Myosin heavy chain profile of equine Gluteus medius muscle following prolonged draught-exercise training and detraining

Fourteen 4-year old Andalusian mares were used to examine the plasticity of myosin heavy chain (MHC) composition in horse skeletal muscle with heavy draught-exercise training and detraining. Seven horses underwent a training programme based on carriage exercises for 8 months. Afterwards, they were kept in paddocks for 3 months. The remaining seven animals were used as control horses. Three gluteus medius muscle biopsies were removed at depths of 20, 40 and 60 mm from each horse before (month 0), during the training (months 3 and 8) and after detraining (month 11). Myosin heavy chain composition was analysed by electrophoresis and immunohistochemically with anti-MHC monoclonal antibodies. Fibre areas, oxidative capacity and capillaries were studied histochemically. After 8 months of training, MHC-IIX and IIX fibres decreased whereas MHC-I and type I and I + IIA fibres increased. Neither MHC-IIA nor the percentage of IIA fibres changed when the data were considered as a whole, but the proportion of MHC-IIA increased in the superficial region of the muscle after 8 months of training. Mean areas of type II fibres were not affected by training and detraining, but the cross-sectional of type I fibres increased after 3 month of training and not further increases were recorded afterward. The percentage of high-oxidative capacity fibres and the number of capillaries per mm² increased with training. Most of these muscular adaptations reverted after detraining. These results indicate that long term draught-exercise training induces a reversible transition of MHC composition in equine muscle in the order IIX IIA I. The physiological implication of these changes is an impact on the velocity of shortening and fatigue resistance of muscle fibres.     

The effect of training and detraining on muscle composition in the horse.

1. Percutaneous needle biopsies were obtained from six limb muscles in six horses before and during a training programme of 10 or 15 weeks designed to involve both aerobic and anaerobic work. In a subsequent detraining period, biopsies were also taken after 5 and 10 weeks. 2. Samples were analysed biochemically for enzyme activity of lactic dehydrogenase (LDH), creatine phosphokinase (CPK), aldolase (ALD), citrate synthase (CS), aspartate aminotransferase (AST), and alanine aminotransferase (ALT) and for glycogen content. Fibre typing was carried out histochemically before and 10 weeks after commencement of training. 3. There was a significant increase in the percentage of high myosin ATPase activity pH 9-4/high oxidative (FTH) fibres with a corresponding decrease in high myosin ATPase activity pH 9-4/low oxidative (FT) fibres and low myosin ATPase activity pH 9-4/high oxidative (ST) fibres after 10 weeks training. 4. During training, enzyme activities increased progressively but at different rates with an approximate twofold increase in all of the enzymes except CPK by the end of the training period. Changes in all the muscles studied were similar. Glycogen content increased by approximately 33% which was significant when all the muscles were considered together. 5. A decrease in enzyme activity occurred after 5 weeks detraining. However at 10 weeks a consistent but inexplicable increase in all enzyme levels, except CS again occurred. 6. It is concluded that training increased greatly the activity of enzymes involved in both aerobic and anaerobic metabolism.     

Effects of transcutaneous short-term electrical stimulation on M. vastus lateralis characteristics of healthy young men

Fifteen healthy, untrained male subjects (mean age +/- SD, 22 +/- 5 years) were used to examine the plasticity of myosin heavy chain phenotype, size, oxidative capacity and capillarization of skeletal muscle fibre types with short-term electrical stimulation (ES). Ten subjects were electro-stimulated on both quadriceps muscles with a frequency of 45-60 Hz, with 12 s of stimulation followed by 8 s of recovery for a total of 30 min per day, 3 days per week for 6 weeks. The remaining five subjects served as controls. Two vastus lateralis muscle biopsy samples were removed from each subject before (week 0) and after (week 6) ES training. A standardized exercise test on a cycle ergometer was performed by each subject before and after the experimental period and several indicators of whole-body aerobic capacity were estimated. The so-called electromyographic threshold was also determined during the tests. Muscle biopsy samples were analysed by electrophoresis, immunohistochemistry and quantitative histochemistry. Myosin heavy chain (MHC) composition, muscle fibre type distribution, fibre areas, oxidative capacity and capillaries of each fibre type were estimated. Muscular changes with ES revealed an increase of fibres expressing MHC-IIA, and a decrease of fibres expressing MHC-IIX and MHC-I, as well as an increase of the oxidative capacity and mean number of capillaries of fast-twitch (type II) fibres with minimal muscle fibre hypertrophy. These adaptations seem related to a bi-directional transformation from both MHC isoforms I and IIX towards the MHC-IIA isoform. The aerobic performance and electromyographic variables at the whole-body level were not altered by ES. These results indicate that the particular short-term ES training protocol tested in the present study induces significant adaptations in histochemical and metabolic machineries of human skeletal muscle. The results also offer new perspectives for realistic applications of ES in various clinical situations and sport training.     

Adaptive changes in work capacity,skeletal muscle capillarization and enzyme levels during training and detraining

Six male subjects exercised on a bicycle ergometer 30 min with left leg and 30 min with right leg 3 times a week for 8 weeks. This training resulted in a 14.6% increase in V?_o2 max with two-leg exercise and a 23.1% increase with one-leg exercise. A significant decrease towards pretraining V?_o2 max was seen during the following 8 weeks of detraining. Muscle biopsy samples were obtained at rest from m. vastus lateralis before and after training and 4 and 8 weeks after training. During training the number of capillaries per mm² and the number of capillaries per fiber increased about 20%. The number of capillaries around each fibre type (CA) increased 20–30%. The average area of each fibre type increased only about 5%. The fibre area per CA decreased by about 10%. During 8 weeks of detraining decreases were seen in the number of capillaries per fibre, CA and in fibre area, while fibre area per CA and number of capillaries per mm² were almost unchanged at the end of the detraining period. Pronounced increases in activities of oxidative enzymes were observed after training, while only minor increases were seen in glycolytic enzyme activities. All enzyme activities decreased towards pre-training levels during detraining. The results indicate that the training-induced improvement in oxidative capacity and in muscle capillarization expressed as capillaries per fibre and CA disappears within 8 weeks after cessation of training. However, the fibre area per CA and number of capillaries per mm² point at a favourable long term effect on the average diffusion distance between capillaries and muscle fibres.     

Biochemical and histochemical adaptation to sprint training in young athletes

The purpose of the present study was to investigate the effects of 8 months of a specific and controlled sprint training programme on three groups of young athletes (two groups of males and one of females). Biopsies of vastus lateralis were taken before and after the period of training. The type percentage and diameter of the fibres, as well as the glycogen content and the activities of the enzymes of glycogen metabolism (glycogen synthase and glycogen phosphorylase), glycolysis (phosphofructokinase, pyruvate kinase, aldolase and lactate dehydrogenase), oxidative metabolism (succinate de-hydrogenase) and creatine kinase and aminotransferases were studied. The results show an increase in the percentage of type I fibres and an increase in the diameter of both fibre types. A significant increase was also observed in glycogen content, and in the activities of glycogen synthase, glycogen phosphorylase, phosphofructokinase, pyruvate kinase, succinate dehydrogenase, aspartate aminotransferase and alanine aminotransferase. We conclude that a long period of sprint training induces a biochemical muscle adaptation to anaerobic exercise. This metabolic adaptation is followed by a morphological adaptation, although this is probably not as specific as the biochemical one.     

Effects of two high-intensity intermittent training programs interspaced by detraining on human skeletal muscle and performance

Summary The purpose of this study was to investigate the effects of repeated high-intensity intermittent training programs interspaced by detraining on human skeletal muscle and performances. First, nineteen subjects were submitted to a 15-week cycle ergometer training program which involved both continuous and high-intensity interval work patterns. Among these 19 subjects, six participated in a second 15-week training program after 7 weeks of detraining. Subjects were tested before and after each training program for maximal aerobic power and maximal short-term ergocycle performances of 10 and 90 s. Muscle biopsy from the vastus lateralis before and after both training programs served for the determination of creatine kinase (CK), hexokinase, phosphofructokinase (PFK), lactate dehydrogenase (LDH), malate dehydrogenase, 3-hydroxyacyl-CoA dehydrogenase (HADH) and oxoglutarate dehydrogenase (OGDH) activities. The first training program induced significant increases in all performances and enzyme activities but not in CK. Seven weeks of detraining provoked significant decreases in maximal aerobic power and maximal 90 s ergocycle performance. While the interruption of training had no effect on glycolytic enzyme markers (PFK and LDH), oxidative enzyme activities (HADH and OGDH) declined. These results suggest that a fairly long interruption in training has negligeable effects on glycolytic enzymes while a persistent training stimulus is required to maintain high oxidative enzyme levels in human skeletal muscle. The degree of adaptation observed after the second training program confirms that the magnitude of the adaptive response to exercise-training is limited.     

Effect of training with different intensities and volumes on muscle fibre enzyme activity and cross sectional area in the m. triceps brachii

This study primarily examined how intermittent versus continuous endurance training, using similar or dissimilar volumes, affected muscle fibre enzyme activities in the triceps brachii muscle. Thirty-two subjects performed either intermittent (60% of 1RM) or continuous (30% of 1RM) elbow extensions 3 times week⁻¹ in a training apparatus. Training was performed until either a low (five) or a high volume (8 weeks) was accumulated. Muscle biopsies from the m. triceps brachii were taken pre- and post training and following 8 weeks of detraining. Marker enzymes for muscle fibre oxidative (succinate dehydrogenase SDH) and glycolytic (glycerophosphate dehydrogenase; α-GPDH) capacity was assessed by histochemistry, and the resulting enzyme activities measured by image analysis. The type of training affected enzyme activities differently. In type 1 fibres, continuous and intermittent training was equally effective in increasing SDH activity, while intermittent training increased SDH activity more than continuous training in type 2 fibres (P < 0.05). Intermittent training increased α-GPDH activity more than continuous training both in type 1 (P < 0.001) and type 2 fibres (P < 0.05), but the increase in glycolytic capacity following intermittent training was larger in type 1 (54%) than in type 2 fibres (23%). There was no effect of training volume on oxidative or glycolytic capacity in either fibre type. Thus, when training intensity is sufficient to stimulate to increases in oxidative and glycolytic capacity, the SDH and α-GPDH response seems to be volume independent. Detraining reduced Post-T enzyme activities to baseline (all; P < 0.01).     

Enzyme adaptations of human skeletal muscle during bicycle short-sprint training and detraining

The effect of sprint training and detraining on supramaximal performances was studied in relation to muscle enzyme adaptations in eight students trained four times a week for 9 weeks on a cycle ergometer. The subjects were tested for peak oxygen uptake (V˙O_{2 peak}), maximal aerobic power (MAP) and maximal short-term power output (W˙_max) before and after training and after 7 weeks of detraining. During these periods, biopsies were taken from vastus lateralis muscle for the determination of creatine kinase (CK), adenylate kinase (AK), glycogen phosphorylase (PHOS), hexokinase (HK), phosphofructokinase (PFK), lactate dehydrogenase (LDH) and its isozymes, 3-hydroxy-acyl-CoA dehydrogenase (HAD) and citrate synthase (CS) activities. Training induced large improvements in W˙_max (28%) with slight increases (3%) in V˙O_{2 peak}} (P < 0.10). This was associated with a greater glycolytic potential as shown by higher activities for PHOS (9%), PFK (17%) and LDH (31%) after training, without changes in CK and oxidative markers (CS and HAD). Detraining induced significant decreases in V˙O_{2 peak} (4%), MAP (5%) and oxidative markers (10–16%), while W˙_max and the anaerobic potential were maintained at a high level. This suggests a high level in supramaximal power output as a result of a muscle glycogenolytic and glycolytic adaptation. A long interruption in training has negligible effects on short-sprint ability and muscle anaerobic potential. On the other hand, a persistent training stimulus is required to maintain high aerobic capacity and muscle oxidative potential. This may contribute to a rapid return to competitive fitness for sprinters and power athletes.     

The effects of long-term low intensity aerobic training and detraining on serum lipid and lipoprotein concentrations in elderly men and women

The effects of long-term low intensity aerobic training and detraining on serum lipid and lipoprotein concentrations were examined in 30 elderly men and women. These subjects were randomly divided into two groups. The training group [n=15; 7 men and 8 women; mean age 75.5 (SD 5.6) years] agreed to take part in physical training using a treadmill with an exercise intensity at the blood lactate concentration threshold for 30 min 3–6 times a week for 9 months. The other group [n=15; 7 men and 8 women; mean age 73.7 (SD 4.4) years] did not perform any particular physical training and was followed as the control. Following this training period the high density lipoprotein-cholesterol (HDL-C) had increased significantly (P<0.01) while the total cholesterol (TC) : HDL-C ratio had decreased significantly (P<0.01) in the training group after 9 months but had not changed in the control group. The TC, triglyceride (TG) and low density lipoprotein-cholesterol (LDL-C) had not changed significantly in either group. No significant difference was seen between the groups throughout the period for TC, LDLC or TG. There was, however, a significant correlation between the initial TC:HDL-C ratio and the change in the TC:HDL-C ratio following 3 months of training (P <0.05). After 1 month of detraining in 5 patients, the HDL-C had decreased significantly (P < 0.05) while the TC:HDL-C had increased significantly in the training group (P<0.01). These results suggested that long-term low intensity aerobic training improved the profile of serum lipid and lipoprotein concentrations, while detraining returned the profile to that of the pretraining levels in elderly persons.     

Effects of 8 weeks' endurance training on skeletal muscle metabolism in 56–70-year-old sedentary men

Summary The effects of 8 weeks' endurance training on muscle metabolism at rest and after a submaximal bicycle ergometer exercise were studied in 31 previously sedentary men, aged 56–70. Training consisted of 3–5 one hour exercise bouts per week including walking-jogging, swimming, gymnastics and ball games. The effects of training were similar to those previously reported for younger men. Mean maximal oxygen uptake increased (11%), as did the resting values for muscle glycogen concentration, the enzymes representing aerobic energy metabolism (malate dehydrogenase, succinate dehydrogenase), and also some of the anaerobic enzymes (creatine phosphokinase, lactate dehydrogenase). Lactate production during submaximal work decreased. The enzyme activities were lower following acute exercise both before and after training.     

Time course of changes in the human Achilles tendon properties and metabolism during training and detraining in vivo

The purpose of this study was to investigate the time course of changes in human tendon properties and metabolism during resistance training and detraining. Nine men (21-27 years) completed 3 months of isometric plantar flexion training and another 3 months of detraining. At the beginning and on every 1 month of training and detraining periods, the stiffness, blood circulation (blood volume and oxygen saturation), serum procollagen type 1 C-peptide (P1P; reflects synthesis of type 1 collagen), echointensity (reflects collagen content), and MRI signal intensity (reflects collagen structure) of the Achilles tendon were measured. Tendon stiffness did not change until 2 months of training, and the increase (50.3%) reached statistical significance at the end of the training period. After 1 month of detraining, tendon stiffness had already decreased to pre-training level. Blood circulation in the tendon did not change during the experimental period. P1P increased significantly after 2 months of training. Echointensity increased significantly by 9.1% after 2 months of training, and remained high throughout the experiment. MRI signal intensity increased by 24.2% after 2 months and by 21.4% after 3 months of training, but decreased to the pre-training level during the detraining period. These results suggested that the collagen synthesis, content, and structure of human tendons changed at the 2-month point of training period. During detraining, the sudden decrease in tendon stiffness might be related to changes in the structure of collagen fibers within the tendon.     

Lysosomal changes related to exercise injuries and training-induced protection in mouse skeletal muscle

Three experiments were designed to study the lysosomal changes associated with the development and maintenance of the endurance training induced resistance against exercise injuries in mouse skeletal muscles. The activities of arylsulphatase, cathepsin C, cathepsin D, and β-glucuronidase were assayed from the red part of mouse quadriceps femoris muscle 4 days after prolonged strenuous running of 4–9 h duration. Exercise injuries were characterized by necrotic fibers and focal inflammation. Strenuous running of untrained mice induced necrotic lesions and a 4–5 fold increase in the activities of lysosomal enzymes. This lysosomal response was considerably reduced already by daily training bouts on the 3 days preceding the strenuous exertion. Simultaneously exercise injuries were markedly reduced. Extending the endurance training program increased the running ability of mice and further reduced the necrotic lesions and lysosomal changes induced by the strenuous exercise. The detraining of 1 week after the termination of regular endurance training considerably increased the degree of exercise induced lysosomal response. The detraining of longer durations further increased the lysosomal response and no effect of prior endurance training existed after 1 month detraining. Our observations suggest that the severity of exercise injuries is related to the strength of the exercise stimulus and the level of preceding physical activity and can be characterized by the lysosomal changes.     

Analysis of myosin heavy chains at the protein level in horse skeletal muscle

Combined methodologies of enzyme-linked immunosorbent assay (ELISA), sodium dodecyl sulphate polyacrilamide gel electrophoresis (SDS-PAGE), immunoblotting, traditional myofibrillar ATPase (mATPase) histochemistry and immunocytochemistry of whole biopsied samples were used to study myosin heavy chain (MHC) isoforms in the equine gluteus medius muscle. The ELISA technique allowed the quantification of the three MHC isoforms known to be present in different horse muscles: slow (MHC-I) and two fast (termed MHC-IIA and MCH-IIX). The SDS-PAGE method resolved MHCs in three bands: MHC-I, MHC-IIX and MHC-IIA from the fastest to the slowest migrating band and a quantification by densitometry for each MHC isoform was also possible. The identity of these three MHCs was confirmed by immunoblots with specific monoclonal antibodies. Five fibre types were defined immunohistochemically according to their MHC content: I, I + IIA, IIA, the hybrid IIAX and IIX. When quantitative data obtained with the four different methodologies were combined and compared, they were consistent and, when considered together, showed significant correlation. Nevertheless, the percentage of MHC-IIA histochemically derived was underestimated, while that of MHC-IIX was overestimated in comparison with the immunocytochemical determination of these MHC isoforms. The percentage of MHC-I obtained by ELISA technique was underestimated. In short, these integrated methods for the analysis of MHCs at the protein level demonstrate that equine skeletal muscle does not express the MHC-IIB, so type II fibres have been misclassified in numerous previous studies based upon the very traditional mATPase histochemistry. They also offer new prospects for muscle fibre typing in equine experimental studies and veterinary medicine.     

Muscle adaptations and glucose control after physical training in insulin-dependent diabetes mellitus

Six men and three women with insulin-dependent diabetes (without complications) participated in physical training three times a week for 20 weeks. Physical training did not change the concentration of fasting blood-glucose, glucose excretion in urine or glucosylated haemoglobin (HbA1). However, the glucose disposal rate during euglycaemic clamp increased after training. In two patients a minor reduction of insulin dosage was necessary to alleviate slight hypoglycaemic episodes. The training resulted in significant increases in quadriceps isometric and dynamic strength and endurance. Maximal oxygen uptake increased by 8%, the activity of glycolytic enzymes in vastus lateralis muscle by 47% for hexokinase, and 30% for tri-osephosphate dehydrogenase and 25% for lactic dehydrogenase, the activity of oxidative enzymes by 42% for citrate synthase and 46% for 3-hydroxy-acyl-CoA-dehydrogenase. The glycogen concentration in the vastus lateralis muscle did not change significantly. Lipoprotein lipase activity did not change in muscle, nor in adipose tissue. The mean muscle fibre area increased by 25% and the area of FTa fibres by 30%. The new formation of capillaries around different muscle fibres was significant for FTb fibres (26%). The proliferation of capillaries, however, appeared to be insufficient to cope with the increased area of muscle fibres. As a result, the mean area of muscle fibre supplied by one capillary (a measure of diffusion distance) significantly increased after training for FTa fibres. It is concluded that with the exception of deficient proliferation of capillaries, patients with insulin-dependent diabetes mellitus show a normal central and peripheral adaptation to physical training. Physical training does not apparently improve blood glucose control in most cases, despite an increased insulin sensitivity.     

Effect of hyperthyroidism on fibre-type composition, fibre area, glycogen content and enzyme activity in human skeletal muscle

Seven hyperthyroid patients were studied by repeated muscle biopsies (vastus lateralis) before and after a period of medical treatment which averaged 10 months. The biopsies were analysed with regard to fibre-type composition, fibre area, capillary density, glycogen content and enzyme activities representing the glycolytic capacity (hexokinase, 6-phosphofructokinase), oxidative capacity (oxoglutarate dehydrogenase, citrate synthase) and Ca2+- and Mg2+-stimulated ATPase in muscle. In the pretreatment biopsy (hyperthyroid state), there was a significantly lower proportion of type I fibres (30% vs. 41%), a higher capillary density (23%), lower glycogen content (33%), and higher hexokinase activity (32%) compared with the post-treatment biopsy. No significant changes in the activity of the remaining enzymes were observed. The present study indicates that hyperthyroidism induces a transformation from type I to type II fibres in human skeletal muscle. The increase in hexokinase activity probably reflects a higher glucose utilization by skeletal muscle in order to compensate partially for the reduced glycogen content.     

Muscle performance,morphology and metabolic capacity during strength training and detraining: A one leg model

Summary To investigate biochemical, histochemical and contractile properties associated with strength training and detraining, six adult males were studied during and after 10 weeks of dynamic strength training for the quadriceps muscle group of one leg, as well as during and after a subsequent 12 weeks of detraining. Peak torque outputs at the velocities tested (0–270·s^–1) were increased (p<0.05) by 39–60% and 12–37% after training for the trained and untrained legs, respectively. No significant changes in peak torques were observed in six control subjects tested at the same times. Significant decreases in strength performance of the trained leg (16–21%) and untrained leg (10–15%) were observed only after 12 weeks of detraining. Training resulted in an increase (p0.05) in the area of FTa (21%) and FTb (18%) fibres, while detraining was associated with a 12% decrease in FTb fibre cross-sectional area. However, fibre area changes were only noted in the trained leg. Neither training nor detaining had any significant effect on the specific activity of magnesium-activated myofibrillar ATPase or on the activities of enzymes of phosphagen, glycolytic or oxidative metabolism in serial muscle biopsy samples from both legs. In the absence of any changes in muscle enzyme activities and with only modest changes in FT fibre areas in the trained leg, the significant alterations in peak torque outputs with both legs suggest that neural adaptations play a prominent role in strength performance with training and detraining.This study was funded by a grant from the Natural Sciences and Engineering Research Council of Canada     

Different metabolic responses to exercise training programmes in single rat muscle fibres

Summary The aim of this report is to elucidate the effects of exercise training on metabolic properties of different muscle fibre types of the rat hindlimb. Single muscle fibres were dissected from soleus (SOL) or extensor digitorum longus (EDL) muscles of Wistar strain male rats trained on a treadmill for 16 weeks. Each fibre was typed histochemically (SO, slow-twitch oxidative; FOG, fast-twitch oxidative glycolytic; FG, fast-twitch glycolytic). Then glycolytic and oxidative enzymes (CK, LDH, PFK, PK, SDH, and MDH) activities were measured biochemically. Slow,-type fibres (SO) were hypertrophied following endurance training and fast-twitch fibres (FOG and FG) were hypertrophied following sprint training. In EDL muscles the distribution of the slow-type fibres was reduced following the sprint training. The activity of glycolytic enzymes increased significantly in the fast-type fibres (FOG and FG) following sprint training, while oxidative enzymes activities increased in both fast (FOG and FG) and slow (SO) muscle fibres following the endurance training. Neither glycolytic nor oxidative enzymes' activities always increased equally in all types of fibre following exercise training. Consequently, the metabolic profiles in each type of single muscle fibre were affected differently by different intensities of exercise training. These results suggest that the functional (enzymes activity) and structural (muscle fibre hypertrophy) changes of skeletal muscle fibre following exercise training appeared gradually, and would be controlled by different factors.     

Selective glycogen depletion pattern in human muscle fibres after exercise of varying intensity and at varying pedalling rates

1. Glycogen depletion pattern in human skeletal muscle fibres was studied after bicycle exercise of varying intensity performed at different pedalling rates. Work intensities studied were equivalent to 30-150% of V(O) (2) max. with pedalling rates of 30-120 rev/min.2. Glycogen depletion increased dramatically with increasing exercise intensity; depletion was 2.7 and 7.4 times greater respectively at workloads demanding 64 and 84% V(O) (2) max. than at workloads calling for 31% V(O) (2) max. Even greater rates of glycogen utilization occurred at supramaximal loads.3. Slow twitch, high oxidative (ST) fibres were the first to lose glycogen (reduced PAS staining) at all workloads below V(O) (2) max. Progressive glycogen depletion occurred in fast twitch (FT) fibres as work continued. Large quantities of glycogen remained in the muscle after 3 hr of exercise at low exercise intensity. This was almost exclusively found in FT fibres. At workloads exceeding maximal aerobic power, there was an initial depletion of glycogen in both fibre types. Varying the pedalling rate and, thus, the total force exerted in each pedal thrust had no effect on the pattern of glycogen depletion in the fibres.4. Results point to primary reliance upon ST fibres during submaximal endurance exercise, FT fibres being recruited after ST fibres are depleted of glycogen. During exertion requiring energy expenditure greater than the maximal aerobic power, both fibre types appeared to be continuously involved in carrying out the exercise.     

Neural and muscular changes to detraining after electrostimulation training

We investigated the effects of 4 weeks of detraining subsequent to an 8-week electrostimulation (ES) training program on changes in muscle strength, neural and muscular properties of the knee extensor muscles. Nine male subjects followed the training program consisting of 32 sessions of isometric ES training over an 8-week period. All subjects were tested before and after 8 weeks of ES training, and were then retested after 4 weeks of detraining. Quadriceps muscle anatomical cross-sectional area (ACSA) was assessed by ultrasonography imaging. The electromyographic (EMG) activity and muscle activation (i.e., by means of the twitch interpolation technique) obtained during maximal voluntary contractions (MVC) were used to examine neural adaptations. After training, the knee extensor voluntary torque increased significantly by 26%. Torque gains were accompanied by an increase in vastii EMG activity normalized to respective M-wave (+43%), muscle activation (+6%) and quadriceps ACSA (+6%). After detraining, knee extensor MVC, vastii EMG activity, muscle activation and quadriceps ACSA decreased significantly by 9%, 20%, 5% and 3%, respectively. Also, the knee extensor MVC values remained significantly elevated (14%) above baseline levels at the end of the detraining period and this was associated with a larger quadriceps ACSA (+3%) but not with a higher neural activation. We concluded that the voluntary torque losses observed after detraining could be attributed to both neural and muscular alterations. Muscle size preservation could explain the higher knee extensor MVC values observed after the cessation of training compared to those obtained before training, therefore indicating that muscle size changes are slower than neural drive reduction.