Gravitational force modulates muscle activity during mechanical oscillation of the tibia in humans

Abstract

Mechanical oscillation (vibration) is an osteogenic stimulus for bone in animal models and may hold promise as an anti-osteoporosis measure in humans with spinal cord injury (SCI). However, the level of reflex induced muscle contractions associated with various loads (g force) during limb segment oscillation is uncertain. The purpose of this study was to determine whether certain gravitational loads (g forces) at a fixed oscillation frequency (30 Hz) increases muscle reflex activity in individuals with and without SCI. Nine healthy subjects and two individuals with SCI sat with their hip and knee joints at 90° and the foot secured on an oscillation platform. Vertical mechanical oscillations were introduced at 0.3, 0.6, 1.2, 3 and 5g force for 20 seconds at 30 Hz. Non-SCI subjects received the oscillation with and without a 5% MVC background contraction. Peak soleus and tibialis anterior (TA) EMG were normalized to M-max. Soleus and TA EMG were < 2.5% of M-max in both SCI and non-SCI subjects. The greatest EMG occurred at the highest acceleration (5g). Low magnitude mechanical oscillation, shown to enhance bone anabolism in animal models, did not elicit high levels of reflex muscle activity in individuals with and without SCI. These findings support the g force modulated background muscle activity during fixed frequency vibration. The magnitude of muscle activity was low and likely does not influence the load during fixed frequency oscillation of the tibia.

Bone architecture adaptations after spinal cord injury: impact of long-term vibration of a constrained lower limb

“…We previously confirmed that the vibration intervention mimicked the mechanical input conditions of previous animal studies [19] and that it did not cause reflexive muscle contractions [30, 31]. We used a novel and sophisticated imaging approach to obtain outstanding imaging resolution for in vivo bone adaptations.…”

“…These parameters were selected to reflect the vibration conditions of recent studies that reported anabolic effects of vibration in the murine tibia [11, 13, 29]. We previously verified that vibration using these parameters does not increase background EMG activity [30] and downregulates monosynaptic reflex activity [31]. Reflex-mediated muscle contractions did not occur, eliminating muscle force as a potential confounding source of mechanical load to the bone.…”

“…To securely couple the limb segment to the vibrating surface, a compressive load of 10–15%of body weight was applied to the top of the knee via a full-contact pad. Technical details of the vibration system have been described elsewhere [19, 30, 31]…”

Potential regenerative rehabilitation technology: implications of mechanical stimuli to tissue health

“…During single limb segment vibration, the activity of the soleus muscle was suppressed [76]. Vibration caused an 83% reduction in the Hoffmann reflex (H-reflex), but limb load facilitated segmental excitability (decreased H-reflex post activation depression) [35,77].…”

Vibration training after chronic spinal cord injury: Evidence for persistent segmental plasticity

“…We have investigated the effect of various levels of gravitational acceleration [4] and of overlaying various limb compressive loads during vibration [16]. It is more difficult to draw mechanistic conclusions from whole-body vibration (WBV) studies, in which vibration and limb compressive load are modified by influences from vestibular activation, visuomotor systems, and additional sources of afferent input (e.g.…”