The “Hybrid training” (HYBT) method utilizing combined electrical stimulation and voluntary muscle contraction has been developed as a muscle training method. It has already been shown that the method is technically sound and clinically effective in healthy young subjects. The purpose of this study was to investigate the effect of the HYBT method on the knee extensor strength considering safety for elderly people. Twenty subjects were randomly divided into two groups: the HYBT group and the weight machine training (WMT) group. All the subjects performed knee flexion and extension for 19 min per session, twice a week for 12 weeks. At the baseline and after the training, the subjects' maximal isometric torque of knee extension and cross-sectional area (CSA) of quadriceps femoris muscle were measured. The subjects completed the study without adverse effects. The knee extension torque significantly increased in both groups (39% in HYBT group and 42% in WMT group, P < 0.05). The CSA of quadriceps whole significantly increased in both groups (9% in HYBT group and 14% in WMT group, P < 0.05). These results indicate that the HYBT method increases muscle strength and mass, and that this method is as effective as the WMT. In addition, unlike the WMT, the HYBT device, which is portable and not large in size, is so easy to handle that it can be placed at the bedside. Therefore, the HYBT has potential to become a safe, effective method of muscle training for elderly people.