Integrated Opto-Biomechatronics For Single Muscle Fibre Structure-Function Assessment: The MyoRobot 3.0

Area of Science:

• Muscle physiology
• Biomechanics
• Biomaterials engineering

Background:

• Accurate assessment of single muscle fiber biomechanical properties is crucial.
• Conventional methods often assume constant fiber geometry and use interchangeable protocols, leading to mechanical inaccuracies.
• Existing techniques lack integrated structural and functional analysis.

Purpose of the Study:

• To introduce the MyoRobot 3.0, a novel biomechatronics platform for integrated structural and functional analysis of single muscle fibers.
• To quantify fiber thinning during passive stretch and its impact on biomechanical measurements.
• To compare continuous stress-strain and stepwise stretch-jump protocols under various conditions.

Main Methods:

• Development of the MyoRobot 3.0, a platform integrating automated force measurements with synchronized fiber-wide optical imaging.
• Utilizing a motorized optical system for continuous diameter tracking during controlled axial stretch.
• Comparison of slow continuous stress-strain recordings with rapid stepwise stretch-jumps under varying calcium concentrations and cross-bridge inhibition.

Main Results:

• Quantified significant muscle fiber diameter decrease ($\sim$ 13-15%) and cross-sectional area loss ($\sim$ 27%) at 40% strain.
• Demonstrated systematic underestimation of restoration stress when fiber thinning is uncorrected.
• Revealed calcium-dependent increases in passive stiffness with continuous protocols, which were absent in stepwise protocols, suggesting time-dependent contributions.

Conclusions:

• The MyoRobot 3.0 corrects major structural errors in muscle fiber biomechanical analysis.
• Enables protocol-resolved analysis, improving the interpretation of passive muscle stiffness.
• Provides new insights into the molecular determinants of passive muscle stiffness by differentiating time-dependent effects.