The effects of surface-induced loads on forearm muscle activity during steering a bicycle

Abstract

On the bicycle, the human upper extremity has two essential functions in steering the bicycle and in supporting the body. Through the handlebar, surface- induced loads are transmitted to the hand and arm of the bicycle rider under vibration exposure conditions. Thus, the purpose of the study was to investigate the effect of vibration exposure on forearm muscle activity for different road surfaces (i.e. smooth road, concrete stone pavement, rough road) and for different bicycles. Ten subjects participated in experiments and two types of bicycles, i.e. Road Bike (RB) and Mountain Bike (MTB) are compared. The acceleration magnitudes were dominant along x and z-axes. The r.m.s acceleration values in the z direction at the stem of MTB were at most 2.56, 7.04 and 10.76 m·s(-2) when pedaling respectively on asphalt road, concrete pavement and rough road. In the case of RB the corresponding values were respectively 4.43, 11.75 and 27.31 m·s(-2). The cumulative normalized muscular activity levels during MTB trials on different surfaces had the same tendency as with acceleration amplitudes and have ranked in the same order from lowest to highest value. Although road bike measurements have resulted in a similar trend of increment, the values computed for rough road trials were higher than those in MTB trials. During rough road measurements on MTB, rmsEMG of extensor muscles reached a value corresponding to approximately 50% of MVC (Maximum Voluntary Contraction). During RB trials performed on rough road conditions, rmsEMG (%MVC) values for the forearm flexor muscles reached 45.8% of their maximal. The level of muscular activity of forearm muscles in controlling handlebar movements has been observed to be enhanced by the increase in the level of vibration exposed on the bicycle. Since repeated forceful gripping and pushing forces to a handle of a vibratory tool can create a risk of developing circulatory, neurological, or musculoskeletal disorder, a bicycle rider can be considered vulnerable to developing vibration related overuse injuries and/or performance diminishing consequences. Key PointsThe muscular activity level in the forearms increases in response to random vibration transmitted to the bicycle to control handlebar movements.The level of vibration transmission depends on irregularities on road surface and bicycle type.A bicycle rider can be considered vulnerable to developing vibration related overuse injuries and/or performance diminishing consequences.

Keywords: Vibration transmission; cycling; electromyography; mountain bike; road bike.

Figure 1.

The placement of the accelerometer and adopted reference coordinate system

Figure 2.

TA time domain illustration of a simultaneously recorded EMG of extensor muscle (upper trace) and an acceleration (lower trace) signal (z-axis) recorded on the stem during a RB trial on the rough road.

Figure 3.

The percentage of max EMG values for right and left flexor and extensor muscles calculated for MTB (a) and RB (b) trials on different road surfaces

Figure 4.

Cumulative summation of normalized muscle activity levels, calculated as the summation of both flexor and extensor muscles in both left and right arms. Average values of the r.m.s EMG as a function of the percentage of the maximal voluntary contraction (%MVC) presented for each muscle.