Mechanical properties of human skeletal muscle from in vitro studies of biopsies

Noninvasive estimation of myosin heavy chain composition in human skeletal muscle

PURPOSE

Information on muscle fiber type composition is of great importance in muscle physiology and athletic performance. Because there are only a few techniques available that noninvasively and accurately provide an estimate of muscle fiber type composition, the development of additional and alternative approaches is required.

METHODS

Twenty-seven participants (21 men, 6 women) with an average age of 43 ± 18 yr, height of 175 ± 7 cm, and mass of 74 ± 12 kg participated in the study. Delay, contraction, and half relaxation times were calculated from tensiomyographic radial twitch responses of the vastus lateralis muscle. Univariate and multiple linear regression analyses were used to correlate the proportion of myosin heavy chain I (%MHC-I) in a biopsy obtained from the same muscle with a single and all three radial twitch parameters.

RESULTS

Delay, contraction, and half relaxation times all correlated with %MHC-I (r = 0.612, 0.878, and 0.669, respectively, at P ≤ 0.001). When all three parameters were included in a multiple linear regression, the correlation with the %MHC-I was even better (R = 0.933, P < 0.001).

CONCLUSIONS

These data suggest that time parameters of the skeletal muscle mechanical radial twitch response, measured with a contact linear displacement sensor, can be used as an accurate noninvasive predictor of the %MHC-I in a muscle.

Myosin light chain phosphorylation in intact human muscle

The phosphate content of the fast (LC2F) and two slow (LC2S and LC2S1) phosphorylatable light chains (P-light chains) in myosin isolated from biopsy samples of rested human vastus lateralis muscle averaged 0.21, 0.28 and 0.25 mol of phosphate per mol of P-light chain, respectively. Following a 10 s maximal contraction, phosphate content was increased by almost 2-fold in the fast and two slow P-light chains. After prolonged, moderate cycling activity phosphate content was only slightly increased in the three P-light chains. These data suggest that, unlike animal skeletal muscle, myosin light chain kinase and phosphatase activities are similar in human fast and slow muscle fibres.

Photoelectric transducer for measuring the length and diameter of elastic vessels

This photoelectric transducer was developed to measure diameter and length variations in arterial segments in vitro and in elastic tubes. Its construction is simple, its nonlinearity small and its sensitivity adequate. The cut-off frequency is 300 Hz and its small phase lag changes linearly with frequency.

Estimation of myodynamic parameter values from observations on isometrically contracting muscle groups

A new method is presented which makes it possible to estimate from a series of experimental observations of isometric maximum-effort muscle torque, a set of myodynamic parameter values for each of a number of muscles contributing collectively to the total torque output. The parameters that can be estimated are: the individual maximum isometric forces; the spreads of the length-tension curves; the relative maximum isometric tendon extensions; and the optimum muscle lengths; most of which parameters could not be estimated previously. The method is described for both penniform and fusiform muscles, and is demonstrated using the human triceps muscle as an example. The values obtained by this method are in general agreement with comparable values obtained by in vitro methods.

Analysis of stretch responses of a myocybernetic model muscle fibre

The characteristics of the response to stretch of a muscle fibre are investigated using a validated model of skeletal muscle. It is found that the model correctly predicts all the peculiar features of the stretch response: the initial rapid force rise; the subsequent slower rise; the slow decay of the force upon termination of the elongation; and the dependence of these phenomena upon stretching velocity and muscle length. The hypothesis explaining these phenomena is discussed in detail.