Measuring of Skeletal Muscles’ Dynamic Properties

Vojko Valencic and Natasa Knez

Faculty of Electrical Engineering, University of Ljubljana, Ljubljana, Slovenia

Abstract: Many studies have been completed to deter-mine how muscles work. One of the aspects to study is themuscle response and how to measure it. The main aim ofthis study was to build a noninvasive measuring systemthat would be simple to use and would measure the muscleresponse as close to the muscle as possible. The measuringmethod was based on a magnetic displacement sensormeasuring the muscle belly response. The sensor wasplaced adjacent to the skin over the muscle and measuredradial movements of the muscle belly. The muscle re-sponse to single twitch electrical stimulation was mea-sured. Different skeletal muscles or muscle groups havedifferent biomechanical characteristics. The rise time ofthe muscle belly response was the characteristic parameter

for this study. The comparison of muscles’ responses interms of the values of their normalized rise time param-eters confirmed their identities as slow or fast muscles. Thevalue of the normalized rise time parameter of a slowmuscle was four times less than the value of the parameterof a fast muscle. Although the question remains as to howthe available measuring techniques provide estimations ofskeletal muscles’ dynamic properties, this proposed mea-suring method contributes to a better understanding ofskeletal muscles’ dynamic properties. It offers a possibleway of studying the muscle structure from the muscle’sresponse to electrical stimulation in a simple noninvasiveway. Key Words: Muscle response—Noninvasive mea-suring system—Skeletal muscle dynamic properties.

Skeletal muscles move the body and increase forcebetween insertions by shortening the muscle fibers.There are some methods for directly measuringmuscle force, but these methods are usually too in-vasive for clinical applications. Some different meth-ods have been proposed and tested for measuringthe muscle force indirectly. In dystrophic patients,muscle responses are measured by measuring thetorque about a specific joint (1,2). The early musclechanges in such patients are evident in the tibialisanterior muscle (2). When the muscle is activated byelectrical stimulation, an antagonistic group ofmuscles responds. This is observed particularly whenantagonistic muscles are weak due to neuromusculardiseases or denervation. In such cases the directmeasurement of muscle responses is obligate. Thetorque about the ankle joint is a result of the con-traction of many muscles. In standard procedures aphysician detects a muscle response visually and bytouching the muscle belly with his fingers. This pro-cedure is the basis for the simple measuring methodbased on a displacement sensor. The displacement

sensor is placed in a position radial to the measuredmuscle and adjacent to the skin above the musclebelly as illustrated in Fig. 1.

The advantages of the displacement sensormethod over the torque measuring method are theselectivity of measured muscle responses and theability to use the same equipment for measuring theresponses of all skeletal muscles lying close enoughto the skin. The sensor measures skeletal muscle re-sponses on the surface of the skin over the muscle.This technique has been tested in previous experi-ments to measure responses of the tibialis anteriormuscle in healthy subjects (3,4) and in the gluteusmaximus muscle in subjects with above-knee ampu-tations (5). This method is used as a standard pro-cedure in the clinical environment for persons afterabove-knee amputation (5).

MATERIALS AND METHODS

For the feasibility study, the muscle responses of ahealthy subject were measured. The responses of 5different muscles were compared. The subject was a25-year-old male. Electrical stimulation was pro-vided by a single DC stimulus of 100 V and 1 msduration. The two surface electrodes (self-adhesive

Received February 1996.Address correspondence and reprint requests to Dr. Natasa

Knez, Faculty of Electrical Engineering, University of Ljubljana,Trzaska 25, 1111 Ljubljana, Slovenia.

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rectangular, 4 × 9 cm) were fixed to the skin 5 cmdistal toward both muscle insertions from the mea-suring point. The joint was left loose so that the mea-surement was not performed in an isometric condi-tion. The displacement sensor was pressed to theskin over the measured muscle with a pressure ofabout 0.2 N/cm2. The sensor was attached to an armfixed to the bed on which the subject was lying dur-ing the measurement. The sensor was placed radialto the skin surface from the measuring point. In pre-vious studies (3), the optimum position of a measur-ing point had been determined. The best results inlong, slim muscles had been achieved at the pointwhere the highest muscle belly displacement hadbeen detected. The measuring point of each mea-sured muscle was usually on the proximal third of itsspecific length. The exact positioning of the sensortip had not been significant when the characteristicsof skeletal muscles had been studied from the shapesof their responses.

The muscles to be measured were chosen by theirmuscle fiber characteristics. The quadriceps and bra-chioradialis muscles have more fast muscle fibers;the soleus muscle is known as a slow muscle. Forcomparison, the gastrocnemius and tibialis anteriormuscles were also chosen. The muscle belly responsehas been analyzed by means of time parameters. Themost significant parameter for this study was therise-time of the muscle response. The rise-time (Dtr)is the time between 10% and 90% of the maximumvalue of the muscle response (Fig. 2).

vr =Ddr

Dtr@mm/s# (1)

Where vr is a velocity parameter of a muscle bellyresponse, Dtr is the rise time, and Ddr is the differ-ence in displacement of a muscle belly during therise time. To compare the rise time of differentmuscles, the rise time has to be normalized to the

maximum value of a certain response (Eq. 2). Thenormalized rise time Dtr is related to the tangent ofthe response rise angle and estimates the velocity ofa muscle belly response.

vrn =vr

dm=

Ddr /Dtrdm

@mm/s/mm# (2)

Where vrn is the normalized velocity parameter ofthe muscle belly response, Dtr is the rise time, Ddr isthe difference in displacement of a muscle belly dur-ing the rise time, and dm is the maximum relativedisplacement of a muscle belly response (Fig. 2).Since Ddr equals 0.8 z dm, the normalized velocity pa-rameter depends only on the rise time:

vrn =0.8

Dtr@mm/s/mm# (3)

RESULTS

The measured responses of 5 muscles were ana-lyzed. The values of the normalized velocity param-eters vrn were studied (Fig. 3). For the quadricepsand brachioradialis muscles, the values of the nor-malized velocity parameters were greater than thosefor other muscles. The 2 muscles are known as fastmuscles. The value of the normalized velocity pa-rameter was 5 times smaller for the slow soleusmuscle. For the gastrocnemius and tibialis anteriormuscles, the values of the normalized velocity pa-

FIG. 1. Shown are the locations for the electrodes and the dis-placement sensor used to measure the muscle belly response toelectrical stimulation for the tibialis anterior muscle.

FIG. 2. This graph shows the muscle belly response to an elec-trical stimulus of 1 ms duration. The two parameters used foranalysis are Dtr, rise time, and Ddr, displacement during a risetime.

FIG. 3. The normalized velocity parameters (vrn) of 5 musclesare compared on this bar graph.

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rameters were greater than those for slow musclesand less than those for fast muscles.

DISCUSSION

Measurements of the muscle belly responses toelectrical stimuli revealed differences amongmuscles. The most significant difference amongmuscle responses was due to their structures. For thequadriceps and brachioradialis muscles, we foundthe normalized velocity parameter vrn to be 4 timesgreater than the parameter for the soleus muscle.Both the quadriceps and brachioradialis muscles areknown as fast muscles with more fast muscle fibers.On the other hand, the muscle soleus is a slowmuscle with a higher portion of slow muscle fibers.The gastrocnemius and tibialis anterior muscles arenot typically slow or fast muscles, and the values oftheir normalized velocity parameters are betweenthe values of the parameters for fast and slowmuscles. From this preliminary study, it can be con-cluded that measuring the muscle belly responsewith a displacement sensor can provide valuable in-formation about muscle contraction characteristics.In future studies, the proposed method needs to beproved by means of histological examination of thetissues of all the muscles under consideration. In this

particular study it was shown that the measured val-ues of a normalized velocity parameter vrn agreewith current knowledge about skeletal muscle dy-namics.

Acknowledgments. The presented study was supportedby the Ministry of Science and Technology of The Repub-lic of Slovenia. We give special thanks to Friderik Knez forhis help with measurements and thanks to Dr. HelenaBurger and Dr. Crt Marincek from The University Reha-bilitation Institute, Republic of Slovenia, where the mea-surements were performed.

REFERENCES

1. Valencic V. Direct measurement of the skeletal muscle tonus,advances in external control of human extremities. Belgrade:Nauka, 1990:575–84.

2. Zupan A, Gregoric M, Valencic V. Long-lasting effects ofelectrical stimulation upon muscles of patients suffering fromprogressive muscular dystrophy. Clin Rehab 1995;9:102–9.

3. Kogovsek N. Measuring and modelling of biomechanical char-acteristics of skeletal muscles. Ljubljana: University ofLjubljana, FER, 1991.

4. Knez N, Valencic V, Burger H, Marincek C. Variability ofMeasurements of skeletal muscles’ responses by means of dis-placement sensor. Proceedings of the Third Electrotechnicaland Computer Science Conference ERK‘94, Portoroz, Slov-enia, 1994;B:340–3.

5. Burger H, Valencic V, Marincek C, Kogovsek N. Propertiesof musculus gluteus maximus in above-knee amputees. ClinBiomech 1996;11:35–8.

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