Effect of varying exercise intensity on glycogen depletion in human muscle fibres

Glycogen depletion of muscle fibre types I, IIA, IIAB and IIB was studied during bicycle exercise at 43% (n = 5), 61% (n = 7) and 91% (n = 5) of VO2max. Glycogen content in individual fibres from vastus lateralis muscles was quantified as optical density of periodic acid-Schiff (PAS) stain. After 60 min at the lowest intensity, glycogen depletion was observed in almost all type I fibres and in about 20% of type IIA fibres. After 60 min exercise at 61% of VO2max, glycogen breakdown was observed in all type I fibres and in about 65% of type IIA fibres. During the first part of exercise at 91% of VO2max, glycogen breakdown was observed in all type I and IIA and in about 50% of type IIAB and IIB fibres. Muscle lactate concentration increased during the the first 5 min of exercise at 91% of VO2max to 15 mmol kg-1 (w/w) and remained thereafter at this level. From start of exercise the average rates of glycogen depletion in type I fibres were about 1.0, 2.0 and 4.3 mmol glucosyl units kg-1 (w/w) min-1 at 43%, 61% and 91% of VO2max. The depletion rates were almost constant with time at the two lower intensities. The results indicate that the number of fibres activated from the start increase gradually in response to increased exercise intensity. The rates of glycogen depletion in type I fibres suggest a progressive tension output of these fibres with increasing intensity.     

Reabsorption ofl-glutamine andl-histidine from various regions of the rat proximal convolution studied by stationary microperfusion: Evidence that the proximal convolution is not homogeneous

Summary The glycogen depletion pattern in human muscle fibers was followed throughout the course of prolonged exercise at a work load requiring 67% of the subjects' maximal aerobic power. Biopsy samples were taken from the vastus lateralis muscle at rest and after 20, 60, 120, and 180 (or when unable to continue at the prescribed load) min of exercise. Muscle fibers were identified as fast twitch (FT) or slow twitch (ST) on the basis of myofibrillar ATPase activity. The glycogen content of muscle samples was determined biochemically. At the end of the exercise total muscle glycogen content was very low. Glycogen was also estimated in the fibers with the PAS stain. ST fibers were the first to become depleted of their glycogen but as the exercise progressed, the FT fibers were also depleted. These data may suggest a preferential utilization of ST fibers during prolonged, intense exercise, with a secondary recruitment of FT occuring as the ST fibers became depleted of their glycogen stores.     

Muscle glycogen depletion patterns in fast twitch fibre subgroups of man during submaximal and supramaximal exercise

Summary Muscle glycogen depletion in ST, FTa and FTb fibres were studied in human subjects undergoing two distinctly different modes of bicycle exercise. Two hours of submaximal exercise (60% of V0₂ max) produced a 77% decline in muscle glycogen concentration accompanied by only minor elevations in muscle and blood lactate levels, whereas 10 one minute supramaximal work bouts resulted in a 52% decrease in total glycogen concentration and substantially elevated muscle and blood lactate contents. Moreover the patterns of glycogen depletion in the two conditions were also distinctly different. Based on the PAS staining intensity, glycogen was depleted the most in ST fibres and least in FTb fibres, during submaximal work. During supramaximal work FTb fibres were the lightest in PAS staining, with little loss of glycogen from ST fibres. In both situations the loss of glycogen in the FTa fibres was intermediate compared to the other two fibre types. These data support a selective recruitment of muscle fibres during work of different intensities, and further, suggest a physiological basis for the subgrouping of FT fibres in man.     

Glycogen synthetase and phosphorylase activity in slow and fast twitch skeletal muscle fibres in man.

Glycogen synthetase (I + D) and phosphorylase (à + b) activity was determined in human skeletal muscle biopsies with different muscle fibre composition and in dissected an pooled batches of the two main muscle fibre types: slow twitch (ST) and fast twitch (FT), respectively. Glycogen synthetase (I + D) activity remained unchanged as the per cent of FT fibres increased, but phosphoyrlase (à + b) activity was found to increase. A similar activity pattern was found in dissected and FT fibres as compared to ST fibres, i.e. the same glycogen synthetase activity but heightened phosphoyrlase activity.     

Glycogen Synthetase and Phosphorylase Activity in Slow and Fast Twitch Skeletal Muscle Fibres in Man

Glycogen synthetase (I + D) and phosphorylase (á+b) activity was determined in human skeletal muscle biopsies with different muscle fibre composition and in dissected and pooled batches of the two main muscle fibre types: slow twitch (ST) and fast twitch (FT), respectively. Glycogen synthetase (I + D) activity remained unchanged as the per cent of FT fibres increased, but phosphorylase (á+b) activity was found to increase. A similar activity pattern was found in dissected and pooled FT fibres as compared to ST fibres, i.e. the same glycogen synthetase activity but heightened phosphorylase activity.     

Muscle glycogen depletion patterns in type I and subgroups of type II fibres during prolonged severe exercise in man

Glycogen depletion of muscle fibre types I, II A, II AB and II B was studied using a histochemical method to quantify glycogen content in individual fibres. The reliability was examined in 29 muscle biopsies, in which total glycogen content was compared to average periodic acid Schiff (PAS) stain intensity in sections from the same samples. Over a wide range of glycogen content (1-252 mmole glucosyl units . kg-1 wet weight) a linear relationship (r = 0.93) was found between the two methods for quantification of muscle glycogen. Glycogen depletion patterns in type I, II A, II AB and II B fibres were studied in 5 subjects during exhaustive bicycle exercise at 75% of VO2 max. At rest before exercise glycogen content was 16% higher in type II subgroups than in type I (p less than 0.05). From start of exercise the same glycogen depletion rate was observed in type I and II A. Glycogen content of Type II AB and II B was unchanged during the first part of exercise. Later a decrease was observed, first in type II AB and finally in II B, suggesting a decrease in threshold force of these fibre types. The results indicate physiological differences between the 3 subgroups of type II fibres in man, whereas at the present exercise intensity type I and II A fibres were recruited simultaneously from start.     

Selective glycogen depletion in skeletal muscle fibres of man following sustained contractions

1. Six healthy males performed sustained contractions with different tensions related to their maximal voluntary contraction (MVC). The isometric exercise consisted of efforts to extend the knee when flexed at an angle of 90 degrees .2. Biopsy samples were taken from the lateral portion of M. quadriceps femoris before and after different periods (6-45 min) during a series of sustained contractions. Total glycogen content was determined on each muscle sample. In order to evaluate whether the glycogen depletion occurred preferentially in slow twitch (ST) or fast twitch (FT) fibres, serial sections of the muscle samples were stained for myofibrillar ATPase and glycogen (PAS reaction).3. In all experiments a selective glycogen depletion was observed. At low tensions, the ST fibres and at higher tensions the FT fibres became glycogen depleted. The critical tension at which this conversion in glycogen depletion from ST to FT fibres took place was 20% MVC.4. It is concluded that at sustained contractions of less than 20% MVC there is a major reliance upon ST fibres and above that level a primary dependence upon FT fibres. It is further suggested that restriction of blood flow and thus availability of oxygen at forces higher than 20% MVC may be the explanation for the present findings.     

Lactate in blood,mixed skeletal muscle,and FT or ST fibres during cycle exercise in man

The relationship between muscle and blood lactate levels during progressively step-wise incrementing cycle exercise has been investigated in 10 male subjects. Steps between power outputs during exercise were 50 W and each stage, from loadless pedalling until voluntary exhaustion, lasted 4 min. Blood samples and biopsies (m. vastus lateralis) were taken for lactate determination at each power output beginning with the exercise intensity perceived by the subject as being “rather moderate”. The ratio muscle: blood lactate was greater than one at all power outputs and increased most markedly at the power output closest to that eliciting 4 mmol × I^-1 blood lactate (W_OBLA). At W_OBLA. blood lactate was positively correlated to muscle lactate concentrations which covaried widely among subjects (mean 8.3. range 4.5–14.4 mmol × kg^-l wet weight). Muscle fibres from the W_OBLA biopsy in 6 subjects were dissected out and identified as fast twitch (FT) or slow twitch (ST). No significant difference in lactate concentration was observed between pools of FT or ST fibres.     

Glycogen breakdown in different human muscle fibre types during exhaustive exercise of short duration.

The rates of glycogen breakdown during exhaustive intense exercise of three different intensities were determined in type I and subgroups of type II fibres. The exercise intensity corresponded to 122 +/- 2, 150 +/- 7 and 194 +/- 7% of VO2max. Muscle biopsies were taken from both legs before and immediately after exhaustion. Muscle lactate concentration increased by 27 +/- 1, 27 +/- 1 and 20 +/- 2 mmol kg-1 wet wt during the exercise at 122, 150 and 194% VO2max, respectively. The rates of glycogen depletion increased in all fibre types with increasing intensity, and the decline in type I fibres was 30-35% less than in type II fibres at all intensities. No differences were observed between the glycogen depletion rates in subgroups of type II fibres (IIA, IIAB and IIB). During the exercise at 194% VO2max, the rates of glycogen breakdown were 0.35 +/- 0.03 and 0.52 +/- 0.05 mmol s-1 kg-1 wet wt in type I and type II fibres, respectively. For both fibre types, the rates were 32 and 69% lower during the exercise at 150 and 122% VO2max. These data indicate that the glycolytic capacity of type I fibres is 30-35% lower than the capacity of type II fibres, in good agreement with the differences in phosphorylase and phosphofructokinase activities (Essén et al. 1975, Harris et al. 1976). The data also indicate that both fibre types contribute significantly to the anaerobic energy release at powers up till almost 200% VO2max.     

Adaptation of skeletal muscles to training

Based on the myosin ATPase reaction, human skeletal muscles are composed of two main fibre types, named slow (ST) and fast (FT) twitch fibres, respectively. With few exceptions, ST and FT fibres are evenly represented in the muscles, however with a large interindividual variation. Endurance athletes tend to have a predominance of ST fibres while sprinters have a predominance of FT fibres. The ST fibres are surrounded by 3-4 capillaries, and they have the largest potential for terminal oxidation and the smallest for glycolysis. Of the FT fibres, two subtypes may be distinguished (a and b), of which no FTb fibres are seen in the endurance trained muscles of athletes. Training also results in an increase in the number of capillaries for all fibre types. FTa fibres have a metabolic potential which is intermediate to that of the ST and FTb fibres. With endurance training, the potential for terminal oxidation increases, resulting in a larger ability to use fat as a fuel during submaximal exercise and in a reduced production of lactate. Thus, training has a glycogen sparing effect and endurance increases. Human intercostal muscles appear to have approximately 60% ST fibres. In the external intercostal muscles, the number of capillaries and the occurrence of FTb fibres is similar to the findings in untrained muscles. In contrast, the internal intercostal muscles placed in the mid-axillary line have no FTb fibres and relatively many capillaries. Thus, these (expiratory) muscles appear to be extensively used.     

Relationship between isometric endurance and fibre types in human leg muscles.

Relationship between isometric endurance performance at 50% of maximal voluntary isometric contraction (MCV) and skeletal muscle fibre composition has been elucidated in 19 physical education students. This was found to be linear and the equation corresponded to: y=9.35 + 1.093x; r=0.70 (endurance time expressed in seconds and fibre composition as percent slow twitch muscle fibres (ST) of the vastus lateralis muscle). As it is assumed from previous studies that similar isometric tensions preferentially recruit fast twitch muscle fibres (FT) and that the muscle at the point of exhaustion exhibits maximal values for lactate accumulation, it is suggested that lactate formed in FT fibres is released and stored in nonrecruited ST fibres. The ability to sustain similar isometric tension would then be depending on how large the fraction of ST fibres is that can serve as a lactaterecipient for lactate producing FT fibres.     

Relationship between Isometric Endurance and Fibre Types in Human Leg Muscles

Relationship between isometric endurance performance at 50% of maximal voluntary isometric contraction (MVC) and skeletal muscle fibre composition has been elucidated in 19 physical education students. This was found to be linear and the equation corresponded to: y=9.35 + 1.093x; r= 0.70 (endurance time expressed in seconds and fibre composition as percent slow twitch muscle fibres (ST) of the vastus lateralis muscle). As it is assumed from previous studies that similar isometric tensions preferentially recruit fast twitch muscle fibres (FT) and that the muscle at the point of exhaustion exhibits maximal values for lactate accumulation, it is suggested that lactate formed in FT fibres is released and stored in nonrecruited ST fibres. The ability to sustain similar isometric tension would then be depending on how large the fraction of ST fibres is that can serve as a lactate recipient for lactate producing FT fibres.     

Muscle fatigue and its relation to lactate accumulation and LDH activity in man

The lactate concentration in different muscle fibre types was determined in biopsy specimens from human vastus lateralis muscle after 30 and 60 s of maximal dynamic leg exercise. In addition, muscle fibre type distribution, total lactate dehydrogenase (LDH) activity, and isozymes of LDH were determined. In accordance with previous studies (Thorstensson and Karlsson 1976, Nilsson et al. 1977) it was found that an increasing proportion of slow twitch (ST) fibres corresponded to better sustained muscle force. Lactate was found preferentially in fast twitch (FT) fibres after 30 s, but after 60 s this difference was abolished. Differences between the two main muscle fibre types in muscle lactate, total LDH activity, and M-LDH activity were correlated to muscle fatigue. It was concluded that lactate or associated pH changes primarily in FT fibres could be one factor responsible for the impaired muscle function.     

Morphology and enzymatic capacity in arm and leg muscles in 78–81 year old men and women

Twelve men and twelve women 78–81 years of age were studied with muscle biopsies from the right vastus lateralis and biceps brachii and with measurements of isometric and isokinetic strength for knee-extension and isometric strength for elbow-flexion. Bicycle ergometry with determination of heart rate and oxygen uptake at submaximal (50 W) and “maximal” work loads was also performed. Body cell mass was estimated from measurements of total body potassium. Muscle fibre composition with respect to slow twitch (ST = type I) and fast twitch (FT = type II) fibres did not differ between the sexes and the younger subjects drawn from population studies. The mean fibre areas averaged in vastus lateralis 4.7 and 3.3 μm²× 10³ in men and women, respectively. This is less than 10 up to 30% of values found in sedentary younger subjects. The decline dominated in FT fibres, especially FTb fibres. In contrast biceps brachii did not show any matched fibre size reduction. The number of atrophic fibres was high and so was the frequency of “enclosed” fibres and areas with type grouping (ST fibres), indicating denervation—reinnervation. Such abnormalities are rarely seen in younger ages. Correlation analysis showed that only a minor part of the reduction in body cell mass with age could be explained by a reduction in fibre areas and that a reduction with age of the number of muscle fibres of both fibre types must be assumed. Positive correlations were observed between muscle strength and mean fibre and FT fibre areas. Comparing the present findings of skeletal muscle morphology to those in younger age groups, it is apparent that during the 8th decade of life major changes do occur. The measurements of aerobic and anaerobic enzymatic activities and of muscle capillary supply showed that levels comparable to those of younger age groups are maintained. Thus, quantitative rather than qualitative changes may explain the reduction in work performance with age.     

Muscle strength and fatigue after selective glycogen depletion in human skeletal muscle fibers

Summary Two groups of male subjects were studied to examine the effects of different exercise protocols on performance of an isokinetic, short-time strength test, the performance of which is related to fast twitch (FT) muscle fiber recruitment. The laboratory group (LG) (n=10) cycled (30 min, 70% VO₂ max), ran (75 min), and performed repeated bouts of sprint cycling and rapid, maximal contractions of the quadriceps. The marathon group (MG) (n=7) participated in and completed Stockholm's Marathon 1979. A strength test was performed before and within 1–2 h after completion of the group exercise protocol. The m. vastus lateralis was biopsied and muscle fibers classified as slow twitch (ST) or FT. After periodic acid-Schiff staining fibers were qualitatively classified as to glycogen content. In LG significant glycogen depletion occurred in both fiber types and in MG predominantly ST fibers were exhausted of glycogen after the exercise protocol. The glycogen exhaustion from both fiber types in LG was associated with impaired maximal muscular strength produced during a single dynamic contraction, as well as with reduced muscle fatigue patterns. When glycogen exhaustion was induced in ST muscle fibers only in the MG, no impairment was observed for maximal muscular strength but fatigue during 50 consecutive contractions was significantly increased.This study was supported in part by Coca-Cola Export Corporation, Sweden     

Concentrative amino acid uptake at the serosal side of colon mucosa

4 healthy men had biopsies taken from the soleus and the lateral head of the gastrocnemius muscle for determination of capillary supply, fibre type distribution and fibre area.In serial transverse sections slow twitch (ST) and fast twitch (FT) fibres were identified histochemically by myofibrillar adenosine triphosphatase stains and capillaries visualized by a periodic acid Schiff stain after digestion of glycogen by -amylase.In soleus, ST and FT fibre type areas were approximately two times greater than in gastrocnemius. FT fibres (11060 m²) were 50% greater than ST fibres (7520 m²) in soleus, whereas no difference between FT and ST fibres was observed in gastrocnemius (4730 m² and 4310 m², respectively). Both muscles were mixed with respect to fibre types but in all subjects soleus had a higher relative content of ST fibres than gastrocnemius. The area occupied by ST fibres relative to total area was 64% in soleus and 50% in gastrocnemius.Capillary density was 288 capillaries·mm^–2 in soleus and 365 in gastrocnemius. Evaluated on the basis of mean number of capillaries in contact with fibres of each type relative to fibre type area (m^–2·10^–3) individual ST fibres had a richer capillary supply than FT fibres in both soleus (0.84 and 0.57, respectively) and gastrocnemius (1.12 and 0.92, respectively).Differences in organization of the capillary network between soleus and gastrocnemius were observed. Capillaries are running mainly parallel to the muscle fibres in gastrocnemius, but tortuously and/or frequently branching in soleus. Tortuously and/or frequently branching capillaries implies nutritional advantages, which will not be reflected in the quantitative data obtained from counts of capillaries made on transverse sectioned muscle. Therefore the absolute values given for soleus and gastrocnemius, respectively, cannot be directly compared.     

Actomyosin ATPase,Myokinase, CPK and LDH in Human Fast and Slow Twitch Muscle Fibres

The enzyme activities of Mg²⁺ stimulated ATPase, creatine phosphokinase (CPK), myokinase (MK) and lactate dehydrogenase (LDH) were determined in pooled fast twitch (FT) and slow twitch (ST) human skeletal muscle fibres, dissected out from freeze-dried muscle biopsy material. All enzymes investigated demonstrated higher activities in FT fibres. The ratio in enzyme activity between fibre types was greatest for Mg²⁺ stimulated ATPase (3:1) and smallest for CPK (1.3: 1). In addition, the isozyme patterns of CPK, MK and LDH were studied by means of isoelectric focusing (CPK and MK) and discelectrophoresis (LDH). A difference was observed between fibre types with respect to the isozyme distribution of MK and LDH, whereas the CPK isozyme pattern was similar in both fibre types. These results on separated human FT and ST fibres were essentially in conformity with what has earlier been indicated from experiments on mixed muscle homogenates.     

Actomyosin ATPase, myokinase, CPK and LDH in human fast and slow twitch muscle fibres.

The enzyme activities of Mg2+ stimulated ATPase, creatine phosphokinase (CPK), myokinase (MK) and lactate dehydrogenase (LDH) were determined in pooled fast twitch (FT) and slow twitch (ST) human skeletal muscle fibers, dissected out from freeze-dried muscle biopsy material. All enzymes investigated demonstrated higher activities in FT fibres. The ratio in enzyme activity between fibre types was greatest for Mg2+ stimulated ATPase (3:1) and smallest for CPK (1.3:1). In addition, the isozyme patterns of CPK, MK and LDH were studied by means of isoelectric focusing (CPK and MK) and discelectrophoresis (LDH). A difference was observed between fibre types with respect to the isozyme distribution of MK and LDH, whereas the CPK isozyme pattern was similar in both fibre types. These results on separated human FT and ST fibres were essentially in conformity with what has earlier been indicated from experiments on mixed muscle homogenates.     

Is there a change in relative muscle fibre composition with age?

Muscle biopsy was taken from the vastus lateralis muscle during surgery of recent fractures of the neck of the femur in 52 patients aged 66-100 years. The percentage of slow twitch (ST) fibres was close to 55% with no significant change with age and no difference between patients with and without clinical diagnoses prior to the fracture. Also subgrouping of fast twitch (FT) fibres showed insignificant changes in this age range. Muscle fibre areas were smaller for FT than for ST fibres. A comparison between proximal and distal sampling site in the muscle did not show any significant difference. Combining these results and previous results from our laboratories no change in relative fibre composition with age could be proven.     

Fibre conduction velocity and fibre composition in human vastus lateralis

Summary The relationship between muscle fibre composition and fibre conduction velocity was investigated in 19 male track athletes, 12 sprinters and 7 distance runners, aged 20–24 years, using needle biopsy samples from vastus lateralis. Cross sectional areas of the fast twitch (FT) and slow twitch (ST) fibres were determined by histochemical analysis. The percentage of FT fibre areas ranged from 22.6 to 93.6%. Sprinters had a higher percentage of FT fibres than distance runners. Muscle fibre conduction velocity was measured with a surface electrode array placed along the muscle fibres, and calculated from the time delay between 2 myoelectric signals recorded during a maximal voluntary contraction. The conduction velocity ranged from 4.13 to 5.20 m·s^–1. A linear correlation between conduction velocity and the relative area of FT fibres was statistically significant (r = 0.84,p<0.01). This correlation indicates that muscle fibre composition can be estimated from muscle fibre conduction velocity measured noninvasively with surface electrodes.