The bigger, the stronger? Insights from muscle architecture and nervous characteristics in obese adolescent girls

Abstract

Background: Young obese youth are generally stronger than lean youth. This has been linked to the loading effect of excess body mass, acting as a training stimulus comparable to strength training. Whether this triggers specific adaptations of the muscle architecture (MA) and voluntary activation (VA) that could account for the higher strength of obese subjects remains unknown.

Methods: MA characteristics (that is, pennation angle (PA), fascicle length (FL) and muscle thickness (MT)) and muscle size (that is, anatomical cross-sectional area (ACSA)) of the knee extensor (KE) and plantar flexor (PF) muscles were evaluated in 12 obese and 12 non-obese adolescent girls (12-15 years). Maximal isometric torque and VA of the KE and PF muscles were also assessed.

Results: Results revealed higher PA (P<0.05), greater MT (P<0.001), ACSA (P<0.01), segmental lean mass (P<0.001) and VA (P<0.001) for KE and PF muscles in obese girls. Moreover, obese individuals produced a higher absolute torque than their lean counterparts on the KE (224.6±39.5 vs 135.7±32.7 N m, respectively; P<0.001) and PF muscles (73.3±16.5 vs 44.5±6.2 N m; P<0.001). Maximal voluntary contraction (MVC) was correlated to PA for the KE (r=0.46-0.57, P<0.05-0.01) and PF muscles (r=0.45-0.55, P<0.05-0.01). MVC was also correlated with VA (KE: r=0.44, P<0.05; PF: r=0.65, P<0.001) and segmental lean mass (KE: r=0.48, P<0.05; PF: r=0.57, P<0.01).

Conclusions: This study highlighted favorable muscular and nervous adaptations to obesity that account for the higher strength of obese youth. The excess of body mass supported during daily activities could act as a chronic training stimulus responsible for these adaptations.