Beyond quadriceps strength: impaired reactive strength capacity during unilateral drop jumping in patellofemoral pain

Abstract Purpose Patellofemoral pain (PFP) affects 23–40% of individuals. Altered ground reaction force (GRF) profiles during jumping are reported in PFP, but discrete peaks might disregard differences elsewhere in the movement. Using statistical parametric mapping (SPM), which tests entire time-normalized curves, we compared force–time curves between people with PFP and controls during unilateral drop jumping and examined relationships between quadriceps strength and instantaneous GRF. Methods Forty-three participants (PFP: n = 20, 14 female; control: n = 23, 16 female) performed unilateral drop jumps from a 0.25 m platform onto force plates (1000 Hz) in this cross-sectional study. Isometric quadriceps strength was assessed at 60° knee flexion. Continuous GRF profiles were analyzed with SPM, and SPM1d regression-related knee extensor torque to instantaneous GRF. Reactive strength index (RSI), contact time (CT), jump height, and peak force were also compared. Results SPM identified significant clusters spanning 32–56% (involved) and 19–42% (uninvolved) of the first landing, where GRF was ~ 0.5 BW lower in PFP than controls ( p < 0.001). Curves did not differ elsewhere, and no limb differences occurred within groups. PFP participants demonstrated longer CT ( p = 0.002, rB = 0.57), lower RSI ( p = 0.003, rB = 0.53), and lower relative peak forces ( p = 0.017, d = 0.75). SPM regression revealed moderate-to-strong negative associations between knee extensor torque and GRF during initial contact in the PFP group only. Conclusion In this cohort, those with PFP generated lower GRF during the amortization-phase which discrete measures alone would not localize. The absence of limb differences is consistent with bilateral deficits, cautioning against limb symmetry indices.