The purpose of this study was to determine whether oscillation parameters, that are known to have bone-anabolic potential (based on animal studies), increase electromyographic activity in individuals with and without SCI. We hypothesized that reflexive EMG activity would be elicited with increasing g force in both cohorts. To our knowledge, this is the first study to quantify reflexive muscle activity during low magnitude, fixed frequency mechanical oscillation of an isolated constrained human limb segment.
Statistically significant increases in background Sol and TA EMG emerged in Non-SCI subjects when the limb underwent oscillation at high amplitudes. However, a more important finding is that these increased background EMG levels were only a small fraction of M-max (< 2.5% in all cases). Thus while increases in SOL and TA EMG were detectable, it is unlikely that this level of muscle contraction could contribute to sizable compressive loads to the tibia.
In human whole body vibration (WBV) studies, EMG is found to be significantly increased when subjects stand on an oscillating surface (Cardinale and Lim, 2003
; Roelants et al., 2006
). Subjects in previous WBV/EMG reports actively stood and/or performed a variety of squat maneuvers, and therefore maintained higher levels of volitional muscle activation than subjects in the present report. In these studies, the application of an oscillatory load triggered peak EMG increases of up to 50% (Abercromby et al., 2007
; Cardinale and Lim, 2003
; Roelants et al., 2006
). We are aware of no previous WBV studies that compare peak EMG during passive standing (no volitional activation) with and without oscillation, a more valid comparison between WBV and the present study. However, as the magnitude of EMG change was considerable in WBV + squat protocols, it seems likely that background EMG and direct oscillation of the head (vestibular system) plays a role in determining whether oscillation will trigger additional reflex activity. This view is congruent with , in which peak EMG increases were more prevalent during a 5% MVC background contraction. We surmise that excitatory central drive, with or without vestibular input to the alpha motor neuron pool, underlies this facilitation.
4.1. Mechanisms of Reflex Responses
In WBV studies, vestibular activation may play a role in reflex responses to oscillation. Amplification of the platform acceleration can be considerable at the ankle, knee, hip, and spine, and standing subjects exposed to platform displacements >0.5mm reported discomfort (Kiiski et al., 2008
). Bite bar studies indicate that during WBV, transmission of accelerations to the head can be considerable (Abercromby et al., 2007
). These factors support that WBV perturbs the head, triggering motor cortical drive to the lower extremity muscles. In the present study, subjects were fully supported in a sitting position with the lower leg constrained on the platform. As transmissibility of accelerations to the head is damped by lower extremity joint flexion (Abercromby et al., 2007
), we anticipate that vestibular inputs were minimal during this study. This lack of excitatory descending drive to the lower extremity motor neuronal pool likely contributed to the minimal increase in background EMG observed during limb oscillation, even at high platform amplitudes.
Methodological factors may have also led to the small responses of background EMG to limb oscillation. Previous WBV studies of surface EMG with power spectral analysis demonstrated sharp peaks that corresponded exactly with the oscillation frequency and its harmonics (Fratini et al., 2008
). These authors suggested that motion artifact is contributory to EMG in WBV studies, particularly when platform displacement is high (4 mm) and frequencies range from 30 to 45 Hz (Fratini et al., 2008
; Hazell et al., 2007
). In the current study, a band-stop filter was applied to eliminate any possible motion artifact associated with the vibration stimulus and its harmonics. This procedure thus decreased the risk of over-estimation of reflex muscle activation (Fratini et al., 2008
While filtering negated the effect of motion artifact in the EMG signal, the level of background EMG appeared to be influenced by the phase of the oscillation (). Particularly during oscillation with a 5% MVC, background EMG increased after the halfway point of the oscillation event, at approximately 16 ms from the onset of the oscillation. The latter half of the oscillation corresponds to the descending movement of the vibration platform. Indeed, this suggests that the accumulation of phase 1 inputs of the oscillation triggers the EMG response observed in the 2nd phase. This most likely does not occur on the first response as only □16 ms separated the phase 1 from phase 2 response. Therefore, the EMG response in phase 2 was triggered by either/both phase 1 or phases 2 events from the previous oscillation cycle. This would give the EMG response either a latency of ~35 ms, timing consistent with short latency responses. The quiescence of EMG during the initial phase is a consistent response, and may represent a strong consistent inhibition from previous cycles of oscillation, but this is speculative and cannot be specifically addressed in this study.
4.2. SCI vs Non-SCI EMG Nesponses
In contrast to non-SCI subjects, subjects with SCI demonstrated no increase in EMG activity at any magnitude of oscillation (). Several possible explanations for this difference exist. Subjects with complete SCI lack descending supra spinal input to the lower extremities, unlike the non-SCI group. However, as previously described, we believe that head perturbation was minimal in this experimental design. None of the subjects with SCI were on anti-spasmodic medication, so this study was not constrained by phar macological management. This feature of diminished EMG during constrained limb segment oscillation differs from previous WBV approaches and should offer unique insights into the effects of oscillation on spinal reflex pathways and bone physiology.
Secondly, SCI is characterized by hyper-reflexia. If rapid muscle length changes were to occur (as has been observed during oscillation on see-saw type systems (Cochrane et al., 2009
)), stretch reflex-mediated contractions in SCI subjects would be expected. The absence of reflex-mediated EMG activity in subjects with SCI supports that the mode of oscillation delivered in the present study did not yield noteworthy changes in soleus or TA muscle length. By extension, changes in muscle length were not likely to be the source of peak EMG increases observed in non-SCI subjects.
The tonic vibration reflex (TVR) is mediated by both monosynaptic and poly-synaptic pathways that result in increased motor unit activation (Martin and Park, 1997
). After incomplete SCI, direct vibration of tendon or of a muscle belly facilitates muscle contraction (Cotey et al., 2009
) and WBV training has recently been shown to augment gait (Ness and Field-Fote, 2009
). In complete SCI however, prolonged application of vibration to tendon appears to inhibit electromyographic activity of plantar flexor muscles (Butler et al., 2006
). This finding is congruent with the blunted EMG response of SCI subjects to whole limb oscillation in the present report.
4.3. Considerations for Future SCI Studies
Compared to Non-SCI subjects, low magnitude fixed frequency mechanical oscillation of the lower leg induced a similar minimal reflexive muscle activity in the two subjects with SCI. Future work should explore the mechanisms of these differences, such as alterations in post-activation depression of spinal pathways. The results of the present study suggest that constrained limb oscillation after SCI, as an experimental anti-osteoporosis intervention, would not induce significant compressive loads due to reflex-mediated muscle contractions, although our conclusions must be tempered as we studied only two subjects with SCI. Recent animal work suggests that limb oscillation in the absence of compressive loads may offer an adequate stimulus to trigger bone anabolism (Garman et al., 2007a
). Therefore, mechanical oscillation could be a powerful osteogenic stimulus for individuals without muscle voluntary contraction, such as those with SCI. Moreover, it may offer a new therapeutic option for people with lower motor neuron injury, who cannot trigger muscle contractions either reflexively or via electrical stimulation. Future studies should proceed to determine if mechanical oscillation can influence bone integrity in individuals with SCI. This study supports that an effect on bone density in future studies will likely be attributable to the mechanical transduction on osteoblasts rather than forces generated through high level muscle contractions.