ACUTE EFFECTS OF EXERCISE ON PASSIVE JOINT STIFFNESS

M. Ricard, D. Butterfield, D. Draper, S. Schulthies, W. Myrer
Physical Education Department, Brigham Young University, Provo, UT 84602

Presented at the 20th Annual Meeting of the American Society of Biomechanics
Atlanta, Georgia. October 17-19, 1996

INTRODUCTION

The purpose of the study was to compare the effects of exercise on passive knee joint stiffness. Passive knee joint stiffness was measured prior to and following a weight training exercise bout.

Figure 1. Diagram of a spring-damper model used to describe the stiffness (k) and damping coefficient (c) of the knee.

Passive stiffness in a human joint is due muscular factors attributed to actin-myosin filaments and connective tissue factors which can be attributed to fascia, ligaments and friction in the joint. The viscoelastic behavior of a human joint can be modeled using a spring-damper model as shown in Figure 1. A torsional spring with linear stiffness can be used to represent the elastic response of the actin-myosin filaments and connective tissue in the knee joint. The time-dependent behavior of connective tissue can be modeled using a viscous damping element as shown in Figure 1. The immediate effects of an exercise bout such as weight lifting are known to increase the stiffness of the joint involved. High specific tensions in the muscle may disrupt the sarcolemma and sarcoplasmic reticulum resulting in subcellular microtrauma to tissue (Armstrong, et al., 1983; Friden, et al., 1983; and Kuipers, et al., 1983). Lakie, et al. (1988) found that joint stiffness increases following active or passive movements. However, little is known about the acute effects exercise on joint stiffness immediately following an exercise bout.

PROCEDURES

Twenty three subjects with normal knees were selected for this study. Passive knee joint stiffness was measured prior to and following a weight training exercise bout. The exercise bout involved 10 sets of 10 single knee extension/flexions at 75% of 1 RM, followed by 10 sets of 10 eccentric contractions. The stiffness of the relaxed knee was measured using a Penny & Giles electrogoniometer. The position of the right knee was sampled for 6 s at 500 Hz.

Figure 2. Typical damped oscillation resulting from allowing the lower leg to swing freely about the knee joint.

The stiffness and damping coefficients of the relaxed knee was determined with the subjects seated on a table. The lower extremity was supported by the researcher with the knee completely extended. Once the subject was completely relaxed the leg-foot system was released allowing the lower limb to freely oscillate until the lower leg comes to rest in the vertical position. Five good trials were obtained prior and after exercise. A typical damped oscillation of the knee is shown in Figure 2. Stiffness was calculated from the first two complete cycles of oscillation using the methods presented by Meirovitch (1975). The logarithmic decrement (d) of the oscillation was calculated by

d = ln (A1 - A2)

where A1 and A2 are the knee joint angles for the first two peaks in the oscillation (Figure 2). The viscous damping factor (df) was calculated as follows.

df = sqrt ( d / (2 pi)^2 + d^2)

where d is the logarithmic decrement of the damped oscillation. The natural frequency of the oscillation (w) was determined from the following equation.

w = d / df x T

where d is the logarithmic decrement, df is the viscous damping factor and T is the time between oscillation peaks. The damping coefficient (c) was obtained as follows.

c = 2 df w I

where df is the viscous damping factor, w is the natural frequency and I is the moment of inertia of the lower leg and foot about the knee joint. The passive stiffness (k) of the knee joint was obtained using the following equation.

2 k = I w

where I is the moment of inertia of the leg-foot and the w is the natural frequency. All five trials for stiffness (k) and damping coefficients (c) were averaged. Dependent t-tests were used to test for significant differences in stiffness and damping after exercise.

RESULTS

The acute effects of exercise on passive knee joint stiffness, damping coefficient and natural frequency are shown in Table 1. Significant increases in passive knee joint stiffness, damping coefficient and natural frequency were observed following exercise (p < .05). Mean passive knee joint stiffness increased by 1.56 N(m/rad immediately following the exercise regime. Single knee extension strength decreased significantly from 37.8 ( 14.87 kg prior to exercise to 14.28 ( 2.42 kg following the exercise bout.

Table 1: Mean and SD for Knee Joint Stiffness and Damping Before and After Exercise

DISCUSSION

Passive knee joint stiffness arises from the combined effects elastic structures including muscle, tendon, articular surface, ligament and skin. In a relaxed joint a considerable portion of joint stiffness can be attributed to passive resistance offered by actin-myosin filaments. Mechanically induced micro-trauma to the myofibrils can be quantified immediately after the exercise based upon morphological changes (Newham, et al., 1983; Brooks, et al., 1995). In the current study knee joint stiffness and damping significantly increased immediately after an exercise. Joint stiffness may increase following exercise as a result of mechanically induced disruption of sarcomeres. The increased damping in the knee joint following exercise may be due to trauma to the myofibrils and inflammation of the joint caused by an increase in extracellular proteins (Armstrong, 1990). In conclusion, the immediate effects of exercise resulted in an increase in damping and passive stiffness in the human knee joint.

REFERENCES

Armstrong, R.B. et al. J. Appl. Physiol. 54, 80-93, 1983.

Armstrong, R.B. Med. Sci. Sports Exer. 22, 429-435, 1990.

Brooks, S.V. et al. J. Physiol. 488, 459-469, 1995.

Friden, J. et al. Int. J. Sports Med. 4, 170-176, 1983.

Kuipers, H. et al. Int. J. Sports Med. 4, 45-51, 1983.

Lakie, M. et al. J. Exp. Physiol. 73, 487-500, 1988.

Meirovitch, L. Elements of Vibration Analysis. McGraw-Hill, 1975.

Newham, D.J. et al. J. Neurol. Sci. 61, 109-122, 1983