Abstract
BackgroundThe vast majority of dynamometer-based animal models for investigation of the response to chronic muscle contraction exposure has been limited to analysis of isometric, lengthening, or shortening contractions in isolation. An exception to this has been the utilization of a rat model to study stretch-shortening contractions (SSCs), a sequence of consecutive isometric, lengthening, and shortening contractions common during daily activity and resistance-type exercise. However, the availability of diverse genetic strains of rats is limited. Therefore, the purpose of the present study was to develop a dynamometer-based SSC training protocol to induce increased muscle mass and performance in plantarflexor muscles of mice.MethodsYoung (3 months old) C57BL/6 mice were subjected to 1 month of plantarflexion SSC training. Hindlimb muscles were analyzed for muscle mass, quantitative morphology, myogenesis/myopathy relevant gene expression, and fiber type distribution.ResultsThe main aim of the research was achieved when training induced a 2-fold increase in plantarflexion peak torque output and a 19% increase in muscle mass for the agonist plantaris (PLT) muscle. In establishing this model, several outcomes emerged which raised the value of the model past that of being a mere recapitulation of the rat model. An increase in the number of muscle fibers per transverse muscle section accounted for the PLT muscle mass gain while the antagonist tibialis anterior (TA) muscle atrophied by 30% with preferential atrophy of type IIb and IIx fibers. These alterations were accompanied by distinct gene expression profiles.ConclusionsThe findings confirm the development of a stretch-shortening contraction training model for the PLT muscle of mice and demonstrate that increased cross-sectional fiber number can occur following high-intensity SSC training. Furthermore, the TA muscle atrophy provides direct evidence for the concept of muscle imbalance in phasic non-weight bearing muscles, a concept largely characterized based on clinical observation of patients. The susceptibility to this imbalance is demonstrated to be selective for the type IIb and IIx muscle fiber types. Overall, the study highlights the importance of considering muscle fiber number modulation and the effect of training on surrounding muscles in exercise comprised of SSCs.
Highlights
The vast majority of dynamometer-based animal models for investigation of the response to chronic muscle contraction exposure has been limited to analysis of isometric, lengthening, or shortening contractions in isolation
Effective Stretch-shortening contraction (SSC) training for enhancement in plantarflexion performance accompanied by differential responses in muscle mass PLT and tibialis anterior (TA) muscles Plantarflexion SSC training increased static performance two-fold in terms of maximum isometric torque and dynamic performance in terms of peak torque during the SSC test (Fig. 1a and b). This increase in plantarflexion performance was accompanied by a 19% increase in PLT muscle mass and a 30% decrease in TA muscle mass with no change in mouse body weight, 25.6 ± 0.6 g in the initial non-trained state and 25.3 ± 0.6 g after training (Fig. 2a)
To assess recovery from fatigue, c an isometric contraction and d a single SSC test were assessed 5 minutes and 7 minutes after a training session of 80 SSCs, respectively. These values were expressed relative to their pre-session values. e For the isometric contraction following a session of 80 SSCs, maximum torque production typically could not be completely maintained for the duration of the contraction
Summary
The vast majority of dynamometer-based animal models for investigation of the response to chronic muscle contraction exposure has been limited to analysis of isometric, lengthening, or shortening contractions in isolation. An exception to this has been the utilization of a rat model to study stretch-shortening contractions (SSCs), a sequence of consecutive isometric, lengthening, and shortening contractions common during daily activity and resistance-type exercise. To investigate the response to contractions with high control and precision, dynamometer based animal studies have been utilized [13] The majority of these studies have investigated isometric, lengthening, or shortening contractions in isolation in order to characterize the impact of each contraction mode alone [13, 14]. This animal model has been limited in regard to diversity of genetically defined strains available
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