This study was conducted to analyze the relation between age, muscle architecture, and muscle strength in children with Erb’s palsy and to compare the architecture parameters in both study groups during contraction and relaxation and also in each group between relaxation and contraction. As the majority of children with Erb’s palsy have abnormal reaching as they reach with an abducted arm, this refers to the weakness of the anterior deltoid muscle that is responsible for shoulder flexion. Our results confirmed the presence of a significant correlation between age, muscle thickness, pennation angle, and muscle strength, but no relationship was detected between muscle thickness and muscle strength in the affected side. Furthermore, a significant difference was found in muscle architecture parameters during contraction and relaxation between the two examined sides and also between contraction and relaxation in each side. The significant correlation between age and muscle architecture parameters found in this study is consistent with a previous report  that showed that pennation angle values increase from birth and stabilize after adolescence. Our results are also consistent with another study  wherein it was found that the muscle thickness increases with age from 2 to 16 years in healthy children. The increase in the muscle architecture parameters with age may be due to the ongoing maturation and development of children as the muscle size increases.
The significant correlation between muscle thickness and pennation angle found in the present study is also reinforced by a previous investigation , which analyzed the relationship between pennation angle and muscle thickness in normal individuals and observed a significant correlation. In addition, another previous study  showed that the pennation angle has a direct relationship with muscle thickness. Furthermore, a correlation between pennation angle and muscle volume has been reported . The results of the present study revealed a significant correlation between pennation angle and muscle strength, which is consistent with the previous study  reporting an increase in the pennation angle after strength training of the vastus lateralis muscle that increases the muscle volume by 10%. However, a negative correlation between pennation angle and joint force has been reported . As the muscle volume increases, the muscle thickness and the pennation angle increase as a normal response to increasing age or to strength training.
The nonsignificant correlation found between muscle thickness and strength in this study is in contrast to a previous report , which showed that muscle strength is related to muscle volume as indicated by the muscle cross-sectional area and thickness. In addition to these results, another study  found that there is a strong relationship between the thickness of the quadriceps muscle and maximum voluntary contraction. According to these results, a nonsignificant correlation exists between muscle thickness and strength of the anterior deltoid muscle in children with Erb’s palsy. However, we observed that muscle thickness was significantly reduced in the affected side. This could be due to the little difference at young age (from 1 to 5 years), but the correlation between muscle thickness and strength may change over time during the growth period due to lower mobility, disuse atrophy, and muscle weakness. Furthermore, these differences were associated with changes in muscle strength; therefore, an appropriate exercise program designed to improve muscle strength during the growth spurt may be required. Therefore, the results of this study could not reveal the time required for muscle thickness to change with age, which indicates that a large sample size is required to generalize this result and make further predictions and assumptions to investigate the relationship of muscle thickness and muscle strength with age in children . Also, this nonsignificant correlation may be due to the lack of activity of these children at this young age, which is exacerbated by the neurological affection that limits the muscle thickness and strength to some extent.
The results of this study also showed that there is a significant difference in muscle architecture parameters during relaxation and contraction between the two examined sides, except for the pennation angle during relaxation that showed no significant difference. The significant difference between both sides in the pennation angles during contraction is consistent with a previous report  wherein the authors claimed that in case of disuse atrophy, the pennation angle of muscle fibers is reduced compared to that in healthy young persons, which is largely due to the decreased amount of contractile tissue. However, this finding does not agree with the results of comparison of pennation angles during relaxation. It has been reported that a decreased muscle size is associated with changes in fascicle length and pennation angle after a time of disuse . On the other hand, in a previous study , the authors claimed that with a decrease in physical activity, there were no significant changes in muscle architecture in the antigravity muscles such as the biceps brachii and tibialis anterior; however, this agrees with our results only in the comparison of pennation angles during relaxation between both groups. This significant difference in muscle architecture between both sides may be due to the muscle weakness and the decrease of functional activities in the affected side. Therefore, we expected a marked difference in the pennation angle during relaxation that exhibited a nonsignificant difference.
In addition, the significant increase in the muscle architecture from relaxation to contraction in each side is in agreement with an earlier report  showing that the pennation angle of a given muscle increases as the muscle fibers shorten, that is, muscle fiber rotation occurs during fiber shortening. Another study  found that the thickness of the proximal semitendinosus muscle increased with contraction on both the unaffected and affected sides after hamstring strain injury. The significant increase in muscle thickness and pennation angle from relaxation to contraction observed in the present study may be due to the increase in muscle size during contraction that alters the muscle architecture.
Limitations of the study
The current study has some limitations. First, measurements were performed only on the middle area of the muscle; it has been shown that the muscle architecture can vary throughout the muscle length. Secondly, the study was limited to measurement of muscle thickness and pennation angle, whereas other parameters such as fascicle length were not measured. Thirdly, the study was limited to evaluation of only the anterior deltoid muscle. Fourthly, the sample size of participant population is small.