If you have access to an isokinetic dynamometer test eccentrically at 90 d/s with knee at 80 degrees of flexion per O’Neil. If you have a HHD you can use a strap to secure it over the knee while seated as in the video below.

O’Neill, S., Barry, S., Watson, P., Plantarflexor strength and endurance deficits associated with mid-portion Achilles tendinopathy: The role of soleus, Physical Therapy in Sports (2019), doi: https://doi.org/10.1016/j.ptsp.2019.03.002.