Neck posture and muscle activity are different when upside down: a human volunteer study

Area of Science:

• Biomechanics
• Human Factors Engineering
• Orthopedics

Background:

• Rollover crashes pose a significant risk for catastrophic neck injuries due to head impacts with the vehicle roof.
• Understanding in vivo neck kinematics and muscle responses during rollover events is crucial for injury prevention.
• Current injury models often lack in vivo data on neck posture and muscle activation in inverted positions.

Purpose of the Study:

• To investigate in vivo neck vertebral alignment and muscle activation patterns in humans when in an inverted (upside-down) position.
• To compare these parameters between upright and inverted postures to inform injury prevention strategies.
• To provide data for improving the accuracy of cadaveric and computational models of rollover crashes.

Main Methods:

• Eleven healthy human subjects (6 female, 5 male) were tested in both upright and inverted seated positions using a custom apparatus.
• Neck vertebral alignment was assessed using fluoroscopy.
• Surface and indwelling electrodes recorded electromyographic (EMG) activity in eight superficial and deep neck muscles.

Main Results:

• In the inverted, relaxed state, subjects exhibited increased neck lordosis, posterior C1 alignment relative to C7, and extended Frankfort plane, with elevated activity in six neck muscles compared to upright.
• When inverted subjects attempted to look forward (neutral head posture), neck flexor activity increased, C7 showed increased flexion, and C1 aligned anterior to C7.
• Significant inter-subject variability was observed in both vertebral alignment and muscle activation.

Conclusions:

• In vivo neck posture and muscle activation differ substantially between upright and inverted conditions.
• The observed variability highlights the need for injury models to account for diverse postures and muscle states.
• Findings provide critical in vivo data to enhance the relevance of computational and cadaveric models for rollover injury research and prevention device development.