK activity (P less than 0.05). However, no significant differences in total CK activities were detected. Additionally, mitochondrial CK and CK-BB isoenzymes were present in muscle homogenates. A significant correlation was observed in the increase in mean serum total CK (3,322 U/l) and CK-MB (174 U/l) activities 24 h after the race (r = 0.98, P less than 0.05). These results show that gastrocnemius muscle adapts to long-distance training and racing with increased CK-MB activities and imply that skeletal muscle is the major source of elevated serum CK-MB activities in marathon runners.     

Metabolic adaptations to training precede changes in muscle mitochondrial capacity.

To determine whether increases in muscle mitochondrial capacity are necessary for the characteristic lower exercise glycogen loss and lactate concentration observed during exercise in the trained state, we have employed a short-term training model involving 2 h of cycling per day at 67% maximal O2 uptake (VO2max) for 5-7 consecutive days. Before and after training, biopsies were extracted from the vastus lateralis of nine male subjects during a continuous exercise challenge consisting of 30 min of work at 67% VO2max followed by 30 min at 76% VO2max. Analysis of samples at 0, 15, 20, and 60 min indicated a pronounced reduction (P less than 0.05) in glycogen utilization after training. Reductions in glycogen utilization were accompanied by reductions (P less than 0.05) in muscle lactate concentration (mmol/kg dry wt) at 15 min [37.4 +/- 9.3 (SE) vs. 20.2 +/- 5.3], 30 min (30.5 +/- 6.9 vs. 17.6 +/- 3.8), and 60 min (26.5 +/- 5.8 vs. 17.8 +/- 3.5) of exercise. Maximal aerobic power, VO2max (l/min) was unaffected by the training (3.99 +/- 0.21 vs. 4.05 +/- 0.26). Measurements of maximal activities of enzymes representative of the citric acid cycle (succinic dehydrogenase and citrate synthase) were similar before and after the training. It is concluded that, in the voluntary exercising human, altered metabolic events are an early adaptive response to training and need not be accompanied by changes in muscle mitochondrial capacity.     

Large energetic adaptations of elderly muscle to resistance and endurance training.

This study determined the cellular energetic and structural adaptations of elderly muscle to exercise training. Forty male and female subjects (69.2 +/- 0.6 yr) were assigned to a control group or 6 mo of endurance (ET) or resistance training (RT). We used magnetic resonance spectroscopy and imaging to characterize energetic properties and size of the quadriceps femoris muscle. The phosphocreatine and pH changes during exercise yielded the muscle oxidative properties, glycolytic ATP synthesis, and contractile ATP demand. Muscle biopsies taken from the same site as the magnetic resonance measurements were used to determine myosin heavy chain isoforms, metabolite concentrations, and mitochondrial volume densities. The ET group showed changes in all energetic pathways: oxidative capacity (+31%), contractile ATP demand (-21%), and glycolytic ATP supply (-56%). The RT group had a large increase in oxidative capacity (57%). Only the RT group exhibited change in structural properties: a rise in mitochondrial volume density (31%) and muscle size (10%). These results demonstrate large energetic, but smaller structural, adaptations by elderly muscle with exercise training. The rise in oxidative properties with both ET and RT suggests that the aerobic pathway is particularly sensitive to exercise training in elderly muscle. Thus elderly muscle remains adaptable to chronic exercise, with large energetic changes accompanying both ET and RT.     

Functional adaptations of rat skeletal muscle arterioles to aerobic exercise training.

This study tested the hypothesis that both structural and functional adaptations of arterioles occur within the skeletal muscle of rats aerobically trained for 8-10 wk with treadmill exercise. The training regimen used has been shown to elicit a 37% increase in plantaris citrate synthase activity but did not result in an elevation in citrate synthase activity in the spinotrapezius or gracilis muscles of rats used in this study. In the in vivo resting spinotrapezius muscle, arteriole diameters were similar in sedentary (SED) and trained (TR) rats. However, large- (1A) and intermediate- (2A) sized arterioles dilated proportionately more in TR than in SED rats during 1- to 8-Hz muscle contractions, even though the passive mechanical properties (circumference-passive wall tension relationships) were similar between groups. Vascular casts demonstrated a trend for an increase in the number of small (3A) arterioles and an approximately 20% increase in the passive diameter of 1A and 2A arterioles in the spinotrapezius muscle of TR rats. In contrast, in the gracilis muscle, arteriole diameters and density were identical in SED and TR rats, but the capillary-to-muscle fiber ratio was approximately 15% higher in TR rats. The results suggest that aerobic exercise training can greatly increase functional vasodilation and induce a slight increase in vascular density in skeletal muscle tissues, even if the oxidative capacity of these tissues is not increased by the training regimen.     

Regionalized adaptations and muscle fiber proliferation in stretch-induced enlargement

The relative contribution of increases in fiber area to stretch-induced muscle enlargement was evaluated in the slow tonic fibers of the anterior latissimus dorsi of adult Japanese quails. A weight corresponding to 10% of the bird's body mass was attached to one wing. Thirty days of stretch in 34 birds averaged 171.8 +/- 13.5% increase in muscle mass and 23.5 +/- 0.8% increase in muscle fiber length. The volume density of noncontractile tissue increased in middle and distal regions of stretch-enlarged muscles. Mean fiber cross-sectional area increased 56.7 +/- 12.3% in the midregion of stretched muscles. Further analysis indicated slow beta-fiber hypertrophy occurred in proximal, middle, and distal regions; however, fast alpha-type fiber hypertrophy was limited to middle regions of stretched muscles. Stretched muscles had a significant increase in the frequency of slow beta-fibers that were less than 500 microns 2 in all regions and fast alpha-type fibers in middle and distal regions. Total fiber number was determined after nitric acid digestion of connective tissue in 10 birds. Fiber number increased 51.8 +/- 19.4% in stretched muscle. These results are the first to clearly show that muscle fiber proliferation contributes substantially to adult skeletal muscle stretch-induced enlargement, although we do not know whether the responses of the slow tonic anterior latissimus dorsi might be similar or different from mammalian twitch muscle.     

Skeletal muscle biochemical adaptations to exercise training in miniature swine

McAllister, Richard M., Brian L. Reiter, John F. Amann, andM. Harold Laughlin. Skeletal muscle biochemical adaptations toexercise training in miniature swine. J. Appl.Physiol. 82(6): 1862-1868, 1997.The primarypurpose of this study was to test the hypothesis that enduranceexercise training induces increased oxidative capacity in porcineskeletal muscle. To test this hypothesis, female miniature swine wereeither trained by treadmill running 5 days/wk over 16-20 wk (Trn;n = 35) or pen confined (Sed;n = 33). Myocardialhypertrophy, lower heart rates during submaximal stages of a maximaltreadmill running test, and increased running time to exhaustion duringthat test were indicative of training efficacy. A variety of skeletalmuscles were sampled and subsequently assayed for the enzymes citratesynthase (CS), 3-hydroxyacyl-CoA dehydrogenase, and lactatedehydrogenase and for antioxidant enzymes. Fiber type composition of arepresentative muscle was also determined histochemically. The largestincrease in CS activity (62%) was found in the gluteus maximus muscle(Sed, 14.7 ± 1.1 µmol · min¹ · g¹;Trn, 23.9 ± 1.0; P < 0.0005).Muscles exhibiting increased CS activity, however, were locatedprimarily in the forelimb; ankle and knee extensor and respiratorymuscles were unchanged with training. Only two muscles exhibited higher3-hydroxyacyl-CoA dehydrogenase activity in Trn compared with Sed.Lactate dehydrogenase activity was unchanged with training, as wereactivities of antioxidant enzymes. Histochemical analysis of thetriceps brachii muscle (long head) revealed lower type IIB fibernumbers in Trn (Sed, 42 ± 6%; Trn, 10 ± 4;P < 0.01) and greater type IID/Xfiber numbers (Sed, 11 ± 2; Trn, 22 ± 3;P < 0.025). These findingsindicate that porcine skeletal muscle adapts to endurance exercisetraining in a manner similar to muscle of humans and other animalmodels, with increased oxidative capacity. Specificmuscles exhibiting these adaptations, however, differ between theminiature swine and other species.

     

The adaptations in muscle architecture following whole body vibration training

Objective:This study aims to investigate the effect of 8-week whole-body vibration (WBV) added to conventional training on muscular architecture, dynamic muscle strength and physical performance compared to controls in young basketball players.Methods:Sixteen young basketball players between the ages of 14-16 years were randomly assigned to whole body vibration group (VG) or control group (CG). Both groups were trained with a conventional program. Pennation angle (PeA), fascicle length and muscle thickness of Rectus Femoris (RF) and Vastus lateralis were measured by ultrasonography. Isokinetic dynamic muscle testing at 180 °/s and 60°/s, squat jump (SJ) and flexibility were evaluated before and after 8 weeks of training programs. Primary outcome measure was the fascicle length.Results:Fascicle length of RF, SJ height and flexibility increased significantly within VG compared to pretraining (p<0.05). SJ height increased in VG compared to CG significantly following training (p<0.05). PeA, fascicle length, muscle thicknesses, strength and flexibility did not differ between groups.Conclusion:Eight weeks of WBV training improved fascicle length of RF, SJ height, and flexibility compared to pre-training. Addition of WBV to conventional training did not cause improvement in muscle architecture, strength and flexibility compared to conventional training alone.     

Biochemical adaptations to training: implications for resisting muscle fatigue

Skeletal muscle activity is invariably associated with a decline in force-generating capacity (fatigue). The build-up of metabolic by-products such as intracellular H+ and inorganic phosphate (Pi) has been shown to be one of the potential mechanisms of muscle fatigue. The use of phosphorus magnetic resonance spectroscopy is a repeatable and useful tool to study the effect of pH and Pi on force development. When maximal exercise is preceded by submaximal exercise to reduce the starting muscle pH and increase Pi, the degree of muscle fatigue correlates more strongly with H2PO4- than pH or Pi alone. However, other studies in humans have found that H2PO4- does not always correlate well with fatigue. The use of ramp exercise protocols allow repeatable and sensitive measurement of changes in muscle metabolism in response to endurance training. Chronic electrical stimulation in dogs and endurance training in humans results in reduced pH and Pi changes at the same exercise intensities. This means that the effect of pH and Pi in depressing force development is reduced, which could partially explain the increased fatigue resistance seen following endurance training.     

Inspiratory muscle adaptations following pressure or flow training in humans

Skeletal muscle adapts differently to training with high forces or with high velocities. The effects of these disparate training protocols on the inspiratory muscles were investigated in ten healthy volunteers. Five subjects trained using high force (pressure) loads (pressure trainers) and five trained using high velocity (flow) loads (flow trainers). Pressure training entailed performing 30 maximal static inspiratory efforts against a closed airway. Flow training entailed performing 30 sets of three maximal dynamic inspiratory efforts against a minimal resistance. Training was supervised and carried out 5 days a week for 6 weeks. Inspiratory flow rates and oesophageal pressure-time curves were measured before and after training. Peak inspiratory pressures during maximal static and dynamic efforts and peak flows during the maximal dynamic efforts were calculated. The time-to-peak pressure and rate of rise in peak pressure during maximal static and dynamic manoeuvres were also calculated before and following training. Maximal static pressure increased in the pressure training group and maximal dynamic pressure increased in the flow training group. Both groups increased the rate of pressure production (dP/dt) during their respective maximal efforts. The post-training decrease in time-to-peak pressure was proportionately greater in the flow trainers than in the pressure trainers. The differences in time-to-peak pressure between the two groups were consistent with the different effects of force and velocity training on the time-to-peak tension of skeletal muscle.     

Sympathetic adaptations to one-legged training.

The purpose of the present study was to determine the effect of leg exercise training on sympathetic nerve responses at rest and during dynamic exercise. Six men were trained by using high-intensity interval and prolonged continuous one-legged cycling 4 day/wk, 40 min/day, for 6 wk. Heart rate, mean arterial pressure (MAP), and muscle sympathetic nerve activity (MSNA; peroneal nerve) were measured during 3 min of upright dynamic one-legged knee extensions at 40 W before and after training. After training, peak oxygen uptake in the trained leg increased 19 +/- 2% (P < 0.01). At rest, heart rate decreased from 77 +/- 3 to 71 +/- 6 beats/min (P < 0.01) with no significant changes in MAP (91 +/- 7 to 91 +/- 11 mmHg) and MSNA (29 +/- 3 to 28 +/- 1 bursts/min). During exercise, both heart rate and MAP were lower after training (108 +/- 5 to 96 +/- 5 beats/min and 132 +/- 8 to 119 +/- 4 mmHg, respectively, during the third minute of exercise; P < 0.01). MSNA decreased similarly from rest during the first 2 min of exercise both before and after training. However, MSNA was significantly less during the third minute of exercise after training (32 +/- 2 to 22 +/- 3 bursts/min; P < 0.01). This training effect on MSNA remained when MSNA was expressed as bursts per 100 heartbeats. Responses to exercise in five untrained control subjects were not different at 0 and 6 wk. These results demonstrate that exercise training prolongs the decrease in MSNA during upright leg exercise and indicates that attenuation of MSNA to exercise reported with forearm training also occurs with leg training.     

Reduction in hybrid single muscle fiber proportions with resistance training in humans.

The purpose of this investigation was to examine the effects of 12 wk of progressive resistance training (PRT) on single muscle fiber myosin heavy chain (MHC; I, I/IIa, I/IIa/IIx, IIa, IIa/IIx, IIx) isoform proportions in young individuals. Young, untrained men (YM; n = 6) and women (YW; n = 6) (age = 22 +/- 1 and 25 +/- 2 yr for YW and YM, respectively) received pre- and post-PRT muscle biopsies from the right vastus lateralis for single muscle fiber MHC distribution by electrophoretic analysis (192 +/- 5 pre- and 183 +/- 6 post-fibers/subject analyzed; 4,495 fibers total). Data are presented as percentages of the total fibers analyzed per subject. The PRT protocol elicited an increase in the pure MHC IIa (Delta = + 24 and + 27; YW and YM, respectively; P < 0.05) with no change in the pure MHC I distribution. The hybrid MHC distributions decreased I/IIa/IIx (Delta = -2; YM and YW; P < 0.05), IIa/IIx (Delta = -13 and -19 for YM and YW, respectively; P < 0.05), and total hybrid fiber proportion (I/IIa + I/IIa/IIx + IIa/IIx) decreased (Delta = -19 and -30 for YM and YW, respectively; P < 0.05) with the training, as did the MHC IIx distribution (Delta = -2; YW only; P < 0.05). Alterations in the predominance of MHC isoforms within hybrid fibers (decrease in MHC I-dominant I/IIa and nondominant MHC IIa/IIx, increase in MHC IIa-dominant IIa/IIx; P < 0.05) appeared to contribute to the increase in the MHC IIa proportion. Electrophoresis of muscle cross sections revealed an approximately 7% increase (P < 0.05) in MHC IIa proportion in both groups, whereas the MHC IIx decrease by 7.5 and 11.6% post-PRT in YW and YM, respectively. MHC I proportions increase in YM by 4.8% (P < 0.05) post-PRT. These findings further support previous resistance training data in young adults with respect to the increase in the MHC IIa proportions but demonstrate that a majority of the change can be attributed to the decrease in single-fiber hybrid proportions.     

Molecular adaptations in human skeletal muscle to endurance training under simulated hypoxic conditions.

This study was performed to explore changes in gene expression as a consequence of exercise training at two levels of intensity under normoxic and normobaric hypoxic conditions (corresponding to an altitude of 3,850 m). Four groups of human subjects trained five times a week for a total of 6 wk on a bicycle ergometer. Muscle biopsies were taken, and performance tests were carried out before and after the training period. Similar increases in maximal O(2) uptake (8.3-13.1%) and maximal power output (11.4-20.8%) were found in all groups. RT-PCR revealed elevated mRNA concentrations of the alpha-subunit of hypoxia-inducible factor 1 (HIF-1) after both high- (+82.4%) and low (+78.4%)-intensity training under hypoxic conditions. The mRNA of HIF-1alpha(736), a splice variant of HIF-1alpha newly detected in human skeletal muscle, was shown to be changed in a similar pattern as HIF-1alpha. Increased mRNA contents of myoglobin (+72.2%) and vascular endothelial growth factor (+52.4%) were evoked only after high-intensity training in hypoxia. Augmented mRNA levels of oxidative enzymes, phosphofructokinase, and heat shock protein 70 were found after high-intensity training under both hypoxic and normoxic conditions. Our findings suggest that HIF-1 is specifically involved in the regulation of muscle adaptations after hypoxia training. Fine-tuning of the training response is recognized at the molecular level, and with less sensitivity also at the structural level, but not at global functional responses like maximal O(2) uptake or maximal power output.     

Human skeletal muscle fiber type alteration with high-intensity intermittent training

The response of muscle fiber type proportions and fiber areas to 15 weeks of strenuous high-intensity intermittent training was investigated in twenty-four carefully ascertained sedentary (14 women and 10 men) and 10 control (4 women and 6 men) subjects. The supervised training program consisted mainly of series of supramaximal exercise lasting 15 s to 90 s on a cycle ergometer. Proportions of muscle fiber type and areas of the fibers were determined from a biopsy of the vastus lateralis before and after the training program. No significant change was observed for any of the histochemical characteristics in the control group. Training significantly increased the proportion of type I and decreased type IIb fibers, the proportion of type IIa remained unchanged. Areas of type I and IIb fibers increased significantly with training. These results suggest that high-intensity intermittent training in humans may alter the proportion of type I and the area of type I and IIb fibers and in consequence that fiber type composition in human vastus lateralis muscle is not determined solely by genetic factors.     

Single muscle fiber contractile properties of young competitive distance runners.

The purpose of this investigation was to characterize the contractile properties of individual slow- and fast-twitch myofibers from highly trained distance runners. Muscle biopsies were obtained from the gastrocnemius of eight competitive runners (Run) and eight recreationally active individuals (Rec). Slow-twitch [myosin heavy chain (MHC) I] and fast-twitch (MHC IIa) myofibers were isolated and analyzed for diameter (microm), peak force (Po; mN), unloaded contraction velocity (Vo; fiber lengths/s), and power. Maximum oxygen uptake was higher (P<0.05) in Run (71+/-1 vs. 47+/-2 ml.kg(-1).min(-1)). Diameter of MHC I and MHC IIa fibers from Run subjects was approximately 20% greater (P<0.05) than Rec. Peak force of the MHC IIa fibers was 31% higher (P<0.05) in Run, whereas Po of MHC I fibers was not different between groups. No differences for specific tension (Po/cross-sectional area) were present between groups for either fiber type. Vo was higher (P<0.05) in MHC I (+70%) and MHC IIa (+18%) fibers from Run subjects. In vitro peak absolute power (microN.s(-1)) of both fiber types was greater (P<0.05) in Run (131 and 85% for MHC I and MHC IIa, respectively). Additionally, normalized power (W/l) of the MHC I fibers was 64% higher in Run, whereas no differences were noted for normalized power of MHC IIa fibers. These data indicate that highly trained endurance runners have elevated contraction velocity in both slow- and fast-twitch myofibers. These characteristics of the fast-twitch muscle fibers have not been previously reported in competitive endurance athletes and may contribute to the high level of running performance in these athletes.     

Predictable adaptations by skeletal muscle mitochondria to different exercise training workloads

1. Mitochondria were isolated according to their cellular location within the fibers of pooled gastrocnemius and plantaris muscle of the rat. This procedure yields two populations of mitochondria which display different biochemical properties. 2. The adaptive response of these mitochondria populations to the chronic exposure to different elevated energy demands (different modes of exercise training) was investigated. 3. The observed changes in mitochondrial protein content and cytochrome oxidase activity in the respective mitochondria population suggests that each population is capable of independent adaptations. 4. The adaptive response of each mitochondria population, furthermore, was predictable with respect to the metabolic energy demand of the exercise training workload.     

Metabolic adaptations in skeletal muscle of streptozotocin-diabetic rats following exercise training

Compensatory metabolic adaptations induced in streptozotocin-diabetic rat skeletal muscle by submaximal endurance training have been investigated. The gastrocnemius muscles of sedentary streptozotocin-diabetic rats were found to have a lower than normal myoglobin content, succinate dehydrogenase activity, and capacity to oxidize pyruvate and palmitate-1-[¹⁴C]. The values of these parameters were significantly increased in the diabetic skeletal muscle by the training program, obtaining levels similar to those of normal sedentary animals.