Symptomatic and functional responses to concentric-eccentric isokinetic versus eccentric-only isotonic exercise

Journal of Athletic Training, Sept-Oct, 2009 by Jeffrey J. Parr, Joshua F. Yarrow, Carolyn M. Garbo, Paul A. Borsa

Determining the symptomatic and functional responses to various resistance-training protocols may enable clinicians to prescribe safer and more effective exercise protocols for both healthy and injured individuals. A primary goal of postinjury rehabilitation is to recover, and possibly to increase, muscle strength after injury (l-3) When prescribing resistance training protocols for healthy or injured populations, resistance exercises that combine concentric and eccentric muscle actions are typically implemented. Both types of muscle actions are generally performed with similar absolute intensities; however, a 40% to 50% greater load can be performed during maximal eccentric muscle actions than during maximal concentric actions. (4,5) This indicates that the eccentric phase of exercise is underloaded throughout typical resistance exercise. Additionally, researchers (6) have shown that when the same amount of torque is produced by a muscle during concentric and eccentric contractions, fewer motor units are recruited during the eccentric contraction. Several authors (1,2,7) have indicated that, when compared with concentric actions of equal absolute exercise intensities, eccentric-only (ECC) actions promote greater neural activation, (1,3,7,8) skeletal muscle hypertrophy, (1,3,7,8) and muscle-tendon and ligament stiffness, indicating that eccentric exercise may be a superior resistance-training method and may also be beneficial during postinjury rehabilitation. Further, Kaminski et al (2) suggested that enhanced eccentric exercise may reduce the risk of musculotendinous injury or reinjury during high-intensity activities by improving the muscle-tendon's ability to withstand force and strain without failing. However, eccentric exercise may also elevate the magnitude of exercise-induced myofiber damage, (2,6,9,10) which may delay or limit full structural and functional recovery during physical rehabilitation and training. Thus, systematic evaluations of the safety and efficacy of exercise protocols that involve overloaded eccentric muscle actions appear necessary before these protocols can be implemented in recreation or rehabilitation settings.

Recently, investigators (1-17) have shown that rehabilitation protocols involving eccentric-resistance exercises, performed with either free weights or an isokinetic dynamometer, are effective in increasing functional capacity and decreasing muscular pain. However, care must be taken when implementing eccentric exercise, as individuals may develop exercise-induced muscle damage and its associated symptoms, including muscle soreness, loss in range of motion (ROM), and reduced muscle strength. (2,6,18) The exact mechanisms underlying delayed-onset muscle soreness (DOMS) remain unknown, but evidence now indicates that DOMS is related to the secondary cascade of tissue damage. Secondary tissue damage occurs after an injury in which damaged cells release chemical mediators, such as cytokines and proteolytic enzymes associated with the acute inflammatory response. (19,20) Studies designed to compare the magnitude of exercise-induced muscle damage and associated symptoms after different modes of eccentric exercise may lead to improvements in the design and implementation of rehabilitative exercise protocols.

Two common exercise protocols that involve overloaded eccentric muscle actions are ECC isotonic exercise, which uses constant supramaximal external resistance, and combined concentric-eccentric (CON-ECC) isokinetic exercise, which uses constant angular velocities. Researchers (11-16) have examined eccentric exercise and overall outcome goals, such as functional capacity and ability to return to activity, but the short-term muscle strength, muscle soreness, and ROM responses to different modes of eccentric exercise have not been investigated in either healthy or injured populations. Therefore, the purpose of our study was to compare functional and symptomatic responses (ie, muscle strength, ROM, and muscle point tenderness) after an ECC isotonic protocol and after a CON-ECC isokinetic exercise protocol in healthy individuals matched for total training volume.

METHODS

Participants

Twenty-four healthy individuals (12 men, 12 women; age = 21.17 [ or -] 2.78 years, height = 171.40 [ or -] 10.09 cm, mass = 72.85 [ or -] 16.32 kg) volunteered for this study. To meet the inclusion criteria, participants had to be aged 18 to 35 years and untrained (ie, no resistance exercise during at least 6 weeks before the study). Untrained participants were studied to avoid the potentially confounding repeated-bout effect. (21,22) Participants were excluded if they (1) had consumed any nutritional supplement intended to enhance exercise performance during the 6 weeks before the study or (2) had any orthopaedic injury that limited exercise performance. All study participants provided written informed consent, and the study was approved by the University of Florida Institutional Review Board.

Study Design