Original Research
Ergonomics & Occupational Health

Biceps Brachii Muscle Contraction During Different Driving Postures

Background and Context

Driver fatigue and musculoskeletal discomfort are significant contributors to road accidents worldwide. In Malaysia, fatigue has been linked to approximately 15.7% of total road accidents, making it one of the leading non-mechanical causes of traffic crashes. Understanding the biomechanical demands placed on drivers during routine vehicle operation — particularly during steering manoeuvres — is essential for designing vehicle interiors that minimise fatigue, reduce injury risk, and promote sustained alertness during long driving sessions.

The upper extremity muscles play a critical role in vehicle control. The biceps brachii, located on the anterior aspect of the upper arm, is a primary elbow flexor and forearm supinator that is actively engaged during steering wheel manipulation. The anterior deltoid contributes to shoulder flexion and forward reaching, while the upper trapezius stabilises the shoulder girdle. Together, these muscles maintain the driver’s hands on the steering wheel and generate the forces required for turning, lane changing, and corrective manoeuvres.

Previous ergonomic research has established that sustained static postures — characteristic of prolonged driving — lead to localised muscle fatigue, which manifests as discomfort, pain, reduced reaction time, and impaired vehicle control. However, the specific relationships between driving posture geometry (particularly elbow and shoulder angles), steering direction, and muscle activation patterns have been insufficiently characterised, particularly for Malaysian anthropometric populations. This study addresses that gap by providing objective electromyographic data on upper extremity muscle behaviour during simulated driving tasks.

Study Design and Methods

The study recruited 14 respondents who underwent surface electromyography (sEMG) measurement using Delsys Incorporation equipment. sEMG is the standard non-invasive technique for assessing muscle electrical activity during voluntary contractions, providing quantifiable data on the timing, intensity, and pattern of muscle recruitment.

Three driving posture parameters were established based on fixed elbow and shoulder angle combinations, designated as Posture A, Posture B, and Posture C. The elbow angles ranged from less than 134° (Posture A) through approximately 134° (Posture B) to greater than 134° (Posture C). These postures were selected to represent the range of driver seating configurations commonly encountered in passenger vehicles, reflecting variations in seat position, steering wheel height, and driver body dimensions.

Electrodes were placed bilaterally on the biceps brachii, anterior deltoid, and upper trapezius muscles following standardised anatomical guidelines. Maximum Voluntary Isometric Contraction (MVIC) tests were performed for each muscle to establish normalisation values. Participants then performed standardised steering wheel turning tasks — both left and right turns — while muscle activity was recorded. Root Mean Square (RMS) values of the sEMG signal were calculated as the primary measure of muscle contraction intensity. A notch filter and appropriate bandpass filtering were applied to the raw signal to remove electrical interference and motion artefacts.

Principal Results

The most striking finding was the pronounced asymmetry in biceps brachii activation during steering wheel turns. When participants turned the steering wheel to the right, the BB muscle showed contraction increases of 3 to 6 times the baseline level. This substantial increase reflects the biomechanical demands of pulling the steering wheel rightward, which requires concentric contraction of the right BB and eccentric control by the left BB to coordinate the movement smoothly.

In contrast, the deltoid anterior and upper trapezius muscles exhibited contraction patterns that were directionally opposite to the steering wheel turning action. When drivers turned the wheel to the left, right-side DA and TU muscle contraction was higher, and vice versa. This pattern is consistent with the stabilising and antagonist roles these muscles play during unilateral arm movements — while the BB on the pulling side generates the primary turning force, the shoulder stabilisers on the contralateral side maintain postural control and prevent the trunk from rotating with the steering input.

Subsequent analysis using multiple linear regression (stepwise method) developed models relating subjective comfort perception to objective muscle contraction measurements. Three models were generated, one for each posture, and the results converged on Posture B (elbow angle approximately 134°) as the configuration producing the optimal balance between muscle effort and perceived comfort. This finding aligns with the ergonomic principle that moderate joint angles — neither fully flexed nor fully extended — tend to produce the least muscular effort and greatest subjective comfort.

Ergonomic Implications for Vehicle Design

The identification of Posture B as the most comfortable driving configuration has direct implications for vehicle seat and steering column design. Modern vehicles increasingly offer adjustable steering columns (tilt and telescoping), seat height adjustment, and seat track movement, enabling drivers to achieve their preferred posture. However, many drivers, particularly in developing countries, may not adjust these settings optimally — either due to lack of awareness, vehicle limitations, or habitual behaviour.

The asymmetric muscle loading patterns observed during steering also have implications for understanding driver fatigue mechanisms. If the BB muscle experiences 3–6 times its resting activation level during right turns, frequent right-turning manoeuvres (such as those encountered in roundabouts or specific route configurations) could accelerate localised fatigue in the dominant steering arm. Vehicle manufacturers could consider this asymmetry when designing power steering assistance algorithms that modulate steering effort based on the direction and magnitude of driver input.

For Malaysian drivers specifically, anthropometric considerations are important. Malaysian body dimensions tend to differ from Western design standards, which may affect the optimal positioning of steering wheels and seats. The data from this study provide Malaysian-specific evidence that can inform vehicle ergonomics standards tailored to the local population.

Relationship to Road Safety

The connection between driving posture, muscle fatigue, and road safety is mediated through several pathways. Fatigued muscles produce slower, less accurate movements, which directly impairs steering precision and reaction time. Discomfort from poor posture causes drivers to shift position frequently, diverting attention from the road. Over extended driving periods, accumulated muscular fatigue can contribute to drowsiness and reduced vigilance. In Malaysia, where road accident mortality rates remain among the highest in Southeast Asia, any intervention that reduces driver fatigue — including optimal seat and steering ergonomics — has potential life-saving significance.

Limitations

Several limitations should be acknowledged. The sample size of 14 participants, while adequate for exploratory electromyographic research, limits the generalisability of the findings. The study was conducted in a simulated laboratory environment rather than during actual on-road driving, which may not fully replicate the vibration, cognitive load, and dynamic steering demands of real driving conditions. Only three discrete posture configurations were tested; a continuous parametric analysis across a wider range of joint angles could reveal more nuanced relationships. The study focused on upper extremity muscles only, without assessing lower back and cervical spine musculature that also contribute significantly to driving comfort and fatigue. Finally, only male participants were included in some versions of this study, and results may not be fully applicable to female drivers, who tend to have different anthropometric proportions and muscle strength characteristics.

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