Studies of the interaction of muscle and tendon properties in understanding human muscle force and power

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In the present study, we measured the contraction-induced shortening (dL) of individual synergistic human muscles in a repeated motor task to assess their contractile behaviour. Ultrasonography was used to obtain dL measurements in the gastrocnemius (GS) and soleus (SOL) muscles of six men performing 11 consecutive isometric plantarflexions. Contractions 1 and 11 were performed with maximal effort, and contractions 2-4, 5-7 and 8-10 were performed with efforts generating 50, 70 and 90%, respectively, of the plantarflexion moment produced in contraction 1. In contractions 5-10, the SOL muscle dL was similar (p > 0.05) to that produced in contraction 1 (approximately 6 mm), indicating that the SOL muscle became fully activated at 70% of the maximum plantarflexion moment. The GS muscle dL in contractions 10 and 11 exceeded by approximately 0.5 mm (p < 0.05) and 1.3 mm (p < 0.01), respectively, that generated in contraction 1 (approximately 10 mm), despite evidence obtained by superimposed stimulation that contraction 1 was produced with full motor unit activation. The consequent paradox that the GS muscle would produce in contractions 10 and 11 a greater activation and therefore more force than its actual potential is resolved when considering the interaction between the time-dependent tensile response of tendon and the performance of muscle as dictated by the sliding filament mechanism of contraction

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Sustaining Human Muscle Power and Resisting Fatigue

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This review paper is based on an invited key-note lecture given by Professor Anthony J Sargeant to the Polish Physiological Society. As a review it deals with one aspect of the research interests and developments initiated by Professor Sargeant over many years.
Journal of Physiology and Pharmacology 2006 Nov;57 Suppl 10:5-16

During human locomotion the ability to generate and sustain mechanical power output is dependent on the organised variability in contractile and metabolic properties of the muscle fibres that comprise the active muscles. In studies of human exercise we have used a micro-dissection technique to obtain fragments of single muscle fibres from needle biopsies before and after exercise. Each fibre fragment is divided into two parts.

One part is used to characterize the fibre type in respect of the heavy chain myosin isoform expressed. The other part of the fragment is analysed for high energy phosphate concentrations. Fibres are classified on the basis of expressing either type I, type IIA, or type IIX myosin heavy chain isoforms. It should be noted however that in the type II population many fibres co-express both IIA and the IIX isoforms and we therefore characterize these fibres on the basis of the degree of co-expression. We have used this technique to examine the time course of high energy phosphate concentration and fatigue in different fibre populations during exercise. The progressive reduction of power during maximal sprint efforts may be interpreted as the cumulative effect of metabolic depletion in successive fibre type populations from IIX to IIXa to IIAx to IIA to I. One important application of the micro-dissection technique is that PCr content may also be used as a very sensitive metabolic marker for fibre type recruitment during very short duration concentric, isometric and eccentric exercise

How are different human muscle fibre types recruited in exercise?

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This study using a PCr/Cr ratio analysis of single human muscle fibre fragments obtained from needle biopsy during exercise was part of along term interest of Professor Anthony J Sargeant. It is based on techniques developed in his research team over many years.
Journal of Applied Physiology
J Appl Physiol. 2007 Nov;103(5)

In the literature, an inconsistency exists in the submaximal exercise intensity at which type II fibers are activated. In the present study, the recruitment of type I and II fibers was investigated from the very beginning and throughout a 45-min cycle exercise at 75% of the maximal oxygen uptake, which corresponded to 38% of the maximal dynamic muscle force. Biopsies of the vastus lateralis muscle were taken from six subjects at rest and during the exercise, two at each time point.

From the first biopsy single fibers were isolated and characterized as type I and II, and phosphocreatine-to-creatine (PCr/Cr) ratios and periodic acid-Schiff (PAS) stain intensities were measured. Cross sections were cut from the second biopsy, individual fibers were characterized as type I and II, and PAS stain intensities were measured. A decline in PCr/Cr ratio and in PAS stain intensity was used as indication of fiber recruitment. Within 1 min of exercise both type I and, although to a lesser extent, type II fibers were recruited. Furthermore, the PCr/Cr ratio revealed that the same proportion of fibers was recruited during the whole 45 min of exercise, indicating a rather constant recruitment. The PAS staining, however, proved inadequate to fully demonstrate fiber recruitment even after 45 min of exercise. We conclude that during cycling exercise a greater proportion of type II fibers is recruited than previously reported for isometric contractions, probably because of the dynamic character of the exercise. Furthermore, the PCr/Cr ratio method is more sensitive in determining fiber activation than the PAS stain intensity method.

What limits Maximum Oxygen Uptake in Human Exercise?

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Anthony J Sargeant proposed this approach to using a reduced muscle mass to examine the limitation to maximum oxygen uptake (100% 0xygen cannot be used – hence 45%) enhanced oxygen increases Maximum Oxygen Uptake in 2-leg exercise where cardiac output is limiting – but does not increase the maximum oxygen uptake in 1-leg exercise where the limitation is not the delivery of oxygen but the muscle enzyme systems capability to take up and utilize the oxygen. This PhD research work was carried out in the laboratory of CTM Davies who in the tradition of the day put his name first on nearly all research leaving his laboratory.

Anthony J Sargeant examined the question: Are human arm muscles less efficient than leg muscles?

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Reported differences in the power output and efficiency of arm work compared to leg work are an artefact. Typical arm work research actually study arm work plus a contribution from upper body muscles. When the upper body contribution is eliminated by restraining movement so that arm cranking is only performed by arm muscles the efficiencies and power are not significantly different to leg work.

Nor-adrenaline and adrenaline responses to different forms and intensities of exercise

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A preliminary report to The Physiological Society (later published in full : see Eur J Appl Physiol Occup Physiol. 1974 Mar 28;32(3):195-206.

What physiological signals determine the perception of exertion?

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Journal of Human Ergology
J Hum Ergol (Tokyo). 1973 Sep;2(1):3-11

This was a by-product of research looking at exercise performed with different muscle masses. It was thought that it might help the understanding of which physiological cues were important in determining the perception of exertion.