The goal of this observational study is to analyze the differences in muscle activation of six muscles of the lower leg (soleus, gastrocnemius medialis, gastrocnemius lateralis, tibialis anterior, peroneus longus, peroneus brevis) in seven possible configurations of a specific instability device, the Blackboard (BB), as well as on the floor, among healthy active subjects.

The main questions it aims to answer are:

1. Which configuration of the BB produces the highest activation for each studied muscle?
2. Which muscle is the most activated in each configuration of the BB?
3. What is the muscle activation pattern obtained in each configuration of the BB?

A single group of participants will be analyzed. Both the studied leg and the order of configurations of the instability device and ground condition will be randomly assigned. Participants will first undergo a 2-minute continuous walking warm-up, followed by a performance of a maximum voluntary isometric contraction (MVIC) for each muscle, during which the maximum activation produced will be recorded. Each participant will be allowed a 30-second familiarization period on the most unstable configuration of the BB. Three repetitions of 20 seconds will be recorded on both the ground and each configuration, with a 30-second rest between repetitions. The central 10 seconds of each trial will be used for subsequent analysis. The mean of the three repetitions for each muscle will be calculated and subsequently processed and normalized by the maximum activation value during the MIVC.

Finally, a statistical analysis of the differences in muscle activation in each configuration will be conducted with the intention of addressing the mentioned questions.

The use of different instability devices is common in the clinical setting, both for preventive and therapeutic purposes. Numerous studies have demonstrated their effectiveness in improving stability and proprioception, although in most cases in a generalized manner. The need to selectively target specific structures and joints has led to the development of specific instability systems, such as the Blackboard (BB).

The BB is an instability device composed of two rectangular wooden boards, joined together, with Velcro on its base where rounded slats can be freely placed to generate the desired instability according to the patient's or subject's requirements. It is known for its portability, small size, and adaptability, and, as demonstrated in a previous study, similar to traditional devices (BOSU or Wobble board) in activating the Peroneus longus muscle, essential for single-leg stability. However, its detailed functioning is still not fully understood, as the muscle activation it produces during single-leg stance in its different configurations has not been analyzed yet.

This study is a single-group cross-sectional observational study and it will include a sample of 30 participants who will be analyzed in a single session, during single-leg stance on the floor and on the different configurations of the BB. Electrode placement will follow SENIAM guidelines. The session will begin with a 2-minute continuous walking warm-up, followed by the recording of muscle activation in a maximum voluntary isometric contraction (MVIC). Subsequently, the participant will be allowed a 30-second familiarization period on the most unstable configuration of the BB (Total - see description below). Muscle activation of six leg muscles (soleus, medial gastrocnemius, lateral gastrocnemius, tibialis anterior, peroneus longus, and peroneus brevis) will be measured during single-leg stance on the different configurations of the BB and on the ground, randomly.

The analyzed variable will be muscle activation, and the MuscleLab 4020e surface electromyography device from Ergotest Technology AS will be used. The maximum value obtained during the central 10 seconds of the measurement (which will last a total of 20 seconds) will be recorded, in three attempts with 30 seconds of rest between them.

The hypothesis of this study is that by selecting a BB configuration that reinforces the expected muscle function based on ankle biomechanics, its activation will increase. Thus, configurations that require movements such as eversion, to increase activation of the peroneus longus and brevis, or supination, aiming to increase activation of the tibialis anterior, among others, will be analyzed. The description of the different configurations is detailed below, for the right foot:

• Floor: single-leg stance on a firm surface, in this case, the floor
• Rear: single-leg stance on the BB, with the front table fixed and the rear table with the slat placed centrally
• Inver: single-leg stance on the BB, with the front table fixed and the rear table with the slat placed laterally (demanding an inversion movement of the heel)
• Ever: single-leg stance on the BB, with the front table fixed and the rear table with the slat placed medially (demanding an eversion movement of the heel)
• Sup: single-leg stance on the BB, with the rear table fixed and the front table with the slat placed laterally (demanding a supination movement of the forefoot)
• Pro: single-leg stance on the BB, with the rear table fixed and the front table with the slat placed medially (demanding a pronation movement of the forefoot)
• Fore: single-leg stance on the BB, with the rear table fixed and the front table with the slat placed centrally
• Total: single-leg stance on the BB, with both the front and rear tables with the slats placed centrally

In all cases, the subject's objective is to ensure that the edges of the BB are not in contact with the floor.

Thus, the aim of this study is to analyze the differences in electromyographic activity of the six studied muscles across the eight different conditions. This includes identifying the most activated muscles in each condition and determining which condition produces the highest activation of each muscle. Additionally, a secondary aim is to establish patterns of muscular activation for each condition, that is, for each requested movement of the foot and ankle.

Inclusion Criteria:

• Not to have experienced lower limb injury or pain within the last year prior to the study
• Self-report as physically active, such that they perform at least 90 minutes of weekly physical activity

Exclusion Criteria:

• Previous participation in any lower limb balance or proprioception exercise program
• The presence of known balance impairments such as vertigo, vestibular or central alterations

The participants are being sourced from the Faculty of Physiotherapy of the University of Valencia. They are undergraduate or master's students who have responded to an email invitation sent by one of the professors from the department involved in the study.