Sex differences in rectus femoris morphology across different knee flexion positions

PURPOSEBACKGROUND: Quadriceps function is an important outcome following lower extremity injury and surgery. Measurements of quadriceps function are particularly helpful initially post surgery, however traditional quadriceps strength measures like isokinetic testing are contraindicated during this time period. Inclusion of dynamic musculoskeletal ultrasound imaging in the clinical setting has been beneficial in understanding quadriceps activation specifically rectus femoris (RF) contraction; however, there is a paucity of literature in this area. The purpose of the current study was to describe the cross-sectional area (CSA) of the RF across varying knee flexion angles.

Methods: Forty-five adult recreational athletes were recruited for the study (21 males, 24 females). All subjects underwent tests of maximal volitional isometric contractions of the knee extensors at 0, 30, 60 and 90 degrees of knee flexion. During the trials, musculoskeletal ultrasound images of the RF at 15 cm from the superior pole of the patella were taken at rest and during contraction for each of the angular positions. Mixed model ANOVAs (angle x sex) were utilized to examine the differences between males and females for different angular positions. These analyses were conducted for the resting CSA, active CSA, and the contractile index (resting - active).

Results: RF cross-sectional area increased with increasing angles of knee flexion for both the resting and active conditions. The contractile index consistently decreased as knee flexion angle increased. No statistically significant interactions or main effects for sex were observed, although differences were observed in the trajectories of the data sets for males and females.

Conclusions: RF CSA is dependent on knee flexion angle in both males and females. As a result, the assessment of RF CSA should be conducted in a standardized position if this variable is to be utilized as a meaningful measure of muscle size during rehabilitation. Additional research should seek out which factors are associated with clinically relevant factors that effect RF CSA across the range of knee flexion.

Level of evidence: 3b.