Across the animal kingdom, muscles adapt to the exercise that they perform. They can do this by changing their intrinsic capacity to generate force as well as changing their actual size, both the cross-sectional area of the muscle and the length of the muscle (e.g. Goldspink, 1985; Lynn & Morgan, 1994). An increase in cross-sectional area adds more sarcomeres in parallel and an increase in length adds more sarcomeres in series. Much is known about the ability of eccentric muscle exercise to damage muscle, producing delayed muscle soreness, and then to initiate changes that ultimately serve to protect the muscle from further damage (e.g. Proske & Morgan, 2001). However, the precise ‘stimulus’ for adaptive lengthening of muscle after eccentric damage has not been clearly established in prior studies of human eccentric exercise.
DETAILS OF THE NEW RESEARCH?
Sharifnezhad, Marzilger and Arampatzis (from the Department of Training and Movement Sciences, Humboldt-Universitat in Berlin) addressed this question. They used 4 exercise protocols which lengthened the knee extensor muscles in different ways. They measured the longitudinal growth of one of the extensor muscles, the vastus lateralis muscle. Importantly, the mechanical stimulus varied by changes in the load, the muscle lengthening velocity, and the muscle length. The interventions lasted 10 weeks which is longer than in many comparable human studies. Group one (10 subjects) exercised the knee extensors of one leg at 65% (low load) of the maximum isometric voluntary contraction (MVC) and the second leg at 100% MVC (high load) with 90 deg/s angular velocity, from 25 to 100 deg knee angle. Group two (10 subjects) exercised one leg at 100% MVC, 90 deg/s, from 25 to 65 deg knee angle (short muscle length). The other leg exercised at 100% MVC, at a high angular velocity of 240 deg/s (high muscle lengthening velocity) from 25 to 100 deg knee angle. The third group served as a control and showed no changes with time. The main outcome measures were the length of muscle fascicles of vastus lateralis at rest (to assess longitudinal muscle growth) and the moment–angle relationship of the knee extensors (to assess voluntary force production at different knee angles). After 10 weeks of training, the two exercise groups (all four legs) increased maximal knee voluntary knee joint moments. But, only the high lengthening velocity intervention changed muscle fascicle length. Across a wide range of knee angles the resting muscle fascicles of vastus lateralis muscle lengthened significantly by ~15% compared to before training. One result that is difficult to explain is that the lengthened muscles did not produce maximal force at longer muscle lengths, a finding that has been seen in some short-term studies of eccentric exercise.
SIGNIFICANCE AND IMPLICATIONS:
The major implication of this study is that only specific types of eccentric loading provide a sufficient stimulus for muscle fascicles to grow longer. Furthermore, the lengthening velocity of the fascicles during the eccentric exercise is probably the key for initiation for the longitudinal muscle growth. The study did not directly assess the mechanisms for the changes but based on studies in animals, it is likely that there was an increase in the number of sarcomeres in series produced by the damaging effect of the lengthening contractions at high angular velocity, especially as the muscle force decreases.
Sharifnezhad A, Marzilger R, Arampatzis A (2014). Effects of load magnitude, muscle length and velocity during eccentric chronic loading on the longitudinal growth of the vastus lateralis muscle. J Exp Biol 217, 2726-33.
Goldspink G (1985). Malleability of the motor system: a comparative approach. J Exp Biol 115, 375-91
Lynn R, Morgan DL (1994). Decline running produces more sarcomeres in rat vastus intermedius muscle fibers than does incline running. J Appl Physiol 77, 1439-44.
Prasartwuth O, Allen TJ, Butler JE, Gandevia SC, Taylor JL (2006). Length-dependent changes in voluntary activation, maximum voluntary torque and twitch responses after eccentric damage in humans. J Physiol 571, 243-52.
Proske U, Morgan DL (2001). Muscle damage from eccentric exercise: mechanism, mechanical signs, adaptation and clinical applications. J Physiol 537, 333-45.