20-Bio-B6 Analytical Biochemistry · May 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format: National Exams, May 2014 — 04-Bio-B6 Bioinstrumentation. Three hours, open book, non-communicating calculator permitted. Six questions of equal value (25 marks each); four constitute a complete paper and only the first four appearing in the answer book are marked. All six are solved here as a complete study resource. Every question is a design/essay question (block-diagram instrumentation-system design, or descriptive explanation).
Reference texts (the books a candidate should have reviewed for this subject):
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Approach. A force channel (isometric load cell) and an EMG channel (surface electrodes over the muscle belly) are acquired simultaneously on a common sample clock; every 5-second epoch the processor computes the EMG mean absolute value (MAV) and the ratio of low-band (20-100 Hz) to high-band (100-250 Hz) power, repeating until the force channel reads zero (the fatigue endpoint).
A strain-gauge load cell is built into the handle/dynamometer the subject grips (or a lever arm loaded by elbow flexion), converting the isometric force into a Wheatstone-bridge imbalance. A bridge amplifier followed by a 0-10 Hz low-pass filter conditions the signal — ample bandwidth for a voluntarily-produced force profile that changes over seconds, not milliseconds — and an ADC sampling at ~50 Hz (comfortably above the 20 Hz Nyquist rate for a 10 Hz signal) digitises it.
A bipolar pair of Ag/AgCl surface electrodes is placed longitudinally over the belly of the biceps brachii (standard SENIAM placement, roughly midway between the muscle's proximal and distal tendon attachments, ~20 mm inter-electrode spacing), with a reference electrode over a nearby bony/inactive site (e.g. the olecranon). A differential instrumentation amplifier (high CMRR, gain ~1000×) bandpass-filtered to 20-250 Hz matches the stated EMG range and rejects DC offset and high-frequency noise, with a 60 Hz notch for mains rejection. An anti-alias filter precedes an ADC sampling at ≥1 kHz — well above the 500 Hz Nyquist rate for the 250 Hz upper band edge, leaving margin for a realisable anti-alias roll-off and good frequency resolution in the epoch FFT.
Force and EMG samples share a common clock so each 5-second epoch (5000 samples at 1 kHz) of EMG is time-aligned with the corresponding force value. For each epoch the processor computes: (1) the mean absolute value $\mathrm{MAV}=\frac{1}{N}\sum_{i=1}^{N}|x_i|$ of the EMG samples, the requested amplitude statistic; and (2) a windowed power spectral density (Welch's method with a Hann window) of the epoch, from which the power in the 20-100 Hz band ($P_{low}$) and the 100-250 Hz band ($P_{high}$) are summed and combined into the ratio $R=P_{low}/P_{high}$. As the contraction fatigues, motor-unit conduction velocity slows and firing becomes more synchronous, shifting spectral energy toward lower frequencies while additional motor-unit recruitment raises the signal amplitude — so both MAV and $R$ are expected to rise, epoch by epoch, as the subject approaches exhaustion. The epoch loop repeats continuously and stops once the force channel reads (near) zero, marking the point at which no further force output can be produced.
A real-time scrolling trace shows the raw force and EMG signals as they are acquired. A second, slower-updating trend display plots MAV, the band-power ratio $R$, and the force value against epoch number/elapsed time on a shared time axis, so the clinician/researcher can see the spectral-compression and amplitude-rise fatigue signature develop in real time. Every epoch's MAV, $R$, force value and timestamp are logged to onboard memory or a host PC for post-session analysis.
Both transducers are non-invasive, skin-contact-only sensors (gel Ag/AgCl electrodes; a smooth, biocompatible grip/lever surface for the load cell). As with every patient/subject-attached channel in this paper, the entire signal path from sensors to any mains-powered instrument is electrically isolated (battery-powered front end or isolation amplifier, per Question 5) so no direct galvanic path exists between the subject and line power.
