## 1 Introduction  
Equestrian helmet certification tests are designed to ensure that a minimum performance and quality level is achieved in terms of helmet crashworthiness and structural integrity. As equestrian sports are high risk [1; 2; 3; 4; 5], with the primary type of accident involving a fall from the horse resulting in a head impact [6], it makes good sense that the main helmet functional test in the standards involves recreating some simplified impact conditions [7; 8; 9]. The next most significant test in most equestrian helmet standards is referred to as the lateral crush test, also referred to as the lateral deformation test or rigidity test. However,unlike impact tests, the origins of which are well documented in the literature [10; 11], the rationale and evidence basis for the crush test are unclear. Essentially, this particular test is formulated as a quasi-static test to represent a horse dynamically falling against or rolling over the head of a helmeted jockey.  
The lateral crush test itself is relatively simple. A helmet is placed between two metal plates and is crushed quasi-statically until a peak force is reached at a specified loading rate (see Figure 1). There is no headform in the helmet. To pass the test, maximum and residual crush limits must not be exceeded. Peak loads are set to be 800 N for both PAS 015 [9] and EN 1384 [8] and 1000 N for the Snell E2016 standard [12]. In all cases, the maximum permitted crush is 30 mm and the residual crush may not exceed 10 mm.  
In discussions with engineers working within the standards industry and with standards committee members, it is understood that the lateral crush tests are used to ensure that the helmet is 'not too soft' and that the structure of the helmet has some'stabilizing effect'. It is not intended to simulate a real-world accident. However, there has been no quantification of what constitutes a helmet that is 'too soft', particularly if its impact performance is sufficient. Additionally, in discussions with the equestrian community, it is clear that the lateral crush test is believed to represent a horse falling onto a helmet. Indeed, the most recent revision of the EN1384 standard was to increase the peak force that could be sustained from 630 N to 800 N. That decision was taken on the basis that it should improve helmet performance in the event of a horse falling onto a rider's head. However, there is no evidence that this change would have any influence on helmet performance.  
Equestrians have a high risk of head injury [13] and the majority of professional jockey fatalities are as a result of head injury sustained from a fall. Additionally, reported rates of concussion or mild traumatic brain injury (mTBI) are higher for equestrians than those in boxing and American football [6]. However, when compared with the number of falls and head impacts, crush injuries, particularly to the head, appear to be rare with few reported in the literature [14]. Nevertheless, in some situations such as cross-country riding or eventing, a horse can somersault during a jump and land on the rider. In such cases the injuries can be catastrophic and sometimes fatal [14]. There may be merit in introducing a more realistic crush test to the standards if the objective of the test is to improve helmet performance while being dynamically crushed. However, there are no primary empirical data on which to base such a test. It is not known what typical loads are applied to the rider's head during such an accident and it is not known by how much a typical equestrian helmet might reduce these loads.  
The aim of this present paper is to address this deficit directly by determining what loads are likely to be applied to the rider's head in the event of a horse falling onto it, and to investigate the  
Figure 1: Lateral crush test.  
extent to which a typical equestrian helmet reduces these loads. It is hoped that these primary data will help to inform and create an evidence basis for future standard lateral crush tests.