Assessment of work-related muscle strain by using surface EMG during test contractions interposed between work periods of simulateted mushroom picking.

Surface electromyograms(EMG) during test contractions (TCs) were studied to assess the muscle strain in simulated mushroom picking. Additionally, the duration of the TC for the effective assessment was investigated. Nine female subjects performed standardized shoulder abduction and a stooped posture for one minute as TCs. Each experiment consisted of a 60-min rest, three work periods (W1-W3), a 30-min rest, and two work periods (W4 and W5) separated by a 30-min rest period. The duration of each work period was about 20 min. A total of 18 TCs was performed between the work periods and every 10 minutes in the rest periods. EMGs were recorded from the trapezius, infraspinatus, deltoid, and erector spinae muscles. The amplitude of EMG (AEMG) and mean power frequency (MPF) of EMG were calculated. Each TC was divided equally into three parts. Ratings of perceived exertion (RPE) in the neck, shoulder and low-back were reported during TCs. The work increased RPE of all the parts. AEMG and RPE were increased and MPF was decreased by W1, W2 and W3 in the neck and shoulder muscles. MPF of the erector spinae was increased by the work. The results were not affected by the duration of TCs and the parts during the TCs. AEMG and MPF fluctuated before W1 although the changes of RPE were small. Averaging several TCs was recommended to get stable results from TCs. EMG changes and appropriate TC conditions were discussed in relation to the adaptation in fatiguing contractions.

The assessment of muscle strain with surface electromyograms during simulated mushroom picking.

Muscle strain was assessed with surface EMG during simulated mushroom picking. Nine female subjects performed five periods of work (W1-W5). The duration of each period was about 20 min. W1, W2 and W3 were separated by a short break of several minutes. W3, W4 and W5 were separated by a rest period of about 43 min. EMGs were recorded from the muscles of trapezius, infraspinatus, deltoid and erector spinae muscles. Amplitude of EMG (AEMG) and mean power frequency (MPF) of EMG were calculated every minute. Ratings of perceived exertion (RPE) were recorded for every 5 min. RPE increased during each work period and with the advance of W1-3. AEMG increased during some of the work periods in the trapezius and infraspinatus. AEMG decreased in a few of the work periods in the other muscles. MPF decreased during some of the work periods in the infraspinatus, deltoid and erector spinae. The increase of AEMG in the trapezius was interpreted as spare capacity for the work. The infraspinatus and deltoid were assessed to be more fatigued than the trapezius. The fatigue level of the erector spinae was low.

Back muscle response to sudden trunk loading can be modified by training among healthcare workers.

STUDY DESIGN: Experimental study of the effect of physical training on the reaction to sudden back loading. OBJECTIVE: To investigate the effect and sustainability of "on the job training" on the reaction to sudden back loading among employees at a geriatric ward. SUMMARY OF BACKGROUND DATA: Available data suggest that a delayed muscle reflex response to sudden trunk loading may increase the risk of low back injuries. We have previously shown that training may alter the response to sudden trunk loading in healthy subjects and decrease the time elapsed until stopping of the forward movement of the trunk (stopping time). Data on the possibilities of a training-induced improvement in the reflex response among workers exposed to sudden trunk loading on the job are, however, nonexistent, and there is no evidence of long-term benefits, i.e., the sustainability of a positive training effect. METHODS: The study included 23 participants and 14 controls. All were healthy without prior history of low back pain (LBP). The training group participated in a total of 18 training sessions during a 9-week period. The training focused on reactions to a variety of sudden trunk loadings. Before and after the training intervention and at a 1-year follow-up, all subjects were tested for their reaction to expected and unexpected sudden trunk loading by applying a horizontal force of 58 N to the upper back of the subjects and measuring the electromyographic (EMG) response from the erector spinae muscles. RESULTS: In the training group, the stopping time and the distance moved after unexpected sudden trunk loading decreased significantly (13%-19%, P = 0.02). The improved stopping time was associated with marked changes in the time-wise distribution of the EMG signal after training. In addition, the follow-up study showed a high sustainability of the training effect. CONCLUSIONS: The results demonstrated a training-induced improvement of the response to sudden trunk loading that may be beneficial in workers, such as nurses, who are exposed to sudden trunk perturbations during patient handling.

Quantitative ultrasound tissue characterization in shoulder and thigh muscles--a new approach.

BACKGROUND: The echogenicity patterns of ultrasound scans contain information of tissue composition in muscles. The aim was: (1) to develop a quantitative ultrasound image analysis to characterize tissue composition in terms of intensity and structure of the ultrasound images, and (2) to use the method for characterization of ultrasound images of the supraspinatus muscle, and the vastus lateralis muscle. METHODS: Computerized texture analyses employing first-order and higher-order grey-scale statistics were developed to objectively characterize ultrasound images of m. supraspinatus and m. vastus lateralis from 9 healthy participants. RESULTS: The mean grey-scale intensity was higher in the vastus lateralis muscle (p < 0.05) than in the supraspinatus muscle (average value of middle measuring site 51.4 compared to 35.0). Furthermore, the number of spatially connected and homogeneous regions (blobs) was higher in the vastus lateralis (p < 0.05) than in the supraspinatus (average for m. vastus lateralis: 0.092 mm(-2) and for m. supraspinatus: 0.016 mm(-2)). CONCLUSION: The higher intensity and the higher number of blobs in the vastus lateralis muscle indicates that the thigh muscle contained more non-contractile components than the supraspinatus muscle, and that the muscle was coarser. The image analyses supplemented each other and gave a more complete description of the tissue composition in the muscle than the mean grey-scale value alone.

Training can modify back muscle response to sudden trunk loading.

Sudden, unexpected loading to the trunk has been reported in the literature as a potential cause of low-back disorders. This study's aim was to investigate the effect of "readiness training" on the response to sudden back loading among untrained healthy individuals. The study included 19 participants and 19 matched controls. All were employees at the National Institute of Occupational Health. The participants received ten 45-min training sessions during a 4-week period. The training focused on reactions to a variety of expected and unexpected sudden trunk loadings, including balance and coordination exercises. Before and after the training, all subjects were tested for reaction to sudden trunk loading (SL). This entailed applying a horizontal force of 58 N to the subject's upper back. Elapsed time--measured between SL and stopping--decreased significantly in the training group (from 337 to 311 ms) compared with the control group. The improved stopping time was associated with a changed EMG signal, characterized by an increase in the early parts of the response (up to 225 ms) and a subsequent decrease. EMG onset latency was unaffected by training. This study is apparently one of the first to demonstrate that the response to sudden trunk loading can be improved in healthy subjects without an increase in pre-activation and associated trunk stiffness. In perspective, the results indicate a possibility for a training-induced reduction of the risk of low-back injuries, e.g., in nurses exposed to sudden trunk perturbations during patient handling.