This paper investigates the effect of resistive forces within a variable recruitment (VR) bundle actuators during recruitment state transition. Due to their versatility in design, ease of manufacturing, high force-to-weight ratio, and inherent compliance, FAMs have become a favorable actuation method for the robotics research community. Recently, researchers have adapted mammalian muscle topology to construct a multi-chamber FAM bundle actuator, consisting of separate units of actuation called motor units (MUs). These bundle actuators have VR functionality in which one or more MUs are sequentially activated according to the load demand. This activation scheme has been shown to have higher actuator efficiencies as compared to a single equivalent cross-sectional area FAM actuator. A characteristic behavior of VR bundles is the interaction between FAM elements in the bundle. Distinctively during recruitment state transition, inactive/low-pressure FAMs buckle outward and are compressed past its free strain due to the higher strain of fully active FAMs. There exists an onset pressure above which such FAMs need to contribute positively to the overall force output of the bundle. This paper presents a realistic scenario in which MU pressure is controlled by a hydraulic servo valve. As a result, the overall bundle force exhibits a sharp decrease during recruitment state transition while the MU being recruited is below the onset pressure.

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