This research published in the Journal of Neurophysiology was carried out in Amsterdam by Jo De Ruiter as part of his doctoral thesis supervised by Professor Anthony J Sargeant and Arnold de Haan. It was part of a series of studies examining the regional differences within a single muscle of physiological properties and hence pattern of recruitment in response to different intensities of exercise.
1. The effect of muscle unit (MU) localization on physiological properties was investigated within the fast-twitch fatigue-resistant (FR) and fast-fatigable (FF) MU populations of rat medial gastrocnemius (MG) muscle. Single MG MUs were functionally isolated by microdissection of the ventral roots. FR and FF MU properties of the most proximal and distal muscle compartments were compared. The most proximal and distal compartment are subvolumes of the MG innervated by the most proximal and distal primary nerve branch, respectively. A subsample of the isolated units was glycogen depleted and muscle cross sections were stained for glycogen and myosin-adenosinetriphosphatase.
2. It was shown that proximal FF and FR units reached optimum length for force production at shorter muscle lengths compared with the distal FR and FF units.
3. The fast MUs of the proximal compartment had small territories that were located close to and/or within the mixed region (containing type I, IIA, IIX, and IIB fibers) of the muscle. The fast MUs of the distal compartment had greater territories that were located in the more superficial muscle part (containing only type IIX and IIB fibers) and in some cases spanned the entire area of the distal muscle compartment.
4. FR and FF MUs consisted of muscle fibers identified histochemically as type IIX and IIB, respectively.
5. Within each of the FR and FF MU populations, MUs that were located in the most proximal muscle compartment were more resistant to fatigue compared with the units located in the most distal compartment.
6. Cross-sectional fiber areas were smaller for the proximal FR and FF fibers, but specific force did not differ among units. Consequently, when account was taken of the innervation ratio, the proximal FR and FF units produced less force than distal units of the same type. Tetanic forces were 87 +/- 27 (SD) mN (proximal FR), 154 +/- 53 (SD) mN (distal FR), 142 +/- 25 (SD) mN (proximal FF), and 229 +/- 86 (SD) mN (distal FF).
7. The present findings suggest that with increasing demand placed on rat MG during in vivo locomotion, recruitment is likely to proceed from proximal to distal muscle parts within the FR and FF MU populations.
This research carried out in Amsterdam under the direction of Professor Anthony J Sargeant demonstrated how within the same anatomical muscle there can be quiet different physiological properties in different areas of the same muscle. This work was part of the PhD research of Jo de Ruiter supervised by Professor Tony Sargeant and Arnold de Haan.
The most proximal and distal motor nerve branches in the rat medial gastrocnemius innervate discrete muscle compartments dominated by fast-twitch oxidative and fast-twitch glycolytic fibers, respectively. The functional consequences of the difference in oxidative capacity between these compartments were investigated. Wistar rats were anesthetized with pentobarbital sodium (90 mg/kg ip). Changes in force of both compartments during 21 isometric contractions (train duration 200 ms, stimulation frequency 120 Hz, 3 s between contractions) were studied in situ with and without blood flow. Without blood flow, force and phosphocreatine declined to a greater extent in the proximal than the distal compartment compared with the run with intact flow. After the protocol without blood flow, when flow was restored, the time constants for force recovery (which were closely associated to the recovery of phosphocreatine) were 37 +/- 7 (SD) (proximal compartment) and 148 +/- 20 s (distal compartment). It was concluded that the proximal compartment had a four times higher oxidative capacity and, therefore, a superior ability for repeated force production.
In this research published in Journal of Physiology Anthony Sargeant and his team describe how the recruitment of different types of muscle fibres with increasing exercise intensity changes the oxygen cost of exercise. Thus the relationship of oxygen uptake and mechanical power output is not constant. This is in contrast to the standard teaching of many physiology textbooks.
1. A slow component to pulmonary oxygen uptake (VO2) is reported during prolonged high power exercise performed at constant power output at, or above, approximately 60% of the maximal oxygen uptake. The magnitude of the slow component is reported to be associated with the intensity of exercise and to be largely accounted for by an increased VO2 across the exercising legs.
2. On the assumption that the control mechanism responsible for the increased VO2 is intensity dependent we hypothesized that it should also be apparent in multi-stage incremental exercise tests with the result that the VO2-power output relationship would be curvilinear.
3. We further hypothesized that the change in the VO2-power output relationship could be related to the hierarchical recruitment of different muscle fibre types with a lower mechanical efficiency.
4. Six subjects each performed five incremental exercise tests, at pedalling rates of 40, 60, 80, 100 and 120 rev min-1, over which range we expected to vary the proportional contribution of different fibre types to the power output. Pulmonary VO2 was determined continuously and arterialized capillary blood was sampled and analysed for blood lactate concentration ([lactate]b).
5. Below the level at which a sustained increase in [lactate]b was observed pulmonary VO2 showed a linear relationship with power output; at high power outputs, however, there was an additional increase in VO2 above that expected from the extrapolation of that linear relationship, leading to a positive curvilinear VO2-power output relationship. 6. No systematic effect on the magnitude or onset of the ‘extra’ VO2 was found in relation to pedalling rate, which suggests that it is not related to the pattern of motor unit recruitment in any simple way.
The ability of generating muscle power power is important whether you are an Olympic athlete, a ballet dancer, or an elderly person wanting to climb the stairs to go to bed. In this comprehensive review of his research Anthony Sargeant points out the importance of different types of muscle fibres that make up the human skeletal muscles that produce power in legs and arms. Tony also points out that in research seeking to measure human muscle power it is essential to measure or control the speed at which the power is generated (this is because power is the product of work and velocity).
Structural and functional determinants of human muscle power
by Anthony J Sargeant
Measurements of human power need to be interpreted in relation to the movement frequency, since that will determine the velocity of contraction of the active muscle and hence the power available according to the power-velocity relationship. Techniques are described which enable movement frequency to be kept constant during human exercise under different conditions. Combined with microdissection and analysis of muscle fibre fragments from needle biopsies obtained pre- and postexercise we have been able ‘to take the muscle apart’, having measured the power output, including the effect of fatigue, under conditions of constant movement frequency. We have shown that fatigue may be the consequence of a metabolic challenge to a relatively small population of fast fatigue-sensitive fibres, as indicated by [ATP] depletion to approximately 30% of resting values in those fibres expressing myosin heavy chain isoform IIX after just 10 s of maximal dynamic exercise. Since these same fibres will have a high maximal velocity of contraction, they also make a disproportionate contribution to power output in relation to their number, especially at faster movement rates. The microdissection technique can also be used to measure phosphocreatine concentration ([PCr]), which is an exquisitely sensitive indicator of muscle fibre activity; thus, in just seven brief maximal contractions [PCr] is depleted to levels < 50% of rest in all muscle fibre types. The technique has been applied to study exercise at different intensities, and to compare recruitment in lengthening, shortening and isometric contractions, thus yielding new information on patterns of recruitment, energy turnover and efficiency.
Anthony Sargeant directed this work on skeletal muscle as Head of the Amsterdam research group. The meticulous work was carried out by Jose Sant’Ana Pereira who was one of Tony Sargeant’s PhD students.
In the present study we report a novel histochemical method which, by sequential pre-incubations in alkaline and acidic media, selectively differentiates muscle fibres expressing myosin heavy chain IIX, on the basis of a specific profile for myofibrillar actomyosin ATPase (mATPase) activity. The enzyme reactions were tested for specificity by means of anti-myosin heavy chain monoclonal antibodies, which were characterized on Western blots of muscle homogenates. Enzyme histochemical reactions with the traditional pH buffers were compared to those of the new method and, in conjunction with the immunoreactions, used to confirm the relationship between MyHC expression and the distinct profiles for mATPase. Immunohistochemical reactions demonstrated that the new method only differentiates those fibres expressing myosin heavy chain IIX. The method revealed a continuum in which the intermediate staining intensities corresponded to hybrid fibres expressing myosin heavy chain IIX in combination with either the IIA or IIB forms. Quantitative histochemistry and immunohistochemistry (by image analysis), used to examine the relationship between staining intensities for mATPase and amounts of myosin heavy chain IIX expression, revealed that the new method discriminates well between hybrid fibres expressing variable amounts of the IIX isoform (r2 = 0.93)
Cycling performance depends upon overcoming air and rolling resistance in this research the results of ‘coasting down’ experiments were used by the authors to calculate these components. The experiments were performed in the massive indoor Flower Hall near Amsterdam on a Sunday morning. Anthony Sargeant was the head of the research department which carried out this work.
To calculate the power output during actual cycling, the air friction force Fa and rolling resistance Fr have to be known. Instead of wind tunnel experiments or towing experiments at steady speed, in this study these friction forces were measured by coasting down experiments. Towing experiments at constant acceleration (increasing velocity) were also done for comparison. From the equation of motion, the velocity-time curve v(t) was obtained. Curve-fitting procedures on experimental data of the velocity v yielded values of the rolling resistance force Fr and of the air friction coefficient k = Fa/v2. For the coasting down experiments, the group mean values per body mass m (N = 7) were km = k/m = (2.15 +/- 0.32) x 10(-3)m-1 and ar = Fr/m = (3.76 +/- 0.18) x 10(-2)ms-2, close to other values from the literature. The curves in the phase plane (velocity vs acceleration) and the small residual sum of squares indicated the validity of the theory. The towing experiments were not congruent with the coasting down experiments. Higher values of the air friction were found, probably due to turbulence of the air.