Aging of the human body is understood as a multifactorial process, which is also associated with a significant decline in neuromuscular functions. In general, skeletal muscles undergo a structural disorganization and functional decline. Especially, the lower limbs are more affected by age-related muscle atrophy compared to upper limbs. The reduction of leg muscle mass and strength is related to dysfunction of mobility and is a predictor of future disabilities. Therefore, an important aspect is to reduce muscle hypotrophy in older individuals. Several studies have documented that conventional physical therapy can reduce age-related muscle loss. The aim of the thesis was to employ neuromuscular electrical stimulation (NMES) as an alternative intervention to improve muscle recovery. It is hypothesized that NMES can successfully substitute the reduced volitional control of the aged muscles and induces persisting improvements in the neuromuscular performance.^ In the presented trial, electrical stimulation was applied to the anterior thigh muscles in sedentary seniors with an age-appropriate lifestyle. The thesis presents the implementation of a portable stimulation system especially designed for the elderly, which integrates specific safety features and a compliance monitoring system. The developed stimulator is easy to handle and reliable protective in case of improper usage. Therefore, the system integrates an active thigh-fitted electrode system, which is controlled with a hand-held remote control-unit. The concept of the novel stimulator is presented in detail, which is based on voltage-controlled output stages, integrated sensor- and measurement instrumentation to support the demanding safety features. In addition, a comprehensive compliance management supports the NMES-training in a home environment. For safe operation, a continuous monitoring of the electrode-tissue interface is essential.^ Therefore, a compact circuit was developed, which can measure the complex impedance by using a sinusoidal excitation signal within a frequency range of 10 kHz to 100 kHz. It was demonstrated that the accuracy of the impedance measurements is around 2.60% for excitation voltages lower than 1 V. The system integrates additionally an accelerometer sensor for detecting the surface vibration during evoked muscle contractions. In a preliminary study, acceleromyography was used to assess the muscle performance in five subjects. The results show that accelerometry is highly sensitive to evoked muscle contraction; therefore, it can be used to detect its activation thresholds. Nevertheless, acceleromyography is not interchangeable with force measurements. In the clinical trial, 16 sedentary elderly (average 73.1 years) performed an NMES-training of the anterior thigh muscles. A battery of functional tests, including maximum voluntary knee extension torque, was the basis for the assessment.^ Additionally, muscle biopsies were taken to investigate structural and molecular changes within the tissue. After NMES-training of 8 or 10 weeks, the lower limb muscle performance was significantly improved. In addition, the maximum voluntary contraction increased significantly by 6%. The biopsies taken after the training did not show any sign of fibrosis and inflammatory cell infiltration. Thus, the results provide evidence that electrical stimulation is a safe and an efficient method to counteract muscle decline associated with aging. Further, the thesis presents a comparative investigation of the two fundamental stimulation techniques, current-controlled (CC) and voltage-controlled (VC) stimulation. Therefore, the stimulation was applied to the anterior thigh muscle using a bipolar electrode set-up. The effect of electrical stimulation was assessed by force measurements and myoelectric signal recordings.^ ^A systematic parameter variation of the intensity and pulse width identified the influence of the electrode impedance on both stimulation types. During CC stimulation, the muscle output was susceptible to the variations of the pulse width and intensity. Whereas, a number of activated nerves during VC stimulation was only affected by the variation of the intensity considering pulse widths larger than 150 s. It can be concluded that NMES-training at home provides significant benefits to patients, including the improvement of life-quality and saving of treatment costs. Electrical stimulation significantly improves the reduced volitional control of the aged muscles. The study provides evidence that NMES-training counteracts muscle decline associated with aging.