The "core" region is depicted as a box or cylinder and includes the spine, hips, pelvis, proximal lower extremities, and abdominal structures. This region consists of the abdominals, gluteals, paraspinals, diaphragm, oblique abdominals, pelvic floor and hip girdle muscles. These muscles provide stabilization on the trunk and spine during movement or at rest. The "core" region consists of 29 pairs of muscles and these muscles are classified according to their anatomical and functional features. Bergmark developed a model by categorizing the "core" muscles as local and global. Local muscles provide stabilization between spinal segments, while global muscles are activated according to the direction of movement. The coordinated work of local and global muscles affects the quality of movements and "core" stability. "Core" muscles influence functional activities and functioning of the extremities by providing a stable basis for extremity movements and force transmission.

While core strength is defined as the muscular control mechanism that ensures functional stabilization of the spinal column, core endurance refers to the continuity of this control. Core endurance, a component of core stabilization, occurs when the lumbo-pelvic-hip muscles perform core contraction for a certain period of time or repeatedly. Although core strength plays a role by increasing internal-abdominal pressure to create resistance, core endurance allows muscles and muscle groups to remain in a stable position for a certain period of time. According to Lehman, core endurance affects spinal stabilization more than muscular strength due to the capacity of local core muscles to stabilize the lumbar spine. The "core" region functions as a connection point between the upper and lower extremities and enables the transfer of force from this region to the extremities. This region is a basic structure where the force required for all movements is generated and transmitted from the proximal segments to the distal segments.

During childhood and adolescence, physical and physiological changes vary suddenly depending on age and gender and last between the ages of 15-17. After the age of 18, change becomes regulated and physiological and performance values reach their maximum between the ages of 20-30. After the age of 30, functional capacity and other physical-physiological characteristics begin to decrease. The most important of these changes is the change in the skeletal-muscular system. Muscles develop from birth and reach their maximum level at the age of 25-30. As age progresses, the strength and cross-sections of muscle groups decrease. This decrease occurs faster in people who live sedentary lives.

The upper extremity is the primary tool humans use to manipulate the environment and has a wide range of capabilities with the same basic anatomical structures as the arm, forearm, hand, and fingers. In the upper extremity, mobility is more important than stability, and therefore the coordinated interaction of the shoulder complex, the functional unit of the upper extremity that enables movement relative to the trunk, provides sufficient stability despite high degrees of mobility. The primary function of the shoulder complex is to position the upper extremity so that the hand can function. The primary role of the elbow joint is to shorten or lengthen the length of the upper extremity. The hand is the structure responsible for performing the functions of the upper extremity and can perform many different activities as a manipulator and a means of communication. These activities require various characteristics such as positioning, strength, and precision, increasing their structural complexity compared to other joints of the upper extremity. However, there is a remarkable degree of synergy between these structures.

Reaction is the process by which an organism perceives and responds to stimuli. This process includes the stages in which muscles transmit impulses to the central nervous system (CNS) via afferent nerves, after the CNS decides which response to give, it sends this information back to the muscles via efferent nerves, and the muscles take action based on this information. Reaction time refers to the time between the moment a stimulus reaches the organism and the organism's response to this stimulus. Reaction time refers to the time it takes for an organism to respond to a particular stimulus and is generally divided into two main categories: simple and complex (complex) reaction time. Simple reaction time refers to the process of responding to a single stimulus. It can affect important physical parameters such as core endurance, upper extremity muscle strength, function and reaction time. However, there are not enough studies in the literature investigating this effect in healthy young adults. Most existing studies focus on the role of core endurance on athlete performance. The relationship between core endurance levels and upper extremity muscle strength, function and reaction time in healthy young adults has not yet been examined. The aim of our research is to examine the effect of core endurance levels on upper extremity function, muscle strength and reaction time in healthy young adults.