Adaptations of mouse skeletal muscle to low-intensity vibration training

Vibration training has emerged as a therapeutic strategy to improve the musculoskeletal system in health and rehabilitation. In health, it is utilized as an exercise modality to better athletic performance (reviewed in (35)). In rehabilitation, bone has been the primary focus because it has been shown to be osteogenic (32). Vibration training is also being explored as an alternative exercise modality for conditions of muscle weakness, such as in geriatrics and with bedrest. The results of these studies have been largely mixed. For example, a meta-analysis showed that whole body vibration training was affective in improving leg muscle strength in older adults (14), while another analysis concluded that there is only weak support for the efficacy of whole body vibration for muscle strength (17). Similarly, some but not all studies on young healthy adults show that vibration training improves muscle function (reviewed in (26). Thus, the broader application of vibration training to musculoskeletal health is limited because its effects on skeletal muscles are not unanimous. One aspect that likely contributes to the various results of vibration training on skeletal muscle function is the magnitude of acceleration, or intensity of the vibration. This parameter is typically expressed as a fraction of gravity (g) and it is a primary factor that dictates the mechanical signal delivered from the oscillating platform to the subject standing on the device. The majority of commercially-available, whole body vibration equipment delivers accelerations >1.0 g, referred to as “high intensity” vibration in this paper. Vibration platforms that deliver accelerations <1.0 g (“low intensity”) have also been investigated. In terms of effects on skeletal muscle, it has been shown that 2–12 months of low intensity vibration training results in increased muscle strength (19), balance (19), grip strength (25), and muscle mass (11, 24). Subjects in those studies were selected based on clinical conditions associated with poor bone health, and it is possible that muscle weakness was also a characteristic of those subjects. Thus, it is not clear if low intensity vibration training has the potential to enhance skeletal muscle of healthy individuals without muscle weakness. Low intensity vibration training is utilized to investigate osteogenic effects in animal models including sheep, rats, and mice (e.g., (28, 29, 36)), but myogenic effects have been much less studied. In two notable studies, BALB/c mice were subjected to low intensity vibration training for a duration of 6 wk. Xie and coworkers reported that cross-sectional areas of soleus muscle and type I and II fibers within that muscle were greater in vibrated than control mice (37). However, Murfee and coworkers reported low intensity vibration reduced the number of arterioles and venules in the distal region of soleus muscle, an undesirable microvascular adaptation (20). Additionally, others have reported that high intensity vibration causes injury to rodent muscle as indicated by fiber swelling and centrally located nuclei (21, 22). However, none of these studies evaluated the extent to which muscle function was beneficially or detrimentally altered with vibration training. Thus, a more comprehensive analysis of skeletal muscle following vibration training, particularly low intensity vibration training, is needed to determine function adaptations. As such, the primary objective of this study was to utilize a low intensity vibration platform, designed specifically for mice, to test the hypothesis that contractility is improved in muscles of the hindlimb in response to vibration training. A subset of mice housed in small cages to evoke physical inactivity was also studied in order to determine if vibration training was more effective under conditions promoting muscle weakness. Traditional exercise training can elicit changes in skeletal muscle strength, oxidative capacity, and fiber type distributions, which in turn can affect muscle’s resistance to fatigue. There are indications that vibration training may influence muscle fatigue (15, 27). Therefore, histological analyses reflecting oxidative capacity, capillarity, and fiber types, as well as functional analyses of muscle fatigability and recovery from fatigue, were assessed to determine if low intensity vibration training can provide a strong enough stimulus to evoke such cellular and parallel functional adaptations in muscles of mice.