David Goldsheyder

Whole-Body Vibration Exposure: A Comprehensive Field Study

A comprehensive field study investigated whole-body vibration exposure levels experienced by the train operators of a large metropolitan subway system. The purposes of the study were to measure mechanical vibrations transmitted to the seated train operators, to calculate daily whole-body vibration exposure levels, and to compare these levels with maximum acceptable exposure levels recommended by the international standard on whole-body vibration (ISO 2631). The study also sought to identify factors that may influence mechanical vibrations transmitted to the operators and quantify their effects on the measured vibration levels. The study was carried out by dividing the subway system into subway lines, each line into southbound and northbound directions, and each direction into station-to-station observations. Triaxial measurements were made on all subway lines and for all car types used in the system. For each line, at least two round trips of data were collected. Time-weighted averages of the two sets of data were used for final presentation. A total of 48 round trips were made and more than 100 hours of vibration data was collected and analyzed. All phases of the study were carried out in accordance with the procedures outlined in ISO 2631. It was determined that 6 out of 20 subway lines had vibration levels higher than daily exposure limits recommended by ISO 2631. It was also determined that train speed was the most significant factor influencing vibration exposure levels.

Testing apparatus and experimental procedure for position specific normalization of electromyographic measurements of distal upper extremity musculature.

OBJECTIVE An apparatus and procedure are described to determine position specific normalization coefficients for surface EMG of upper extremity musculature. STUDY DESIGN Thirty-nine subjects were tested three times. Repeatability of EMG measurements across test sessions was determined by computing intraclass correlation coefficients. Two-way analysis of variance was used to test upper extremity position dependent differences in EMG measurements. BACKGROUND EMG measurements are susceptible to error from skin movement and muscle length changes, both of which may occur when upper extremity positions vary. Normalization of the EMG signal without consideration for such positional influences may lead to erroneous conclusions regarding muscle activation during functional tasks. METHOD An apparatus was designed that allowed subjects to perform three repetitions of maximum elbow flexion, forearm pronation, wrist extension, and wrist flexion with the forearm in neutral and pronated positions. Surface EMG was sampled from eight muscles. Mean EMG on maximum voluntary contraction was computed, and resting EMG was subtracted to obtain EMG normalization coefficients. RESULTS Upper extremity position affected the EMG normalization coefficient for biceps brachii, which was lower in the pronated position, and extensor carpi radialis, which was higher in the pronated position (P<0.00625). CONCLUSIONS The apparatus accommodates various combined positions of the elbow, forearm and wrist. The normalization procedure is efficient for testing subjects who are being observed during functional tasks. Only two muscles were affected by upper extremity position, but group trends were not always consistent with individual behavior. This method would ensure the use of appropriate EMG normalization coefficients regardless of individual variation. RELEVANCE This method is effective for normalizing EMG signals using task specific upper extremity positions. It may be used to test isometric exertions of distal upper extremity musculature for clinical and research purposes.

Mechanical Properties of Biological Tissues

The material response discussed in the previous chapters was limited to the response of elastic materials, in particular to linearly elastic materials. Most metals, for example, exhibit linearly elastic behavior when they are subjected to relatively low stresses at room temperature. They undergo plastic deformations at high stress levels. For an elastic material, the relationship between stress and strain can be expressed in the following general form:

Self-Reported Musculoskeletal Symptoms among Mason Tenders and Perceived Job-Related Characteristics

\sigma =\sigma \left(\epsilon \right).

Effect of subway car design on vibration exposure

Equation (15.1) states that the normal stress σ is a function of normal strain ϵ only. The relationship between the shear stress τ and shear strain γ can be expressed in a similar manner. For a linearly elastic material, stress is linearly proportional to strain, and in the case of normal stress and strain, the constant of proportionality is the elastic modulus E of the material (Fig. 15.1):

Stress and Strain

\sigma =E\epsilon .