Muscle endurance will be evaluated on two exercises: Biering-Sorensen Test and Prone Plank Test. Each test will be assessed in different days following a randomized order.

All patients will be encouraged to achieve the maximum time to task failure, stopped only by muscular fatigue or pain precluding to maintain the position. Each exercise will perform under two conditions.

On the control condition, the patients will perform both tests without any cognitive condition, while on the experimental condition they will be instructed to perform both tests while performing a mathematical subtracting task (starting at 300, by sevens).

The starting condition will be randomly assigned with a 5-minute rest prior to the next condition. Time to task failure and pain perception using a visual analog scale will be assessed.

Inclusion Criteria:

• Age between 20-50 years.
• Surgery of lumbar discectomy or lumbar arthrodesis with at least 3 months of evolution.
• Acceptance to participate in the study.

Exclusion Criteria:

• History of previous surgery in the lower extremities.
• Recent trauma (<6 months).
• Use of corticosteroids.
• Pregnancy.
• Cauda equina syndrome.
• Marrow-level lesions.
• Progressive neurological involvement.
• History of rheumatological diseases.
• Inability to perform the test measurement.

Muscle endurance will be evaluated on two exercises: Biering-Sorensen Test and Prone Plank Test.