Mechanical testing of fibres determines how much power a fibre can generate once it is activated. The technique further determines which component is affected by a disease, environmental factor or exercise training.
How does it work?
The permeabilised single muscle fibre system consists of:
- a force transducer – to measure how much force in milliNewton is produced (5 mNewton max),
- a step motor lever arm – to stretch or shorten the fibre, and
- 8x 200µl temperature controlled baths (10 – 50˚C).
The fibre is mounted between the force transducer and motor lever using tin foil clips. The length of the fibre, its diameter and sarcomere spacing is determined with a calibrated microscope and the fibre is ready to be subjected to various chemicals in the different baths. Muscle fibres can be submerged into 8 different temperature controlled baths, which can be set between 10˚C and 50˚C.
Each bath can contain solutions that block contraction (e.g. inhibitors), activate the fibre (e.g. ATP, calcium) or enhance contraction (e.g. drugs). The lever arm connected to the motor is able to control the length of the muscle fibre by stretching it or allowing the fibre to shorten during contraction. This lever can be programmed to produce rapid stretching or shortening (within 1 ms) or gradually over extended periods.
The whole system is mounted on an inverted microscope equipped with a pre-calibrated CMOS camera, allowing for the accurate measurement of the specimen length, diameter and sarcomere spacing.
What properties can be measured?
The following properties of single muscle fibres can be measured and include:
- force (F) – absolute force (in mN) and specific force (in kN/m²)
- elastic component of the fibre – to determine stiffness of the fibre
- shortening velocity (V) – speed of fibre contraction (in fibre lengths/second
- power output – determined from specific force and shortening velocity (kN/m².FL/s)
- calcium sensitivity of fibres – a property of the troponin molecules
- effects of drugs on contractility parameters
- many more…
The upper limit of the force transducer is 5.1 mNewton, with a maximum recording frequency of 1 000 Hz. For a detailed background on the above properties, refer to the Muscle 101 section covering muscle fibre performance.
What materials can be analysed?
Creativity would be at the order of the day. This system is specifically designed to withstand forces less than 5.1 mNewton. We have recently joined forces with the University of Kentucky to measure the contractility of very small pieces of cardiac tissue.
Other tissues (e.g. small pieces of heart valve) or synthetics that cannot produce force (e.g. hydragels, thin synthetic filaments) could be attached to the force transducer and lever arm. By using the lever arm to actively stretch the connected substance, parameters such as elasticity, length-tension curves or breaking force could be measured.