Agonist muscle adaptation accompanied by antagonist muscle atrophy in the hindlimb of mice following stretch-shortening contraction training

Abstract

Background: 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. 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.

Methods: Young (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.

Results: The 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.

Conclusions: The 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.

Keywords: Biomechanics; C57BL6 mice; Dorsiflexor muscles; Muscle fiber type; Muscle physiology; Myosin heavy chain; Plantarflexor muscles; Stretch-shortening contractions.

Fig. 1

Plantarflexion SSC training enhanced plantarflexor muscle performance. Values of a static performance in terms of maximum isometric torque and b dynamic performance in terms of peak torque during a single SSC test were assessed. Sample sizes were N = 9 per group. Values are means ± SE. *Different from non-trained value, P < 0.05

Fig. 2

The agonist PLT muscle increased muscle mass with plantarflexion SSC training whereas the antagonist TA muscle atrophied. Values for a muscle mass (normalized by tibia length) b number of muscle fibers per transverse muscle section, and c mean muscle fiber area were determined. For PLT muscles, an increase in the number of muscle fibers in cross-section accompanied the increase in mass. For TA muscles, atrophy was observed at the individual muscle fiber level. N = 9 per group with exception of total muscle fiber counts for PLT muscles. A subpopulation of all the PLT muscle sections (N = 3 to 8 per group) fulfilled the requirement for direct fiber counts – the requirement that all regions of the muscle section be free of technical defects typically incurred from prepping and cryosectioning such a relatively small muscle. Other measures (e.g. muscle fiber area) were obtained from all of the PLT muscle sections (N = 9 per group) because these relied on the sampling approach of stereological quantitative morphology. Values are means ± SE. *Different from non-trained value, P < 0.05

Fig. 3

Altered muscle fiber morphology was most noticeable in TA muscle sections following plantarflexion SSC training especially in terms of an increased proportion of centrally nucleated muscle fibers. Transverse sections of PLT and TA muscles were stained with hematoxylin and eosin. Scale bar = 100 μm

Fig. 4

Plantarflexion SSC training improved plantarflexion recovery from fatigue. Values for peak torque were evaluated for the first set of 10 contractions. a With training, absolute values improved for all contractions of the first set. b When peak torques were normalized to that of the first contraction, training had no effect early in the set followed by a marginal effect late in the set. 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. However, a training-induced improvement in the degree to which torque could be maintained was noticed by reference to raw torque traces. f The extent to which torque was maintained during a post-session isometric contraction was quantified by expressing the torque at the termination of muscle activation as a percentage of the maximum torque value at the onset of contraction. Sample sizes were N = 9 per group. Values are means ± SE. *Different from non-trained value, P < 0.05

Fig. 5

Differential fiber type distribution and size outcomes following plantarflexion SSC training for PLT and TA muscles. Laminin (green) and multiple MHC isoforms – I (blue), IIa (red), IIb (green) and IIx (negative for staining) – were identified by immunofluorescence. For PLT muscles, a shift from IIb to IIx fibers was apparent with no overt type II fiber size changes. For TA muscles, a shift from type IIb to IIx and IIa fibers accompanied by decreases in fiber sizes especially for type IIb and IIx fibers were noticeable. Scale bar = 50 μm

Fig. 6

Quantification of fiber type distribution and size for PLT and TA muscles. a Frequency distributions for fiber type are presented as percentage of total number of fibers counted for each group (N = 9 muscles per group). Chi-square analysis was performed to determine alterations in distribution with training. b Mean muscle fiber size for each fiber type. Values are means ± SE. *Different from non-trained value, P < 0.05