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Safe Neuromuscular Stimulator Designed for Elderly People

An age-related reduction of muscle mass and strength is often associated with reduced mobility.
Lauretani et al. [1] reports a decrease of isometric muscle strength of 51% and 43% in 75-85 year old women and men, respectively, compared to young subjects (20-29 years). This decrease is often related to dysfunction of mobility in the elderly and is a predictor of future disabilities. Muscle training can dramatically improve the muscle strength, power and functional abilities of elder individuals [2,3].

A neuromuscular electrical stimulator (NMES) for muscle training of elderly people, with the following design rules, is introduced:

Simple usability with reduced input options
Safety protocol that protects the elderly even in case of improper handling
Storage of compliance data on a memory card

The safety protocol is based on measurements of the electrode impedance and functional feedback such as evoked myoelectric signal and accelerometric detected muscle motion.

Neuromuscular electrical stimulation of the thigh

Muscle training is performed on the anterior and posterior thigh with large electrodes (10x13cm). The 4-channel stimulator provides simulta-neous stimulation of both legs.

Figure 1: Electrical Stimulation of quadriceps muscle and hamstrings on both legs

Hardware components

The portable transcutaneous stimulator (Fig. 2) consists of a central unit and four stimulation channels. Each channel is designed modular and includes an output stage and measurement module. The multi-processor structure breaks down the design to simple tasks and implements them on several microcontrollers.

Central control unit
A microcontroller controls all major tasks of the stimulator to provide a stand-alone operation. The stimulation protocol and the compliance data are stored on an SD card. USB and Bluetooth are used for the stimulator-computer communication. Via these data-links, the stimulator is programmed and the compliance data can be stored on a computer.

Measurement module

electrode impedance; stimulation process will be terminated if an error occurs
evoked myoelectric signal (M-wave)
acceleration signal of muscle twitches; the three dimensional acceleration signals are used to determine the thigh orientation and the twitch response.

Output stage
The constant voltage stimulation impulses are generated with a full bridge circuit (A3953, Allegro MicroSystems Inc.) supplied by a DC/DC step-up converter. Two capacitors protect the subject against DC current.

Figure 2: Block diagram of the electrical circuit

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[1] Lauretani F, Russo CR, Bandinelli S et al., Age-associated changes in skeletal muscles and their effect on mobility: an operational diagnosis of sarcopenia, J Appl Physiol, vol. 95, no. 5, pp. 1851-1860, Nov, 2003.

[2] Macaluso A, and De Vito G, Muscle strength, power and adaptations to resistance training in older people, Eur J Appl Physiol, vol. 91, no. 4, pp. 450-72, Apr, 2004.

[3] Kern H, Kovarik J, Franz C et al., Effects of 8 weeks of vibration training at different frequencies (1 or 15 Hz) in senior sportsmen on torque and force development and of 1 year of training on muscle fibers, Neurol Res, vol. 32, no. 1, pp. 26-31, Feb, 2010.

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