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Selection of fuels for muscle contraction

Even at rest, skeletal muscle accounts for more than one-fifth of total energy expenditure; obviously this increases greatly with exercise.

restingMuscle can use a variety of fuels, depending on the intensity of work being performed, the duration of the exercise and also whether the individual is in the fed or fasting state:

plasma glucose
muscle glycogen
triacylglycerol from plasma lipoproteins
triacylglycerol from adipose tissue reserves within the muscle
plasma non-esterified fatty acids
plasma ketone bodies


Skeletal muscle contains two types of fibres:



Experiment 1: Is ATP used in muscle contraction?

gastrocnemiusRats were anaesthetised and the gastrocnemius muscle on one hind limb was exposed and subjected to electrical stimulation for three minutes to cause contractions. The animals were killed, and both hind limbs were immediately immersed in liquid nitrogen. The gastrocnemius muscles from both limbs were dissected out, and the concentrations of ATP, ADP, creatine phosphate and creatine were measured.

µmol /g muscle unstimulated (at rest) after stimulation
ATP 5 4.9
ADP 0.01 0.11
creatine phosphate 17 1.0
creatine 0.1 16.1


What conclusions can you draw from these results?

Only a very small amount of ATP is apparently consumed (and the fall is accounted for by increased ADP). However, a considerable amount of creatine phosphate is consumed (and again the difference is accounted for by increased creatine).

Muscle contraction actually uses a large amount of ATP, which is required immediately. However, there is a significant time lag before the rate of oxidation of metabolic fuels increases to supply ATP. Remember that the total body pool of ATP is very small, but turns over rapidly.

Creatine phosphate acts as an intermediate store of phosphate to rephosphorylate ADP to ATP until metabolic activity increases in response to increased demand for muscle contraction.creatine






This means that the concentration of ATP remains more or less constant, but creatine phosphate is depleted. After the muscle contraction ceases, or when the rate of oxidation of metabolic fuels has increased sufficiently, creatine will be rephosphorylated to creatine phosphate.

Creatine phosphate also acts to shuttle phosphate from the sites of ATP formation (from glycolysis in the cytosol and oxidative phosphorylation in mitochondria) to the sites in the cell where it is required for muscle contraction.

Creatinine is formed by non-enzymic cyclisation of creatine or creatine phosphate; it is a metabolically useless product, and is excreted in the urine. creatinine

A 70 kg man excretes approximately 16 mmol of creatinine per day; a 70 kg woman approximately 10 mmol / day.

Can you account for this gender difference in creatinine excretion?

Creatinine formation depends on total creatine content of the body, mainly in muscle; higher creatinine excretion reflects high proportion of muscle in males.

Why is it usual to express urinary excretion of various metabolites per mol of creatinine rather than per litre of urine?

Urine volume, and hence concentration of metabolites, is highly variable; creatinine excretion is reasonably constant from day to day.

Urinary excretion of creatine is normally < 400 µmol /day. Under what conditions would you expect creatine excretion to be increased significantly?

Any condition leading to net loss of muscle. This may be muscle atrophy in disease or as a result of prolonged bed rest, or may be the normal loss of myometrium in the female menstrual cycle.

Experiment 2: Glucose utilisation by muscle

anaestshetisedFasting dogs was anaesthetised and the femoral artery and popliteal vein were cannulated to permit measurement of arterio-venous differences across the gastrocnemius-plantaris muscle group, at rest and after electrical stimulation to twitch 1 or 5 times per second, for 30 minutes.

The table shows glucose and oxygen uptake into the muscle, and lactate output from the muscle under these conditions.

nmol /g muscle /min
at rest
1 twitch / sec
5 twitches /sec
glucose uptake
oxygen uptake
lactate output
ratio lactate output : glucose uptake


A muscle biopsy sample was taken from both the stimulated and unstimulated leg for measurement of glycogen (µmol glucose equivalent /g muscle).

  glycogen (┬Ámol glucose equivalent /g muscle)
1 twitch /sec
5 twitches /sec

From data reported by Chaplet CK & Stainsby WN (1968). Carbohydrate metabolism in contracting dog skeletal muscle in situ. American Journal of Physiology 215: 995-1004.

What conclusions can you draw from these results?

See the answer