Ultra-wideband radar to measure in vivo muscle forces

Accurate measures of muscle forces are critical for engineers, clinicians, and biomechanists, yet direct measurement is highly invasive, and current estimation methods remain limited in accuracy. Here, we demonstrate the application of ultra-wideband radar sensors to noninvasively estimate muscle forces by measuring changes in the electromagnetic properties of contracting muscles, in muscles with different structural properties, during various static and dynamic conditions, and in the presence of fatigue. First, we show that ultra-wideband radar scans of muscle can reliably track isometric force in a unipennate knee extensor (vastus lateralis) and a bipennate ankle dorsiflexor (tibialis anterior). Next, we integrate radar signals in machine learning and linear models to estimate muscle forces during fatiguing isometric and dynamic knee extension contractions, with exceptional accuracy. Last, we identify radar frequency-dependent effects of the relationship between muscle forces and ultra-wideband radar signals, with these effects being independent of physiological and structural features known to influence muscle force. Together, these findings establish ultra-wideband radar as a noninvasive sensor capable of quantifying in vivo muscle forces, with the potential for wearable assistive technologies, biomechanics, and rehabilitation.