Adaptational responses of the human Achilles tendon by modulation of the applied cyclic strain magnitude. Arampatzis A1, Karamanidis K, Albracht K. J Exp Biol. 2007 Aug;210(Pt 15):2743-53. [FREE FULL TEXT]
Tendons are able to remodel their mechanical and morphological properties in response to mechanical loading. However, there is little information about the effects of controlled modulation in cyclic strain magnitude applied to the tendon on the adaptation of tendon’s properties in vivo. The present study investigated whether the magnitude of the mechanical load induced as cyclic strain applied to the Achilles tendon may have a threshold in order to trigger adaptation effects on tendon mechanical and morphological properties. Twenty-one adults (experimental group, N=11; control group, N=10) participated in the study. The participants of the experimental group exercised one leg at low-magnitude tendon strain (2.85+/-0.99%) and the other leg at high-magnitude tendon strain (4.55+/-1.38%) of similar frequency and volume. After 14 weeks of exercise intervention we found a decrease in strain at a given tendon force, an increase in tendon-aponeurosis stiffness and tendon elastic modulus and a region-specific hypertrophy of the Achilles tendon only in the leg exercised at high strain magnitude. These findings provide evidence of the existence of a threshold or set-point at the applied strain magnitude at which the transduction of the mechanical stimulus may influence the tensional homeostasis of the tendons. The results further show that the mechanical load exerted on the Achilles tendon during the low-strain-magnitude exercise is not a sufficient stimulus for triggering further adaptation effects on the Achilles tendon than the stimulus provided by the mechanical load applied during daily activities.
I like this article. The researchers had subjects train isometrically, four times per week, doing 5 sets of 3 second holds at either 55% of max voluntary contraction (MVC) strength on one leg, while the other leg (on the same subject) did the same with 90% MVC. They set it up so that volume was the same, however they calculated volume as an “integral of the plantar flexion movement over time” which had the high intensity (90% side) do only 4 reps per side, while the low intensity (55%) side did 7 reps. I would have thought to keep volume the same by having them keep the reps the same, but they did so by keeping the area under the curve (load/time) the same, which sounds fair enough, and even with the repetitions and time under tension reduced in the high intensity group, only the high intensity group showed positive adaptations of tendon stiffness. Both sides got stronger, but the high intensity group more so. A cross-section area of the tendons, only in the high intensity side, with this increase being significant only in the central part of the tendon (near 20%), less than significantly in the proximal tendon (~10%), and not much at all in the distal tendon (~5%). The increase in tendon stiffness in the high intensity group (26.5%), however, was greater than the overall increase in cross section area. As such, the researchers thought that with the higher intensity contractions, there was fair amount or remodeling and reorganization of the tendon-collagen matrix leading to improved tensile properties without a lot of tendon hypertrophy, which I think is particularly interesting in people with tendinopathy who often have thickened tendons, which are often shown to reduce (become more normal) with higher intensity resistance exercise. So likely the tendons thicken up a bit with adequate training, but get thicker still when tendinopathy results in irregular collagen formation, at which point normalization of the tendon structure results in a lesser tendon diameter.
Also in the tendinopathy research, there have been a fair number of papers coming out which are starting to hone in on the ideal combination of exercise intensity (weight or percent tension) frequency (days per week) and volume being (sets, reps, or time under tension), and exercise type (eccentric, concentric, or isometric). I think it’s much analogous to weight training to increase muscle strength where there are a great deal of variables in sets, reps, weights which all work, some better than others, but no know ideal for everyone in every situation. Also much like strength training for muscle gains, if the resistance level isn’t sufficiently higher than what the tendon receives during daily activities, it isn’t going to get any stronger. For tendinitis/tendinopathy, my go-to program is still 3-5 sets (usually just 3) of 15 reps of increasing intensity (working to as heavy as possible), fairly regular repetition speed, and with normal concentric/eccentric type contractions much as illustrated in my shooter’s elbow blog. Rather than 4 days per week in this study I prefer daily exercise, which is noticeably more effective in my office but also backed up by research. Still this paper shows 4 days per week is still pretty good.
Still this paper was cool in that it demonstrated 90% MVC for a total of 60 seconds contraction time (5 sets, 3 seconds, 4 reps) was able to positively increase muscle and tendon properties better than the 55% MVC for 105 seconds (5 sets, 3 seconds, 7 reps). It also explains why one of my more recent tennis elbow patients failed to improve after several months of treatment at some other hand physical therapy clinic, where he said the heaviest weight he handled on wrist curls and reverse wrist curls was 2 lb (which was probably only 15% MVC, so they weren’t even in the ballpark). He said they had him do only 1 lb for several weeks prior. Such low intensities are sure not to hurt anything, but sure not to help either as this study attests. So if you are a certified hand therapist, I suggest you put down the splint material for a day and read up on strength and conditioning, your patients will thank you. Anyway, day 1, I think I started him with 5, 8, and 10 lb (he felt immediately better) and worked up from there to 20 something and within 3 weeks he was 90% better. So yeah, my experience agrees with the study findings, light weights for tendons generally equals no improvement. On the contrary heavy weights (within reason of course) helps a lot. It’s science.
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Chad Reilly is a Physical Therapist, obtaining his Master’s in Physical Therapy from Northern Arizona University. He graduated Summa Cum Laude with a B.S. Exercise Science also from NAU. He is a Certified Strength and Conditioning Specialist, and holds a USA Weightlifting Club Coach Certification as well as a NASM Personal Training Certificate. Chad completed Yoga Teacher Training at Sampoorna Yoga in Goa, India.