Extended Abstract
1. Introduction
One of the most common daily motor tasks is stair ascent and descent walk, which requires more muscle activity and range of motion in the joints of the lower limbs than normal walking [1]. In walking up and down stairs, in addition to applying force to move forward, more force is needed to move the body upward, where the knee and ankle muscles are responsible for creating this force when ascending [2, 3]. During walking, the joints are exposed to repeated impact forces at the moment the heel makes contact with the ground, which over time can lead to destructive changes in them. These impact forces during walking on stairs, especially when descending, can be even more severe [1]; so, a suitable shoe can reduce these impact forces and their effect. The choice of footwear is different between people. Some prefer to wear formal shoes with relatively hard heels; some prefer to wear shoes with high heels, and others prefer sneakers. The type of shoe can affect muscle activity and postural alignment, the distribution of force on the sole of the foot, and the ground reaction force, and if not suitable, it can lead to clinical problems in the lower extremity [10].
So far, several studies have been performed on the kinematics, kinetics, and electromyography of muscles when walking up and down stairs [13-16]. The electromyographic activity of the lower extremity muscles while walking on stairs can provide an overview of how these muscles and the forces applied to the lower extremity joints work. When climbing stairs, a lot of pressure is put on the knee joint, which is caused by the contraction of the quadriceps muscles [17]. The present study aimed to examine the effect of show type and load carrying on Electromyographic (EMG) activity of lower extremity muscles during stair ascent and descent walk.
2. Participants and Methods
In this quasi-experimental study, from among male students of Islamic Azad University, Hamedan Branch, 9 aged 20-30 years were randomly and voluntarily selected after ensuring their general health and no history of any injuries, including fracture in limbs, especially lower limbs and spine. A digital scale and a stadiometer (Seca model), were used to measure the weight and height of the subjects, respectively. The 16-channel EMG system (MA 300, Motion Lab Systems, US) was used to record the activity of internal and external twin muscles, the vastus medialis, vastus lateralis, and rectus femoris muscles. These points were selected according to McGill (1996)’s protocol. After installing the electrodes, Maximal Voluntary Isometric Contraction (MVIC) of the muscles was performed to normalize the data. The participants were first tested without load carrying and performing the stair ascent and descent task with bare feet. Then, it was done with the load (carrying a box weighing 15% of the body weight) and performing the task once wearing athletic shoes and once with formal shoes. Subjects were asked to use the most convenient form and the usual speed to complete the task.

3. Results
The results showed that the load factor significantly increases the intensity of muscle activity (Figure 1) indicating that carrying loads during stair ascent and descent task significantly increases EMG activity of lower extremity muscles (F=25.2, P=0.0001). Moreover, the intensity of EMG activity during ascending and descending stairs was significantly different. As shown in Figure 2, its intensity in ascending was higher than in descending (F=58.01, P=0.0001). Regarding the shoe factor, results showed that the intensity of EMG activity in the three cases of barefoot, athletic shoes and formal shoes did not differ significantly (F=2.86, P=0.08). Factor analysis showed that the interaction between the two factors of load and direction (ascending and descending) is not significant; this means that the effect of load carrying on the intensity of EMG activity during ascending and descending stairs was similar. Furthermore, the intensity of EMG activity in different muscles changed equally in both conditions of with and without load carrying (F=1.53, P=0.35); i.e., although the intensity of EMG activity under load carrying was increased significantly, but it increased the intensity of EMG activity in all muscles.
4. Discussion
The load factor has a significant effect on EMG activity of lower extremity muscles; i.e., the intensity of EMG activity increases by load carrying. The direction factor (ascending and descending) has also a significant effect on EMG activity of muscles, where its intensity during ascending stairs increased by about 45% compared to its intensity when descending. In addition to increased intensity, the pattern of EMG activity varied in both directions where the activity of internal and external twin muscles as well as vastus medialis and vastus lateralis muscles during climbing the stairs increased while the activity of rectus femoris muscle was reduced.
5. Conclusion
Based on these results, it can be said that, by descending the stairs, since the intensity of activity in vastus medialis muscle increases more than in vastus lateralis muscle, it causes the patella to be pulled inward more leading to the abrasion of the internal cartilage of patellofemoral joint in the long term. Therefore, it can be concluded that stair ascent and descent can be considered as a risk factor for osteoarthritis.
Ethical Considerations
Compliance with ethical guidelines
All subjects participated in the present study voluntarily after signing a consent form. 
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Authors' contributions
All authors contributed equally in preparing this article. 
Conflicts of interest
The authors declared no conflict of interest