Head impact biomechanics research is performed at K.U.Leuven (Belgium) since 1998 in a project to construct a safer and more comfortable bicycle helmet. One of the goals of this research project is to construct a new tolerance criterion curve for brain injury.

Historically, the Head Injury Criterion (HIC) is the most widely used criterion to quantify head lesion risks. It is based on the relation between linear acceleration and impact pulse duration from human skull fracture tests. It is widely stated that the HIC needs reconsidering as a criterion for possible brain injuries.

From an epidemiological K.U.Leuven study, it is found that the most frequently observed brain injuries in bicycle-related crashes are contusions, acute subdural haematoma (ASDH), and diffuse axonal injury (DAI). Therefore, three lesion specific tolerance curves, expressing peak rotational acceleration vs. pulse duration, are constructed. For contusions, a linear visco-elastic model of the human brain is used. For ASDH, human cadaveric test data from Löwenhielm (Löwenhielm, 1978) is extended with data from K.U.Leuven cadaveric impact tests. For the DAI injury criterion, a proposed DAI tolerance curve from Margulies and Thibault (Margulies et al., 1992) is used.

The three tolerance curves give a limit for the peak rotational acceleration of the head as a function of the duration of the impact pulse, and show a similar pattern. Combining these three tolerance limits results in a new integrated tolerance criterion, referred to as BICLE (Brain Injury Curve LEuven). The BICLE, as the Wayne State Tolerance Curve, represents a relation between a motion parameter and the amount of time this parameter is exerted on the head.

The BICLE has certain limitations as it does not discriminate between different impact directions and is at the moment constructed from a relatively limited amount of data. Nonetheless it is an easy to use reference as it constitutes a worst case scenario for brain injury as a whole.