Abstract
Stretch reflex is an important factor that influences the biomechanical response of the human body under whole-body vibration. However, there is a lack of quantitative evaluation at lower frequencies. Thus, the aim of this study was to investigate the effects of vibration on the stretch reflex and, in particular, to explore the quantitative relationship between dynamic muscle responses and low-frequency vibrations. The gastrocnemius muscle of 45 Sprague-Dawley rats was dissected. Sinusoidal vibrations of five discrete frequencies (2~16 Hz) with peak-to-peak amplitudes of 1 mm were applied to the gastrocnemius muscles with 2 mm or 3 mm prelengthening. Variables including dynamic muscle force, vibration acceleration, and displacement were recorded in two conditions, with and without the stretch reflex. Results showed that the dynamic muscle forces decreased by 20% on average for the 2 mm prelengthening group after the stretch reflex was blocked and by 24% for the 3 mm prelengthening group. Statistical analysis indicated that the amplitude of dynamic muscle force in the “with stretch reflex” condition was significantly larger than that in the “without stretch reflex” condition (p < 0.001). The tension-length curve was found to be a nonlinear hysteresis loop that changed with frequency. The phase difference between the dynamic muscle force and the length change was affected significantly by vibration frequency (p < 0.01), and the minimum frequency was 4–8 Hz. Experimental results of this study could benefit musculoskeletal model by providing a theoretical support to build a stretch reflex model for low-frequency vibration.
Highlights
Muscle fatigue caused by prolonged driving is an important factor affecting driving comfort and resulting in lower back pain [1]
Dynamic muscle response force, muscle length, and vibration acceleration were recorded in two conditions: “with stretch reflex” (WSR) condition (Figure 1(a)), meaning the stretch reflex arc was intact for the lower-extremity muscles, and “without stretch reflex” (WOSR) condition (Figure 1(b)), meaning the stretch reflex arc was blocked by cutting off the sciatic nerve after experimentation in the WSR condition
Three-way ANOVA was applied to phase difference, and result showed that there is no significance between WSR and WOSR conditions (F (1,14) = 0.86, 0 = 0.3357)
Summary
Muscle fatigue caused by prolonged driving is an important factor affecting driving comfort and resulting in lower back pain [1]. Whole-body vibration exposure plays a key role in producing muscle fatigue and accelerating the fatigue process [2, 3]. Several studies have examined the effect of muscle contractions on whole-body vibration response. Huang and Griffin [5] and Nikooyan and Zadpoor [6] reported that the activation of muscles could have a significant effect on human mechanical response under whole-body vibration. Kitazaki and Griffin [7] used a human finite element model with a muscle module to examine vibration comfort. Electromyography (EMG) data was used as activation inputs when predicting muscle response.
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