The influence of sagittal trunk posture on the magnitude and rate of patellofemoral joint stress during stair ascent in asymptomatic females

Background: Excessive patellofemoral joint stress (PFJS) is thought to be a contributory factor to patellofemoral pain (PFP). Thus, treatment strategies that minimize PFJS rate and magnitude during painful activities like stair ascent may be useful for optimizing outcomes for PFP patients. Sagittal plane trunk posture has been shown to influence PFJS during running although it is unknown if a similar relationship exists during stair ascent. Research Question: Does altering sagittal plane trunk posture affect PFJS rate and/or magnitude during stair ascent? Methods: Twenty asymptomatic females (23.4 +/- 2.5 yr; height: 164.4 +/- 7.9 cm; mass: 63.0 +/- 12.2 kg) performed 5 stair ascent trials (96 steps/min) during 3 conditions: self-selected trunk (SS), flexed trunk (FLX), and extended trunk (EXT). Three-dimensional kinematics (200 Hz) and ground reaction forces (2000 Hz) were collected during each trial. A previously described mathematical model was used to calculate PFJS that included subject-specific and non-subject-specific model inputs. Dependent variables included sagittal plane trunk angle, and the rates and magnitudes of PFJS, patellofemoral joint reaction force (PFJRF), and PFJ contact area during the stance phase of stair ascent. Results: Compared to SS, peak PFJS decreased during FLX (mean difference (MD) = 2.6 MPa; p < 0.001; 95%CI = 2.2 to 2.9; effect size (ES) = 5.2) and increased during EXT (MD = -3.3 MPa; p < 0.001; 95%CI = -3.9 to -2.6; ES = -3.4). Similarly, PFJS rate decreased during FLX (MD = 17.8 MPa/sec; p < 0.001; 95%CI = 13.6 to 21.9; ES = 3.6) and increased during EXT (MD = -14 MPa/sec; 95%CI = -19.4 to -8.7; p < 0.001; ES = -2.2). Significance: Sagittal plane trunk posture influences PFJS rate and magnitude during stair ascent in asymptomatic females. Increasing and decreasing forward trunk flexion resulted in decreased and increased PFJS respectively. Future studies should examine the effects of these movement strategy modifications on pain and function in patients with PFP.