Magnetic field effects on the vestibular system: calculation of the pressure on the cupula due to ionic current-induced Lorentz force.

Large static magnetic fields may be employed in magnetic resonance imaging (MRI). At high magnetic field strengths (usually from about 3 T and above) it is possible for humans to perceive a number of effects. One such effect is mild vertigo. Recently, Roberts et al (2011 Current Biology 21 1635-40) proposed a Lorentz-force mechanism resulting from the ionic currents occurring naturally in the endolymph of the vestibular system. In the present work a more detailed calculation of the forces and resulting pressures in the vestibular system is carried out using a numerical model. Firstly, realistic 3D finite element conductivity and fluid maps of the utricle and a single semi-circular canal containing the current sources (dark cells) and sinks (hair cells) of the utricle and ampulla were constructed. Secondly, the electrical current densities in the fluid are calculated. Thirdly, the developed Lorentz force is used directly in the Navier-Stokes equation and the trans-cupular pressure is computed. Since the driving force field is relatively large in comparison with the advective acceleration, we demonstrate that it is possible to perform an approximation in the Navier-Stokes equations that reduces the problem to solving a simpler Poisson equation. This simplification allows rapid and easy calculation for many different directions of applied magnetic field. At 7 T a maximum cupula pressure difference of 1.6 mPa was calculated for the combined ampullar (0.7 µA) and utricular (3.31 µA) distributed current sources, assuming a hair-cell resting current of 100 pA per unit. These pressure values are up to an order of magnitude lower than those proposed by Roberts et al using a simplistic model and calculation, and are in good agreement with the estimated pressure values for nystagmus velocities in caloric experiments. This modeling work supports the hypothesis that the Lorentz force mechanism is a significant contributor to the perception of magnetic field induced vertigo.

Gastrocnemius muscle-tendon behaviour during walking in young and older adults.

AIM: Age-related differences in muscle architectural and tendon mechanical properties have been observed in vivo under static conditions and during single joint contractions. The aim of this study was to determine if there are age-related differences in gastrocnemius fascicle-tendon interactions during a fundamental locomotor task - walking. METHODS: Eight young adults (YA; 27 +/- 4 years) and eight older adults (OA; 77 +/- 4 years) walked on a treadmill at 1.11 m s(-1) whilst length changes in the gastrocnemius lateralis muscle tendon complex (MTC), fascicles and tendinous tissue (TT) were determined from joint angles, ultrasonography and a geometric MTC model (combining MTC and fascicle measurements) respectively. RESULTS: There was no age-related difference in lengthening of the MTC during stance. However, the fascicle and TT contribution to MTC lengthening was altered; TT lengthening was larger in OA than in YA (P = 0.05) and fascicle lengthening was less in OA than YA (P < 0.05). There were no differences between groups in MTC, fascicle or TT shortening amplitude during push-off. CONCLUSION: The observations are consistent with previous reports of increased compliance of TT in older adults.

Metabolic cost, mechanical work, and efficiency during walking in young and older men.

AIM: To investigate mechanical work, efficiency, and antagonist muscle co-activation with a view to better understand the cause of the elevated metabolic cost of walking (C(W)) in older adults. METHODS: Metabolic, mechanical and electromyographic measurements were made as healthy young (YOU; n = 12, age = 27 +/- 3 years) and older (OLD; n = 20, age = 74 +/- 3 years) men of equivalent body mass and leg length walked on a treadmill at four speeds (ranging from 0.83 to 1.67 m s(-1)). RESULTS: Net (above resting) C(W), determined by indirect calorimetry was 31% higher (average across speeds) in OLD (P < 0.05). The integrity of the passive pendulum like interchange of mechanical energies of the centre of mass (COM(B)), an energy-saving mechanism, was maintained in OLD. Furthermore, total mechanical work, determined from fluctuations in mechanical energy of COM(B) and of body segments relative to COM(B), was not significantly elevated in OLD. This resulted in a lower efficiency in OLD (-17%, P < 0.05). Co-activation, temporally quantified from electromyography recordings, was 31% higher in OLD for antagonist muscles of the thigh (P < 0.05). Thigh co-activation was moderately correlated with C(W) at three speeds (r = 0.38-0.52, P < 0.05). CONCLUSION: Healthy septuagenarians with no gait impairment have an elevated C(W) which is not explained by an elevation in whole body mechanical work. Increased antagonist muscle co-activation (possibly an adaptation to ensure adequate joint stability) may offer partial explanation of the elevated C(W).