一張一弛[icchouisshi] tension and relaxation
弛む[yurumu] to become loose
弛み[tarumi] slack
弛む[tarumu] to slacken
中弛み[nakadarumi] a slump

弛緩[shikan] relaxation (e.g. of muscles)
弛張振動[shichoushindou] relaxation oscillation
筋弛緩剤[kinshikanzai] muscle-relaxant drug
弛まない[tayumanai] untiring
腕弛るい[kattarui] listless