Author(s): MERSMANN, F., DOMROES, T., BOHM, S., ARAMPATZIS, A., Institution: HUMBOLDT-UNIVERSITÄT ZU BERLIN, Country: GERMANY, Abstract-ID: 2272

INTRODUCTION:
The strain of a tendon under load is a key determinant of its mechanical loading. Strains between 4.5% and 6.5% appear to effectively promote tendon adaptation [1], whereas strains > 9% combined with high loading volumes may represent a risk factor for overuse injuries [2]. However, the actual strain of tendons in vivo across various exercise are widely unkown. Therefore, the present study investigated patellar tendon strain as an indicator of its mechanical demand in vivo during exercise variations on a knee extension machine and a leg press.
METHODS:
Using ultrasound imaging, patellar tendon strain was measured in 22 trained individuals during unilateral exercise execution in both machines at 40%, 60%, and 80% of the concentric one-repetition maximum (1RM), as well as isolated loading during the flexion phase (i.e., predominantly eccentric contraction) at 100% and 120% 1RM. Additionally, the electromyographic (EMG) activity of the knee extensors and antagonistic musculature was recorded. Data analysis was performed using linear mixed models. This included testing the feasibility of predicting the resulting strain based on the load and the tendon strain measured during a maximal fixed-end contraction (εmax).
RESULTS:
A significant effect of load and movement direction was observed for both machines and exercise variations, with higher strains at higher loads (e.g. in the leg press 3.2% and 5.5% at 40% and 80% 1RM, respectively) and during extension compared to flexion (e.g. 6.0 vs. 4.9% at 80% 1RM). Similar patterns were observed regarding agonistic and antagonistic EMG activity. However, individual prediction of strain using εmax was not possible with high accuracy (R2 ≤ 0.51).
CONCLUSION:
To achieve patellar tendon strains of 4.5% to 6.5% effective for adaptive responses, unilateral exercises on the knee extension machine and leg press require loads of approximately 70% and 80% 1RM, respectively. In this context, strain during knee extension tends to be in the upper part of the target range, while strain during knee flexion is in the lower part, as higher resultant moments are required to overcome the same load during knee extension. Furthermore, antagonist muscle activation increases the moments which the knee extensors need to generate for a given resultant joint moment. The higher antagonistic muscle activation, additionally contributes to the higher strains observed during extension and complicates the predictability of strain using εmax.

1. Arampatzis et al., J Biomech, 43:3073-79, 2010.
2. Mersmann et al., Sports Med Open, 9:83, 2023.