The fatigue of the trunk extensor muscles is often reported in people with the chronic low back pain (LBP). Evidences have showed that the strengthening muscular by the training of the lumbar spinal muscles is efficient to decrease the pain and disability symptoms of patients with the chronic LBP. Also, some studies have reported improve of the strength and endurance of the back muscles from of this type of training muscular. However, the exercise prescription for the trunk extensor muscles during the training is still not clear in the literature. In fact, no study has showed any differences between one and three sets of specific lumbar exercise with regard to better prescription of training in healthy subjects (e.g; preventive action) as well as in LBP subjects (e.g; intervention action).

The purpose of this study is to compare an endurance trunk extensor training using one and three sets of exercise repetition on gains of strength and endurance muscular in healthy subjects.

Forty-two healthy voluntaries (from 18 to 35 years) of both the sexes will participate of this study. All participants will sign an informed consent form approved by the North of Parana University Ethics Committee for Research on Human Subjects. The sample will be randomized in three groups by the allocation from printed cards sequentially numbered in opaque envelopes: (G1, n=14) the participants will perform three sets of 15-20 repetitions of the trunk extensor exercise on a roman chair machine with the hips at 45 degrees relative to horizontal; performing trunk flexion-extension cycles for a total of 4 seconds (2 s concentric and 2 s eccentric contraction); (G2) the participants will perform only one set of 15-20 repetitions of the trunk extensor exercise on a roman chair machine with the hips at 45 degrees relative to horizontal; performing also trunk flexion-extension cycles for a total of 4 seconds (2 s concentric and 2 s eccentric); and (G3) the participants will be the control group, which not participating of any exercise program in any time during the study. A total of 10 weeks of training will be performed in the study, by 2 x week with the progression of load volume (increase the load if the total repetition of 20 is reached).

Before and after the lumbar training, all participants will be assessed by the endurance muscular (Isometric Sorensen test using time-limit, Dynamic endurance using the number of repetition, Borg fatigue perception scale and EMG surface of trunk extensor muscles by computation of median frequency fatigue estimate); and by the strength muscular (back isometric maximal voluntary contraction using a dynamometer on a roman chair). An evaluator trained will perform all testing assessment, in baseline time and after weeks of training. The evaluator will be blinded with regard the groups on assessment. Also, an exercise professional of sport will be responsible by exercise prescription to prescript the training at the Fitness center in the UNOPAR University; This professional will be also blinded to the groups.

An ANOVA two-way to compare the effects in the three groups (G1, G2, and G3) and the two times session-1: baseline and session-2: after 10 weeks); and the effects of interaction (Groups X Times) also will be tested. When necessary a Tukey post hoc analysis will be performed to localize the differences on the three groups. The effect size on each group will be calculated to determine the rate of changes from the training intervention.

Inclusion Criteria:

• Healthy subjects only and not using ergogenic substances and anabolic steroids, and drugs use for increasing ot the muscular performance.
• All subjects need to be voluntaries.

Exclusion Criteria:

• Any contraindication to physical exercise performance in accord with the American College of Sports Medicine Standard Position;
• Several spinal pathologies (fractures, tumors, and inflammation), nerve root disorders (disc herniation and ,spondylolisthesis with neurological compromise, spinal stenosis, and others), several cardiorespiratory diseases, previous back surgery or pregnancy;
• Mental or physical illnesses problems that interfere with the exercise;
• Any surgery in the spine in the last 6 months;
• Upper body mass (trunk, upper limbs and head) greater than 50% of the total strength of the trunk extensor muscles from first day the data collection.
• Subjects enrolled in any exercise program outside of the study.