During a strong voluntary contraction of the biceps

PHYSICS · Multi-part · Fermi University Physics · Electricity, Magnetism & Electromagnetism · Biophysics & Medical Physics · Bioelectricity, the senses and the physics of the body

Forces, Machines and Signals, Ch Muscle, heart and the fields outside the body

Problem

During a strong voluntary contraction of the biceps, a surface EMG electrode placed directly over the muscle belly, at an effective perpendicular distance of

2.0\,\mathrm{cm}

from the centre of the active motor units, records a peak potential of

2.0\,\mathrm{mV}

. Model the many active motor units together as a single current dipole in an unbounded, uniform conductor of conductivity

0.20\,\mathrm{S\,m^{-1}}

(the same volume-conductor model used for the torso). ((a)) Estimate the dipole moment of the contracting muscle. ((b)) A magnetomyography sensor cannot sit on the skin; its pickup coil is held

5.0\,\mathrm{cm}

from the same source. Estimate the peak magnetic field it would record, using the order-of-magnitude Biot-Savart estimate

B\sim \mu_0 p/4\pi r^2

. ((c)) Compare this field with the heart's estimated field of about

250\,\mathrm{pT}

(measured at

50

100\,\mathrm{pT}

) and with urban magnetic noise of order

10\,\mathrm{nT}

, and say what instrumentation this comparison implies is needed. State one respect in which modelling the whole contracting muscle as a single dipole is unrealistic.

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