Eccentric exercise: many questions unanswered.

LOCOMOTION IS ARGUABLY one of the most ubiquitous and important manifestations of life, equally important to feeding as to escape predators. Evolution came up with three molecular motors to power motion: the actomyosin system Levitsky (11); the microtubule, dynein-based flagellar motors of eukaryotes (13); and the flagellar rotary motors of bacteria (15). All three systems serve propulsion, i.e., they impart kinetic energy to move something with regard to the environment. Thus all three motor systems were primarily “designed” to work concentrically. With the advent of more complex organisms containing joints, the evolutionary opportunity arose to make locomotion more efficient by storing actomyosin-generated kinetic energy in elastic materials such as abductin, resilin, and elastin (for scallops, see Ref. 3). In a similar vein, sharks use their entire cartilaginous vertebral column as a spring during locomotion (14). Although information on elastic storage of energy in locomotion of aquatic animals is scant, this has extensively been studied in organisms moving over ground (1). During walking, the inverted pendulum mechanism allows for exchanging kinetic with potential energy, whereas during running, potential and kinetic energy is stored as elastic strain energy in tendons and ligaments of the limb to be reused with the next step. In this context, muscles are activated while lengthening or keeping the same length. To our knowledge, the capability to perform negative work (eccentric work) is a unique evolved feature of actomyosin-based muscle systems. Constant length (isometric) and shortening (concentric) muscle contractions are adequately described on the molecular level by the sliding filament and cross-bridge theory (7, 8). This is not the case for eccentric contractions where the cross-bridge model fails to explain many of the observed phenomena (5). Eccentric contractions have a number of characteristics that set them apart from concentric contractions. Most importantly, with concentric contractions, force decreases with increasing shortening (6). In eccentric contractions, force increases first and then stays constant with increasing lengthening velocity. As a consequence, much larger forces can be generated during active lengthening than during shortening. This exposes muscle and tendon tissues to potentially damaging mechanical stress. Performing concentric and eccentric work on a motor driven ergometer, Bigland-Ritchie and Woods (2) further demonstrated that eccentric work required four times less metabolic energy and only half the electromyographically observed neuronal activation than concentric work. These exciting phenomena set eccentric contractions apart. However, despite their essential role in locomotion, eccentric contractions have remained massively under researched. This series of mini-reviews describes current thinking both with regard to the fundamental mechanical and neuronal properties of eccentric muscle contractions as well as the application of eccentric contractions in rehabilitation and in sports. In the first paper of this series of highlighted topics, Herzog (5) looks at the molecular mechanisms that have been invoked to explain force production in actively lengthening muscles. As mentioned above, the sliding filament and cross-bridge theories fail to explain many observations relevant to eccentric muscle contractions. Specifically, Herzog makes the case that “residual force enhancement,” i.e., the persistent increase of force after active muscle lengthening above values that would be expected based on sarcomere length remains unexplained with crossbridge theory. Herzog discusses competing theories that have been advanced to explain residual force enhancement such as modifications of the cross-bridge kinetics and the “sarcomere length nonuniformity” theories. He suggests that a mechanism based on the active involvement of the cytoskeletal protein titin could explain the disputed experimental findings. Herzog also admits that details of active force regulation during muscle lengthening by actin-titin binding remain unclear and need further research. Duchateau and Baudry (4) review the evidence pertaining to the neuronal control of eccentric contractions. There is considerable evidence indicating that it is difficult, at least in untrained individuals, to maximally activate muscles in eccentric contractions. This opens the possibility that spinal or cortical mechanisms could constrain motor-unit discharge rate. The evidence that the orderly recruitment pattern of motor units can be reversed in eccentric contractions remains controversial. By contrast, a decreased discharge rate of motor units during eccentric contractions is commonly observed and may constrain muscle activity during active lengthening. Several lines of evidence point to different cortical mechanisms in concentric and eccentric contractions; larger cortical excitability, greater involvement of brain areas, and different modulations of intracortical and interhemispheric coordination have been invoked. On the spinal level, motor-evoked potentials (MEP) are reduced both with transcranial magnetic stimulation (TMS) and electrical stimulation of the cortex (TES) in eccentric compared with concentric contractions. This indicates that a number of different preand postsynaptic mechanisms on the level of the motoneuron could be involved. The authors of the review (4) come to the conclusion that despite more information on the neuronal control of eccentric contractions being available today, the exact mechanisms involved on cortical and spinal levels remain unknown. The potential as well as the use of eccentric contractions in rehabilitation settings is reviewed by LaStayo et al. (10). These authors contend that the major defining properties of eccentric muscle contractions, the potential for high muscle force proAddress for reprint requests and other correspondence: H. Hoppeler, Univ. of Bern, Bern, Switzerland (e-mail: [email protected]). J Appl Physiol 116: 1405–1406, 2014; doi:10.1152/japplphysiol.00239.2014. Editorial