FATIGUE PROCESS OF TYPE I AND TYPE II MUSCLE FIBERS

K. Kerševan, V. Valenčič and N. Knez

Laboratory of Biomedical Visualization and Muscle Biomechanics

University of Ljubljana, Faculty of Electrical Engineering, Slovenia

SUMMARY

Skeletal muscles’ fatigue process was examined in muscles biceps brachii and tibialis anterior in 9 male subjects, aged 21 – 24 years. Measurements were performed under isometric conditions. Pulse trains (0.1ims) of different stimulation frequencies and voltage (40–50 V above threshold) were used for 10 s bipolar electrical stimulation. The non-invasive tensiomyographic (TMG) method was used for detection of muscle responses to electrical stimulation. Using pulse trains of different frequencies enabled separate observation of fatigue and tetanization process for both muscle fibre types:

— Type I (fatigue resistant) muscle fibres exerted and retained force throughout the 10 s electrical stimulation. Fatigue process was observed at stimulation frequency of 2 Hz but the type I muscle fibre fatigue did not occur due to their fatigue resistant character. Complete tetanus of type I muscle fibres was noticed at stimulation frequencies of 4 – 6 Hz .

— Fatigue process of type II (fast fatigue) muscle fibres ended before the electrical stimulation was over and was noticed at stimulation frequencies of 7.5 – 9 Hz. Complete tetanus of type II muscle fibres was noticed at stimulation frequencies higher than 18 Hz.

Separate observation of type II muscle fibres enables more efficient treatment and observation of pathological changes in dystrophic, atrophic and denervated patients and also helps professional athletes and their trainers to improve their training technique.

STATE OF THE ART

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Fig. 1: Twitch summation and tetanization in human biceps brachii The diversity of skeletal muscles, which is reflected by the heterogeneity and spatial arrangement of their individual fibres, enables numerous movements of different velocities, forces and endurances, as well as adaptation to altered demands of usage. Both clinical and sport physiologists have always been interested in studying muscle adaptation process as a result of either pathological changes or targeted training process.

Type II muscle fibre fatigue process was observed in unfused tetanus. When a series of stimuli were given, each stimulus elicited a single twitch response so that a series of twitch responses was produced. As the stimulating frequency increased, the twitches began to sum and the response profile changed from unfused to fused tetanus (Fig. 1). For this purpose two muscles were selected with respect to percentage of type I muscle fibres they comprise: muscle biceps brachii (BB):iapprox. 52 % and muscle tibialis anterior (TA): approx. 77 % /1/; according to dissimilar percentage of type I muscle fibres we expected dissimilarities in type II muscle fibre fatigue process as well.

MATERIAL AND METHODS

TMG recordings

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Fig. 2: Principle of TMG measuring method: when muscle contracts, its belly enlarges. Radial enlargements can be measured by displacement sensor. In order to record unfused tetanus, tensiomyographic (TMG) measuring method was used. This is a non-invasive, selective method for detection of skeletal muscles’ contractile properties /2/. In previous studies it has been established that the radial displacement of muscle belly is proportional to muscle force /3/ as well as to percentage of type I muscle fibers /1/.

TMG method is based on muscle contracting principle under isometric conditions: when the muscle is contracted it increases the force between insertions and its middle part – muscle belly – is thickened. Displacement of a muscle belly is measured with the displacement sensor, positioned radial to the skin above the observed muscle (Fig. 2).

For the presented study, the measured subject was sitting on a measuring chair with his measured arm/leg fastened to the frame with one or two bands in order to achieve isometric condition. Measurements were performed with an inductive sensor incorporating a spring of 0.17 N/mm, which provides an initial pressure of approximately 1.5 x 10^–2 N/mm^–2 on a tip area of 113 mm².

Measuring point for each muscle was determined anatomically on the basis of the anatomic guide for electromyographers /4/ – BB (right side): midpoint of the line between lateral head of calvicula and head of radius; TA (left side): four fingerbreadths below tibial tuberosity and one fingerbreadth lateral to tibial crest.

Each muscle was stimulated with pulse trains of 0.1ims duration, stimulation frequencies ranging from 5–25 Hz and voltage ranging from 40–50 V above threshold. For 10 s stimulation, two self-adhesive electrodes were used, placed symmetrically to the sensor: the anode was placed distally and the cathode proximally, 20–50 mm from the measuring point. The stimulator used was a Grass 8800 stimulator, with voltage output through an insulation unit. The measured muscle responses were stored and analysed using a PC.

Data analysis

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Fig. 3: Time course of muscle response to pulse train in human biceps brachii (left) and tibialis anterior (right)

In unfused tetanus, the dynamics of type II fibre fatigue process (oscillating part of TMG response) was observed (Fig. 3) and described with the following parameters: t – fatigue process time constant, t_F – fatigue process duration, f_F – frequency, at which type II fibres fatigue process was observed,

f_T – fused tetanus frequency and d_PPmax – maximum value of peak to peak oscillation. According to Thomas, Johansson & Bigland-Ritchie (1991) /5/, fast-twitch motor units exhibit a sag profile in the unfused tetanus. In the presented study, the sag profile was eliminated from data analysis (Fig. 4).

Before the data was analysed, the oscillating part of TMG response was rectified and course of amplitudes was fitted by exponent curve (Fig. 5):

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Fig. 5: Time course of amplitudes in rectified oscillation Text Box:

Fig. 4: The sag profile was eliminated from data analysis

RESULTS

The dynamics of type II fibre fatigue process in human BB and TA was observed and fatigue process parameters were compared (Table 1):

Muscle
Biceps brachii (right side)	3.94 s	8.1 s	8.6 Hz	19.6 Hz	1.9 mm
Tibialis anterior (left side)	1.02 s	6.4 s	7.7 Hz	18.3 Hz	0.2 mm

Table 1:Aaverage values of type II fibres fatigue process parameters for two different human skeletal muscles

DISCUSSION

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Rectus femoris (after poliomyelitis) 35.5 s 8.3 Hz
Rectus fempris (healthy subjects) 73.3s 12.3 Hz
Statistical significance (P) <0.05 0.02

Table 2: Comparison of average values of type II fibres fatigue process parameters with patients after poliomyelitis and control group The main objective of this study was to find out whether the TMG method is suitable for monitoring the unfused tetanus and whether it provides any information on skeletal muscles’ structural or functional changes. According to both muscles’ sizes and to the percentage of type I muscle fibres comprised in them, the above listed results were expected: (1) the greater the amount of type II muscle fibres (in BB), the longer the duration and the greater the time constant of the fatigue process; (2) in TA, both frequency-dependent phenomena occurred at lower stimulation frequencies – again, the reason might be the percentage of type II fibres. In case of a different protocol (e.g. supra-maximum stimulation and fixed stimulation frequency), with respect to their diameter, maximum value of peak-to-peak oscillation could be used as an indicator for the type II fibres quantity (Fig. 5) – however, the physiological properties of the measured muscle have to be taken into account (muscle’s volume, fascia’s thickness, etc.). Type I muscle fibres fatigue was not noticed during the 10 s stimulation.

This procedure (supra-maximum stimulation (amplitude), variable stimulating frequency) has already been evaluated in clinical environment with patients after poliomyelitis in muscle rectus femoris. When compared to EMG output, TMG method provided useful data on muscle dysfunction as well (Table 2). Apart from that, TMG method is easy to apply, the same set of equipment is suitable for measurements of all surface skeletal muscles and measuring results are available immediately after the measurement.

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Fig.6: Responses to electrical stimulation (supra-maximum, 10 Hz) before (upper) and after (lower) targeted training process, m. VL In the sport field, the described procedure (supra-maximum stimulation, fixed stimulating frequency) is used to monitor the adaptation of the motor system to specific training process. These adaptations can be extensive and have been shown to affect most aspects of the system, both morphological and functional. TMG method’s selectivity enables observation of a single muscle within a given muscle group (Fig. 6). In Fig. 6 the effect of specific training process on muscle vastus lateralis is presented: initial muscular status (upper) and muscular status after specific 10 day training process (lower).

REFERENCES

/1/ Dahmane R., Valenčič V., Knez N. and Eržen I., Evaluation of the ability for non-invasive estimation of the muscle contractile properties on the basis of the muscle belly response, Medical & Biological Engineering & Computing, Vol. 39, 2001, 51-55

/2/ Kogovšek N. and Valenčič V., Measuring of skeletal muscles’ dynamic properties, Artificial organs, Vol. 33, No. 3, 1997, 240-242

/3/ Valenčič V., Direct measurement of the skeletal muscle tonus, Advances in external control of human extremities, Vol. 10, 1990, 575-584

/4/ Delagi E. F., Perotto A., Iazzetti J. and Morrison D., Anatomic guide for the electromyographer: the limbs, Charles C. Thomas, Springfield, Illinois, USA, 1975

/5/ Enoka R. M., Neuromechanical Basis of Kinesiology, Second edition, Human Kinetics, The Cleveland Clinic Foundation, 1994

/6/ Bottinelli R. and Reggiani C., Human skeletal muscle fibres: molecular and functional diversity, Progress in Biophysics & Molecular Biology, Vol. 73, 2000, 195-262

/7/ Salmons S. and Vrbova G., The influence of activity on some contractile characteristics of mammalian fast and slow muscles, Journal of Physiology, Vol. 201, No. 3, 1969, 535-549

ACKNOWLEDGEMENTS

The presented study was supported by the Slovenian Ministry of Education, Science & Sport, Foundation for financing sport organizations in Republic of Slovenia and Our Space Ltd.