The effects of additional weight on early motor skills of infants have been increasingly studied. During the reaching behavior, the additional weight has been shown benefits in certain kinematic variables in full term and preterm infants. There is a growing interest in the study of populations at risk for motor development, especially considering interventions based on functional activities with the purpose of improving the neuromotor control, functional capacity and muscle strength. The aim of the present study is to investigate the effect of a functional training with additional weight in the reaching behavior of preterm infants with low birth weight and to compare the effects with a full term population. A controlled and randomized trial will be carried out with full term infants of adequate birth weight and preterm infants with low birth weight at 6 months of chronological or corrected age. The participants will be recruited by convenience from the charts of a Public Hospital. Participants will be randomly allocated into four groups: 1) adequate weight placebo group (AWPG); 2) adequate weight intervention group (AWIG); 2) low weight placebo group (LWPG) and 4) low weight intervention group (LWIG). . The assessor will be blinded (no knowledge of groups allocation). All infants will undergo a training program of eight sessions (2x/week; 4 weeks). The AWIG and LWIG will receive the training with an additional weight (characterized by a bracelet with a weight of 20% of the mass of the upper limbs)and the AWPG and LWPG will receive the same training, however, without the additional weight (only the bracelets).. All infants will be assessed in three different moments: 1) Pre-intervention: baseline, before the start of the intervention program; 2) Post-intervention: evaluation performed after the end of the program; and 3) Follow-up: evaluation performed seven days after the post-intervention assessment. The following kinematic dependent variables will be calculated: straightness index, mean velocity and motion units and electromyographic variables: pattern of activation and recruitment of muscle fibers - Biceps brachialis, Triceps brachialis, Deltoid and Pectoralis Major muscles; co-activation between biceps and triceps. The Qualisys Track Manager (QTM) and a wireless surface electromyography with 8-channels will be used. If normality assumptions are met, a mixed 2X2 ANOVA with repeated measures will be applied, in order to verify differences between the intervention programs, considering the dependent variables. Otherwise, non-parametric procedures of the same nature will be adopted. The significance will be set at 5% (P<0.05).

Inclusion Criteria:

• Adequate weight infants: healthy infants born with gestational age between 38-42 weeks and adequate birth weight (> 2500 grams);
• Low weight infants: healthy infants born with gestational age between 32-36 weeks and 6 days and with low birth weight (between 1500 and 2500 grams).

Exclusion Criteria:

• Anoxiated infants;
• Signs of neurological impairment (eg, hypoxic-ischemic encephalopathies grade I, II and III, intracranial hemorrhage and neonatal seizures);
• Congenital malformations (eg myelomeningocele and achondroplasias);
• Syndromes (e.g Down Syndrome);
• Sensorial alterations (visual and auditory);
• Cardiorespiratory difficulties;
• Orthopedic impairments (e.g, congenital clubfoot);
• Apgar below 8 and 10 in the first and fifth minutes, respectively.
• Infants who do not attend any of the evaluations, who do not complete at least 80% of the training or who cry uncontrollably, not allowing at least 10 reachings in the evaluations and training. Those that present intercurrences that may compromise normal neurosensorimotor development (for example, recurrent pneumonia, kidney and intestinal infection).

The outcome assessor won't have any knowledge about the study purposes and will be unaware of the group allocation.