Deficits In Performance Fatigability And Contractile Function Of The Plantar Flexor Muscles In Achilles Tendinopathy

Abstract

Achilles tendinopathy (AT) is an overuse condition resulting in pain and stiffness of the Achilles tendon. While experts agree that strength and endurance deficits persist in AT, this claim lacks empirical evidence. PURPOSE: To determine whether individuals with AT present with deficits in strength and fatigability compared to healthy controls (CON) during a single-leg heel raise (SLHR) performed to task failure. METHODS: 6 people with AT (3 male, 26.8±8.9 yrs) and 6 controls (CON, 3 male, 21.9±1.8 yrs) performed maximal voluntary isometric contraction (MVIC) of the plantar flexor muscles before and immediately after SLHR repetitions performed to task failure (test of fatigability). Electrical stimulation of the tibial nerve was used to evoke twitch contractions of the plantar flexor muscles before and after the fatigability test at rest and during MVICs to determine contractile properties and voluntary activation, respectively. RESULTS: At baseline, the AT and CON groups exhibited similar plantar flexor strength (MVIC), voluntary activation and resting twitch amplitude. However, the AT group performed fewer SLHR repetitions than CON (33 vs 59, p=0.009). At task failure of the SLHR task, the reduction in plantar flexor MVIC (17.3% AT vs 23.7% CON, p=0.32) and voluntary activation (5.8% AT vs 7.5% CON, p=0.78) was similar for the two groups. However, persons with AT demonstrated larger reductions in resting twitch amplitude (34.4 Nm to 31.5 Nm, 8.4% reduction, p=0.047), while CON demonstrated no change (p=0.23). CONCLUSION: The plantar flexor muscles of persons with AT were more fatigable for a SLHR task compared with strength-matched controls. Deficits in contractile function rather than the ability to centrally drive the muscle appear to be responsible for deficits in endurance in people with AT. *This work was supported by a Promotion of Doctoral Studies Level I Scholarship from the Foundation for Physical Therapy and by Marquette University’s Exercise and Rehabilitation Sciences Graduate Program.