In almost all sports using a hand or hands, muscles of forearms (musculus brachioradialis, flexor carpi radialis muscles, long palmar muscles, flexor digitorum superficialis muscles, flexor carpi ulnaris muscles, extensor carpi radialis longus muscles, extensor carpi radialis brevis, and common digital extensor muscles) play critical roles, and it is inevitable to develop pertinent muscles individually in accordance with the purpose in order to improve one's skills in an objective sport. It is well known that developing muscles of the lower part of the body is also quite effective means for most sports.
The only method of developing muscles is so-called strength training of expanding and contracting muscles against loads or resistance. Most of conventional strength training activities typically practiced have no clear purpose. Some of the training activities use a special tool under the guidance of a professional trainer, but a trainee needs to go to special facilities to perform such training activities.
Various tools for easily performing strength training at home have been conventionally proposed, but any of them has no clear purpose as described above and most of them just give temporary ease of mind. For example, Patent Document 1 discloses a tool that is held in a hand and moved in water against resistance for developing muscles of arms. This tool includes a plurality of resistance plates and has an advantage that it can vary the resistance in water by adjusting the manner in which these resistance plates are superimposed on each other.    [Patent Document 1] Japanese Unexamined Utility Model Registration Application Publication No. H3-118766.