Passive tension in rat hindlimb during suspension unloading and recovery: muscle/joint contributions

Muscle/joint stiffness associated with disuse conditions or weightlessness may seriously impair movement and work capacity. The purposes of this study were 1) to develop a noninvasive model to measure rat hindlimb passive tension, 2) to describe changes in passive tension (i.e., flexibility) during whole body suspension and weight-bearing recovery, and 3) to determine relative contributions of the posterior hindlimb to passive tension. Male Sprague-Dawley rats were suspended (14 days) and reloaded (14 days). On days 0, 7, 14, 17, 21, and 28, animals were anesthetized and hindlimb passive tension was measured during ankle dorsiflexion. Seven days of suspension significantly increased passive tension. Recovery of passive tension occurred by 14 days of weight bearing. In suspended animals, increased passive tension was due to musculotendinous units (75%) rather than to the joint (25%). Increased passive tension did not appear to be due to a shorter muscle, but changes in muscle architecture, cytoskeletal proteins, or viscoelastic properties of the muscle and its connective tissue elements cannot be excluded.