SLR - June 2015 - Chelsea Viola
Reference: Chaudhry S, Morrissey D, Woledge RC, Bader DL, Screen HR. Eccentric and concentric loading of the triceps surae: an in vivo study of dynamic muscle and tendon biomechanical parameters. J Appl Biomech. 2015 Apr;31(2):69-78.Scientific Literature Review
Reviewed By: Chelsea Viola, DPM
Residency Program: Morristown Medical Center, Morristown, NJ
Podiatric Relevance: Achilles tendinopathy can be difficult to manage with many options available for treatment including immobilization, eccentric exercise, concentric exercise, and surgical intervention. Even with surgical intervention, rehabilitation will likely include a variety of exercises, including isometric. It has been shown through various studies that eccentric exercise is superior to concentric exercise with success rates of 50-60 two to four in sedentary patients and 60-80 two to four in athletes; however, the mechanism of these observed effects remains unknown. The aim of the current study was to investigate the biomechanical characteristics of eccentric and concentric exercises in order to identify the differences in tendon load response.
Methods: Eleven healthy volunteers (six male, five female; mean [SD] age 26.5 year [1.9]; body mass 65.92 kg [10.5]; height 1.73 m [0.08]) were recruited with written consent. All subjects stated they were “moderately active” which was defined as 2-3 hours of physical activity per week. None of the volunteers actively participated in any organized competitive sport. None of the volunteers had a history of tendon injury or any systemic disease. In order to perform the eccentric and concentric exercises, the subjects stood on a force plate and performed heel rise exercises. EMG electrodes were placed on the belly of the lateral gastrocnemius, medial gastrocnemius, soleus, and tibialis anterior in order to measure muscle activation throughout the course of the exercise. Tendon force was calculated from torque around the ankle. Continual tendon elongation was measured via ultrasonography. Tendon length was defined as the distance between the Achilles tendon insertion and the distal muscle-tendon junction (MTJ) of the medial gastrocnemius. For each exercise task, only cycles within 10% of the prescribed speed were retained for analysis.
Results: There was less than a 1 percent difference in the tendon forces calculated from torque around the ankle with and without the inclusion of inertial effects, hence inertia around the foot was ignored. No significant difference in force-displacement behavior during the two exercises, eccentric and concentric, for most subjects, was seen in this study. Range of motion during eccentric and concentric exercise remained similar; therefore, no significant variation in tendon stiffness was determined. Tendon apparent stiffness varies across the subject population but not between exercise types: concentric 62.79 N·mm-1 (9.37) and eccentric 58.89 N·mm-1 (9.21). For some individuals, there is a clear difference in tendon behavior between eccentric and concentric exercise, which necessitated the need for normalizing force and displacement. Once this had been achieved, there were clear temporal differences between eccentric and concentric exercise for both tendon force and length. However, after carrying out paired comparisons of eccentric across all subjects, no significant differences were observed in maximum extension or stiffness. EMG data revealed that muscle activation during the cycle and maximal muscle activation values were significantly higher in concentric anterior compartment (0.030 mV [SE 0.004]) and posterior compartment (0.140 mV [SE 0.014]) than eccentric (0.025mV [SE 0.003] anterior and (posterior??) 0.118 mV [SE 0.011]). A greater amplitude of higher frequency force perturbations were present during eccentric at 10 Hz and showed negative correlation with tendon stiffness at 10 Hz.
Conclusion: In concentric movement, the calf muscles are activated from full dorsiflexion to plantarflexion as they accelerate the subject upwards. This results in tendon force being at its peak at the start of concentric movement. Once the subject approaches full plantarflexion, the force reduces. In contrast, eccentric movement requires the subject to lower to dorsiflexion “under control.” This activates the stretch of the Achilles tendon and the calf muscle is lengthened. At the end of the movement, in full dorsiflexion, the maximal force occurs in order to decelerate the subject against gravity. It is important to note that during this entire exercise, the Achilles tendon is maximally loaded. Muscle contraction then likely acts to strain the tendon. This study did observe different temporal strains for the two exercises; however, when the data were normalized, this difference disappeared. Mechanical loading and strain is an important stimulus for tendon repair; however, differences in strain alone are unlikely to be the sole trigger for repair during eccentric exercise. In reference to the perturbations noted at 10 Hz, it is important to note that not every subject showed the 10 Hz perturbations so it cannot be determined from this study that perturbations are beneficial for tendon repair. The perturbations could simply indicate muscle weakness or poorly synchronized muscle activity. This study has greatly highlighted the need to carry out carefully controlled training studies. Subject choice could not be more important. Utilizing the athletic population may prove beneficial as muscle strength prior to the study can be tested to determine likeness. Those with similar muscle strength should be chosen; therefore, being able to determine not only if the perturbations exist, but also if they are a significant recruiter to tendon repair. It has been shown that controlled eccentric and concentric exercise is necessary in order to fully investigate the underlying mechanisms that make eccentric exercise the best type for tendon repair.