Frequency Spectrum Analysis Of Ground Reaction Force And Load Rate In Runners With Achilles Tendinopathy And Tibial Stress Fracture

PURPOSE: Running-related injuries (RRIs) frequently affect structures such as the Achilles tendon and tibia. Ground reaction force (GRF) and load rate (LR) are key metrics in biomechanical studies of RRIs, reflecting the magnitude and rate of impact forces. However, traditional analyses may overlook frequency-specific vibrations, which offer insight into tissue responses and injury mechanisms. This study investigated the amplitude-frequency spectra of GRF and LR in runners recovering from Achilles tendinopathy (AT) and tibial stress fractures (TSF) to explore potential injury-specific vibrational signatures. METHODS: This study included 58 runners: AT (n = 12), TSF (n = 17), and matched controls (n = 29). For each participant, net GRF data from the left and right sides were averaged across 10+ steps, and then transformed using fast Fourier transforms (FFTs) to convert time-domain signals to the frequency domain. The GRF derivative, LR, was similarly transformed to capture higher-frequency vibrations. Frequency spectra were binned into 2 Hz intervals to assess group differences in spectral amplitude. RESULTS: Amplitude spectra displayed a consistent peak around 100 Hz across groups. The AT group showed higher amplitudes in the 8-40 Hz range in GRF frequencies than controls. In the LR FFT, a 40% increase in right-side amplitude within this range corresponded with the predominance of right-sided AT cases. These elevated amplitudes align with soft tissue vibrational frequencies, suggesting altered loading patterns. The TSF group showed a unique peak near 250 Hz, which may indicate bone-specific adaptations or injury responses. CONCLUSIONS: Spectral analysis of GRF and LR revealed distinct vibrational characteristics associated with AT and TSF. Frequencies related to soft tissue vibrations were more prominent in the AT group. In contrast, frequencies at the expected magnitude for the natural resonant frequency of bone were uniquely present in TSF cases. This approach highlights injury-specific vibrational signatures that could inform biomechanical assessment and rehabilitation strategies targeting frequency-specific adaptations in injured runners.