# Criterion-Related Validity and Reliability of the Front Plank Test in Adults: The ADULT-FIT Project
## 1 Introduction  
The trunk musculature comprises abdominal wall muscles located in the abdomen, such as the rectus abdominis, which is responsible for trunk flexion, and the transverse abdominis, which primary functions to increase intra-abdominal pressure and provide spinal stability. The rotator muscles, such as the internal and external oblique, are responsible for trunk rotation, while the extensor muscles, including the multifidus, paravertebral, and quadratus lumborum muscles, contribute to trunk extension [1,2]. Studies have demonstrated the importance of trunk musculature in health and performance. Related to health, the focus has been on improving spinal stability and thus reducing pain in patients with low back pain [3]. Concerning performance, it has been related to the transfer of forces in sports gestures involving kinetic chains, such as handball throwing or tennis serving [4,5].  
Assessment of trunk musculature has been performed using diverse methodologies [6, 7, 8], with isokinetic devices being the gold standard [9,10]. This method assesses the isometric and isokinetic strength of trunk muscles in different positions depending on the device type [11]. These tests have high validity and reliability [12]; however, they require specific equipment, learning, and familiarization with the measuring device, are expensive, have low portability, and have space limitations [13,14]. An alternative to laboratory tests is field-based fitness tests, in which the objective is to assess the trunk musculature without the need to use a specific device or equipment. Field-based fitness tests are cost-effective and do not require much familiarization [15].  
Several field-based fitness tests have been used to assess trunk musculature [16,17]. One of these tests is the front plank test (FPT), which is not only a test to assess trunk musculature but is also one of the most used exercises in training [18]. Additionally, various studies have used the FPT in CrossFit contexts to analyze muscle function and evaluate the endurance of core-stabilizing muscles [19,20]. A recent systematic review found that only one study examined the criterion-related validity of the FPT among young adults, with limited validity. The study included in the systematic review analyzed the validity in healthy young adults, providing validity to the FPT with surface electromyography (sEMG) of muscle activation [21]. However, other studies, instead of using a gold standard such as electromyography or isokinetic devices, have attempted to validate this test using variables such as physical activity level, body composition variables, and relating it to similar exercises such as the modified V-sit [17,22,23]. Therefore, more studies are needed to examine the criterion-related validity of this test in other age ranges and populations [24]. Moreover, reliability has been studied in children (8 to 12 years), young adults (18 to under 34 years), and older adults (60 to 79 years) but not in adults ranging in age from 34 to 60 years [17,22,25,26]; a recent systematic review showed a moderate reliability in this test in young adults [27].  
Considering the above, it is necessary to study the criterion-related validity and reliability of the trunk musculature assessment in adults. Therefore, the aims were to analyze the criterion-related validity and reliability of the FPT for evaluating trunk musculature in adults according to sex and age groups. We hypothesized that FPT can demonstrate high criterion validity and reliability for assessing trunk musculature in adults.