The Orthopaedic Research Institute-Ankle Strength Testing System: inter-rater and intra-rater reliability testing

Achilles tendinosis is a degenerative overuse tendinopathy involving the primary ankle plantarflexors, namely the soleus and gastrocnemius muscles forming the tendo Achilles. The Orthopaedic Research Institute-Ankle Strength Testing System (ORI-ASTS) was designed to record objective measurements of force generated with a resisted ankle plantarflexion test. Testing normal subjects (n=6) was used to establish the reliability of the ORI-ASTS for measuring ankle plantarflexion force. Testing patients with Achilles tendonitis (n=5) over time and comparing Achilles tendon analogue pain scores to ankle plantarflexion force measurements was used to assess the validity of the ORI-ASTS for monitoring progression of Achilles tendinopathy. Inter-rater reliability of the ORI-ASTS with normal subjects was excellent, with single measure intraclass correlation coefficients (ICC) for right mean peak force of 0.92, left mean peak force of 0.96, right mean total force of 0.89, left mean total force of 0.91. The ORI-ASTS also had excellent intra-rater reliability for normal subjects with the following single measure ICC scores: right mean force 0.96, left mean force 0.92, right mean total force 0.97, left mean total force 0.92. The relative technical errors of measurement were calculated from these results, and ranged from 1.0% to 2.7%. Testing patients with Achilles tendinosis (n=5), and comparing analogue activity pain scores to ORI-ASTS testing demonstrated a strong negative relationship between the two parameters (Spearman Rho -0.87 to -1.0, Kendall tau b -0.82 to -1.0). The relationship was statistically significant at the p=0.01 level for two of the five patients. The ORI-ASTS shows excellent reliability for testing ankle plantarflexor force, and appears valid for objectively assessing and monitoring patients with Achilles tendinosis.