The kinematics of the human knee joint and the strain of the ligaments as a function of flexion are determined analytically and experimentally. The experimental results were obtained in 13 tests of four knee joints in which the strain in each of the two collateral and two cruciate ligaments was measured with mercury strain gauges while the tibia was rotated through a flexion angle of 130 deg. The values of the relative ligament strain obtained from the analytical model are in good agreement with the experimental results.

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The influence of vertical sinusoidal vibration on the visual acuity of twelve human subjects, sitting restrained and without padding in an airplane seat mounted on a shake table, was investigated in the frequency range of 1 to 20 c/s. A. novel visual acuity tester was developed to achieve high accuracy of measurement and to obtain a great many observations in short time periods. The decrement of visual acuity normalized to a shake table acceleration of one g (vector) was determined. Maximum decrements occurred at those frequencies where resonances of the whole body and organ complexes had been determined by other methods. Below 12 c/s the decrements were due mainly to the physiological stress produced in the body and above 12 c/s to the image displacement on the retina which had more and more of a blurring effect. Measurements one minute after cessation of vibration indicated residual effects only at frequencies up to 12 c/s, peaks being at the same resonant frequencies.

The effects of forced vibration upon the human body were determined in the form of circumferential strain of chest, abdomen, pelvis and thigh. Sixteen human subjects, in the sitting erect, sitting relaxed and semi-supine positions, were exposed to vertical sinusoidal frequencies above two c/s at vector accelerations below 1 g. The maximum body strain occurred at 6.7 c/s in the semi-supine position and between 4 c/s and 6 c/s in the sitting positions. In the semi-supine position the mean strain of chest and abdomen at resonance was found to be 0.040 and 0.053 in./in./g, respectively, when standardized to a table acceleration of 1 g. In the sitting positions this strain was only half as large. The strain of the pelvis, on the other hand, was 0.013 in./in./g in the semi-supine posture, and 0.032 in./in./g in the sitting positions. All subjects showed maximum body strain near 7 c/s in the semi-supine posture. Since tolerance to vibration decreases with increasing body strain, the human body on a space couch must be protected against vibrations transverse to the couch axis in the range around 7 c/s.

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