Influence Of Forward Trunk Lean On Achilles Tendon Force During Stair Ascent In Asymptomatic Females

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PURPOSE: Research indicates that forward trunk lean (FTL) decreases patellofemoral joint stress during stair ascent and may benefit people with patellofemoral pain. However, FTL can shift the body’s center of mass anteriorly, increasing the Achilles tendon force (ATF) and the risk for Achilles tendinopathy. Thus a better understanding of the relationship between FTL and ATF is warranted. The purpose of the current study was to examine the effect of FTL on the ATF during stair ascent. It was hypothesized that FTL would cause the ATF to increase. METHODS: Twenty asymptomatic females (age: 23.4 ± 2.5 yr; BMI: 23.1 ± 3.0 kg/m2) participated. Trunk and lower limb kinematic (200 Hz) and ground reaction force data (2000 Hz) were recorded as subjects performed 5 stair ascent trials (96 steps/minute) during 2 conditions: self-selected (SS) and FTL postures. A previous ATF model that included subject-specific (sagittal ankle angle and moment) and non-subject-specific (Achilles tendon moment arm length) inputs were used to estimate ATF. Dependent variables included peak ATF and sagittal trunk and ankle angles, ankle plantar flexion moment, and vertical ground reaction force at time of peak ATF. RESULTS: Compared to SS, FTL caused a decrease in peak ATF (1.8 ± 0.2 vs 2.0 ± 0.2 N/Kg; p = 0.004), ankle plantar flexion moment (1.1 ± 0.1 vs 1.2 ± 0.1 Nm/kg; p = 0.006), and vertical ground reaction force (668.1 ± 116.2 vs 628.8 ± 136.9 N; p < 0.001). In contrast, an increase was observed for the sagittal trunk angle (16.0 ± 7.5 vs 50.2 ± 8.9°; p < 0.001). The sagittal ankle angle (6.5 ± 4.4 vs 7.0 ± 5.8°; p = 0.186) was similar between conditions. CONCLUSIONS: These findings fail to support the hypothesis that FTL causes an increased ATF. The observed decrease in ATF was due to an 8% decrease in the ankle plantar flexion moment which is consistent with previous research that examined FTL and ankle mechanics during walking. Additionally, this observed change in ankle moment was associated with a 6% decrease in vertical ground reaction force which is consistent with previous research examining the effect of FTL on landing biomechanics. Increased FTL during stair ascent decreases ATF and may be considered as a strategy for treating or preventing Achilles tendon injuries associated with excessive loading.

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