Impedance measurements of biological tissue are commonly used in bio-engineering and life sciences to assess the condition and composition of tissue in a non-invasive, reproducible way. In addition, bio-impedance measurements, also known as bioelectrical impedance analysis (BIA) or electrical impedance myography (EIM), can be used to monitor processes such as breathing pulse and muscle contraction.
This blog post presents measurements taken on a human forearm with the 5 MHz MFIA Impedance Analyzer. Although the MFIA is not certified for diagnostic or clinical use, it can still be used in a development lab to measure bio-impedance and to characterize how impedance changes interact with devices such as wearable sensors. This post outlines how the MFIA resolves even small changes in the impedance due to muscle contraction. We present measurements taken from a human forearm both at rest and during a fist clench.
The impedance measurement of the forearm is affected by many parameters including electrode position , and subcutaneous fat levels . However, for this measurement, we keep these parameters fixed and focus on the relative changes in impedance due to the muscle movement.
Figure 1: The photo shows the MFIA connected to an human forearm in a four-electrode configuration. 4 x 1m BNC cables (Amphenol, Digikey part no: ACX-2386-ND) were connected to Ambu Blue Sensor electrodes (Ambu Blue Sensor Q-00-A) via BNC-banana converters (Pomona 1894 Digikey part no:501-1323-ND). The spacing between each electrode was approximately 2 cm. Please note: the MFIA is for laboratory research use only and is not for diagnostic procedures.
Starting with a Sweep
Measuring in 4-electrode configuration reduces the measurement error due to the skin-electrode impedance. The MFIA can measure either in 2-electrode or 4-electrode and can be toggled with a single click. This means both sweeps can be taken in subsequent sweeps to investigate the electrode impedance.
Figure 2 shows an impedance sweep from 100 Hz to 5 MHz, displaying absolute impedance, phase and the current passing through the sample. The current does not rise above 100 uA, and yet even at this low signal level we still achieve excellent phase precision.
Figure 2: Data from the LabOne Sweeper module showing a frequency sweep (bode plot) from 100 Hz to 5 MHz. The test signal voltage was 100 mV and the three traces in the figure are phase (green trace), absolute impedance (blue trace) and the ac current (rms) flowing through the sample (red trace). Please note: the MFIA is for laboratory research use only and is not for diagnostic procedures.
Once More with Movement
The frequency sweep taken in Figure 2 measures the forearm at rest, and shows some non-linear phase behaviour as the frequency rises. This is not surprising as the equivalent circuit model is not a simple RC circuit. The scope of this blog is to present the suitability of the MFIA to measure bio-impedance under muscle movement, so we choose an arbitrary frequency value of 100 kHz to investigate the impedance change when clenching a fist.
To do this, we use the LabOne Plotter tool (one of the three time-domain measurement tools that come as standard with the MFIA). Sitting at 100 kHz and using a test signal of 100 mV, we start the Plotter and track the impedance changes before, during and after the hand movement to form a fist in one smooth movement. Initially with the subject at rest, the phase and absolute impedance are stable at -5.7 degrees and 34 Ohms respectively. Once the fist is clenched, the phase jumps to -4.2 degrees for the duration of the clench and drops back close to the initial at-rest value of -5.7 degrees. The absolute impedance (absZ) follows accordingly, rising from 33.98 Ohm to 35.61 Ohm during the clench. After the fist is released, the values recover close to the at-rest values.
Figure 3: LabOne Plotter tool data. Time domain impedance measurement over a period of 60 seconds (x-axis). The plot shows the phase trace in green and the absolute impedance (abs(Z)) in red. The fist clench lasted for approximately 20 seconds and during which a clear change in phase from -4.2 to -5.7 degrees for the duration can be observed. After releasing the fist both signals recovers close to the at-rest value. The absolute impedance value increases from 33.98 Ohm to 35.61 Ohm during the clench and then recovers to a value close to the at-rest value. Please note: the MFIA is for laboratory research use only and is not for diagnostic procedures.
The results in Figure 3 show how well the MFIA can track changes in impedance and phase for a given movement. This is very useful for developing wearable sensors. The change shown in Figure 3 is relatively large, approximately 1.5 degrees. This raises the question: what is the smallest phase shift the MFIA can track in such a 4-electrode setup? To test this, we measure the phase in the at-rest state. Figure 4 shows the phase trace averaged over 1 second. The standard deviation (measured in real time using the plotter cursor tools) is just 2.1 millidegree, meaning the MFIA can measure phase shifts of this order on a similar time scale. This opens the opportunity to measure such changes in impedance due to smaller muscle movement, for example, single finger movements.
Figure 4: LabOne Plotter data. Phase trace at 100 kHz of the forearm at rest. The cursor tools of the plotter show the standard deviation of the phase to be 2 millidegree, averaged over 1 second. Please note: the MFIA is for laboratory research use only and is not for diagnostic procedures.
Impedance Response at Other Frequencies
The impedance response at 100 kHz nicely tracked the fist clench movement and return to rest. Now we look at the response for difference frequencies. We continue to use the plotter tool, and add a trace to show the test signal frequency. Figure 5 shows the impedance response to a fist clench (each of circa 10 seconds) for three frequencies; 500 kHz, 10 kHz and 100 kHz.
The absolute impedance of each fist clench shows a similar impedance change, of approximately 1 Ohm. However, the phase shift is positive only for high frequencies (500 kHz and 100 kHz). At 10 kHz the phase shift is negative. This behavior can be explained by the various capacitive elements at play when measuring the impedance of the forearm. The data taken on the MFIA can be exported in Matlab or ZView format for further investigation into the equivalent circuit model of the forearm (not presented in this blog).
Figure 5: LabOne Plotter tool Screenshot. Three 10-second fist clenches each taken at a different test frequency (500 kHz, 10 kHz and 100 kHz). Absolute impedance shown in blue, test frequency in green and phase in purple. Please note: the MFIA is for laboratory research use only and is not for diagnostic procedures.
Use with Live Subjects
The MFIA is not certified for clinical use, and should only be used for research purposes. Please check and adhere to your local regulations when connecting the MFIA to a live subject. The current used during these measurements was below 100 uA, as shown by the red trace of Figure 2. A series resistor can also be used to limit the current coming out of the Hcur. The MFIA can be powered by standard mains supply or a third-party DC power supply.
Bio-impedance measurements are an excellent way to non-invasively sense hand movements. Even though the MFIA is not certified for clinical use, it can be used in a development lab to take such measurements with high precision (please ensure to adhere to your local regulations). It also offers the flexibility to select test frequency and amplitude, and to measure in both 4-terminal or 2-terminal setups. The time domain and frequency domain tools of the LabOne interface can be used to characterize and optimize the bio-impedance changes due to a hand movement, for investigation of the tissue itself, or to develop bio-impedance-based wearable sensors. The high phase resolution and speed of measurement of the MFIA will allow you to acquire better bio-impedance data.
Acknowledgments: Many thanks to Magdalena Marszalek for advising on this blog post and for helping with the measurements.
 B. Sanchez, A. Pacheck & S. B. Rutkove Guidelines to electrode positioning for human and animal electrical impedance myography research Scientific Reports volume 6, Article number: 32615 (2016).
 M. Jafarpoor, J. Li, J. K. White, S. B. Rutkove Optimizing Electrode Configuration for Electrical Impedance Measurements of Muscle via the Finite Element Method IEEE Trans Biomed Eng. 2013 May ; 60(5): 1446–1452.