Hypothesis / aims of study
Dynamics of the storage and voiding phases vary throughout the day. Each micturition cycle represents a continuous adjustment of the bladder's mechanical strategy to efficiently accommodate and void urine depending on current physiological demands and circadian rhythms. Bladder diary (BD) is a "black box" of the micturition cycle, capturing these transitions. This study aims to create a biomechanical framework for evaluation of the dynamics of the storage and voiding phases through a novel interpretation of BD data using the physical concepts of Mechanical Advantage (MA) and Displacement Advantage (DA)
Study design, materials and methods
MA is the ratio of output force to input force, reflecting the ability of a mechanical system to amplify force. DA is the ratio of output displacement to input displacement, reflecting the system's capacity to transform small input movements into larger output displacement. MA>1 means that the system amplifies force but typically at the cost of displacement. When DA>1 the system amplifies movement but usually at the expense of force. In rigid systems MA = 1/DA. However, in non-linear flexible systems, these relations are more complex. Structurally, the bladder operates as a type of Hydrostatic Skeleton (HS) (1). Force and displacement here occur in more than one direction changing its shape and volume. Under these conditions bladder's mechanical efficiency during Storage and Voiding is achieved via Variable Gearing—a principle observed in HS that allows dynamic adjustment of MA and DA, i.e., mutual interconversion to achieve optimal efficiency (2).
We developed a framework that quantifies bladder storage and voiding efficiency through the concepts of MA and DA. This framework calculates two indices: Storage Efficiency Index (SEI) and Voiding Efficiency Index (VEI). SEI = MA_storage × DA_storage and VEI = MA_voiding × DA_voiding. Parameters used in these equations were derived from BD and normalized to physiological limits. We applied a normal voiding time of ~25s (male) and ~20s (female), with voided volume range 50–500 mL. Time units were standardized, and clamping via min(x,1) ensured efficiency remained within 0–100% for physiological realism.
Interpretation of results
Efficiency is not disease-specific. In the context of SEI and VEI, it describes how well the bladder performs but does not explain why function is impaired. Unlike traditional parameters that assess individual voids, efficiency indices provide a full-day functional assessment. SEI measures the bladder’s ability to store urine before voiding. High SEI: Fewer voids with optimal volumes → suggests good compliance and/or behavioral adaptation. Low SEI: Frequent, small-volume voids → suggests inefficient storage due to urgency, sensory dysfunction, low compliance, or habitual voiding. VEI measures how effectively the bladder empties relative to its capacity. High VEI: Efficient, near-complete emptying → suggests good detrusor contractility and/or low outlet resistance. Low VEI: Slow or incomplete emptying → suggests weak detrusor contractions, outlet obstruction, or voiding dyssynergia.
Concluding message
The proposed framework offers a novel biomechanical approach to assessing bladder storage and voiding phases dynamically. While extensive clinical validation is pending, developed indices leverage established BD measurements, reinterpreting them through the MA and DA concept. The dedicated calculator can assist in initial clinical decision-making and progress evaluation. Limitations: absence of pressure-volume and post-void residual variables, simplified equations. However, this approach is adaptable to analyze data from more sophisticated future devices that will continuously monitor bladder pressure, volume, and residual urine.