European Journal of

Eur J Appl Physiol (1984) 52:400-406

AhPys plied

iology

and Occupational Physiology 9 Springer-Verlag 1984

Biomechanical characteristics of human ankle-joint muscles A. S. Aruin I and V. M. Zatsiorsky 2 The Moscow Institute of Electronic Computer Building1, Central Institute of Physical Culture2

Summary. Equivalent biomechanical characteristics of h u m a n ankle-joint muscles have been determined by impact and vibration tests. The estimate of the stiffness and damping coefficients has yielded, respectively, (2.67 + 0.48) x 104 N 9 m -1 and (811.58 + 201.3) N 9 s 9 m --1 by impact actions, n = 126; (1.49 -4- 0.35) • 104 N 9 m -1 and (430.1 + 36.1) N 9 s 9 m -1 - by vibration actions, n = 7. The characteristics of the ankle-joint muscles of subjects representing different kinds of sports have proved to be different.

works the angular stiffness of the ankle-joint muscles (dimension N . m . rad -1) was investigated (Hunter and K e a r n e y 1982). The purpose of the present work is to measure the biomechanical characteristics of the ankle-joint muscles (their equivalent stiffness and damping) by methods based on impact and vibration actions.

Key words: H u m a n ankle stiffness and damping Methods of impact and vibration tests

1. Impact method

Introduction

Material and methods

The earlier method described by Cavagna (1970) was used with some modifications (Fig. 1). Experiments were carried out on a strain-gauge platform (1) which was sensitive to the vertical component of the acting force and whose natural frequency was over 60 Hz.

W h e n a foot contacts the floor during running or jumping, it takes up considerable load. The mechanical energy of a h u m a n body is dissipated in muscles and tendons and, secondly, m a y be accumulated in the f o r m of elastic strain energy, thus raising the efficiency of muscle work due to energy recuperation (Cavagna 1970; Aruin et al. 1977a; Aruin et al. 1979; Zatsiorsky and Yakunin 1980). The amounts of dissipated and recuperated energies are dependent, in addition to other factors, on the biomechanical characteristics of the muscles of the lower extremities. These characteristics (stiffness and damping coefficients) were studied by the method of d a m p e d vibrations (impact action) on five h u m a n subjects (Cavagna 1970 ). In his experiments, as in the present work, the linear stiffness in N 9 m -1 and damping in N . s . m -1 were estimated. In a n u m b e r of recent Offprint requests to: A. S. Aruin, Institute of Electronic Computer Building USSR, Moscow, 109028, Bolshoy Vuzovskii Lane 3/12

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Fig. l. Method of impact action. Block diagram of installation. Explanations are in the text

A. S. Aruin and V. M. Zatsiorsky: Biomechanical characteristics of human ankle-joint muscles Each subject performed 15 small vertical jumps with subsequent falls on the balls of the feet without bending the knees and with the lower limbs being tense. The subject's orthograde position was controlled by goniometric transducers (2, 3, 4) attached to the hip, knee, and ankle joints. The activity of muscles was controlled by an EMG amplifier (5) with an input resistance of 1.2 MOhms. EMG signals were sensed by surface electrodes attached to tibialis anterior, biceps femoris, rectus femoris, gastrocnemius (caput laterale) muscles. Information was recorded by a rotating-mirror oscillograph (8). The force values involved were transferred by means of an analog-to-digital converter (6) into an electronic computer which determined the natural frequency, stiffness and damping coefficients (Aruin et al. 1977b). One hundred and eighty-five male athletes representing nine kinds of sports underwent experiments. All subjects were specially trained to keep their balance when landing on the platform after jumps, and to maintain a certain tension of the ankle-joint muscles in the orthograde position. In handling the information obtained, only one attempt was selected from fifteen. The selection criterion was the absence of changes in the angular position of the hip and knee joints. Those subjects who could not meet this requirement were rejected. As a result, the remaining number of subjects was 126. In addition, the maximum moment of force of the foot flexors and maximum static force of the latter of each subject were measured by a strain-measuring device.

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vibrations of the table and the forced vibrations generated on the distal section of the shin, on the knee-joint, loins (at the sacrum), between the shoulder-blades (at the level of the 4th thoracic vertebra) and on the head of the subject were measured by KD-35 piezoelectric accelerometers (4) of RFT company. Upon amplification by integrating amplifiers (5) (types SM-231, SM-241, RFT company), the signals were recorded by a K-115 rotating-mirror oscillograph (6). Transducers were attached to the subject's body by means of rubber plaits and special-purpose mechanical devices which enabled the reliable attachment of the transducers and their required orientation in space. The head transducer was secured by a metal yoke. All attaching devices possessed natural frequencies lying beyond the frequency range in question. The angular position of the joints of the lower limbs was controlled with goniometric transducers (7) mounted on the ankle, knee, and hip joints (Aruin et al. 1978). Experiments were carried out on seven healthy athletes aged from 19 to 22, their average weight being 69.5 +_ 6.3 kg and average height 1.733 _+0.04 m. Each subject was on the vibrator table in the orthograde position on tip-toes with straightened and strained lower limbs (the angle in the ankle-joint being 110 _+ 1.27~ In four of the five experiments, additional loads (8) of 5, 10, 15, 20 kg were fastened to the subject's loins. The amplitudes of vibrations of the human body and the vibrator platform were recorded, and the amplitude-frequency characteristic curves (AFC) of the system under study were plotted. The analysis of these curves made it possible to establish a model of the human body.

2, Vibration method Experiments were performed on a complex installation whose block diagram is shown in Fig. 2. A subject was placed on the table of an electrodynamic vibrator. The latter was excited by a harmonic signal in the frequency range 2 to 70 Hz. Frequencies from 2 to 20 Hz were varied automatically by a programming device; frequencies from 20 to 70 Hz were assigned discretely. The time of automatic frequency change was chosen to meet the inequality

3. Model

t,