isometric

mastery
UK /ˌaɪ.sə(ʊ)ˈmɛt.ɹɪk/ US /ˌaɪ.soʊˈmɛt.ɹɪk/adjnoun

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One of the most _____ isometric exercises for the deskbound is to sit up quite straight and tighten the large muscles of the buttocks.