Effects of different vibration frequencies, amplitudes and contraction levels on lower limb muscles during graded isometric contractions superimposed on whole body vibration stimulation

Background: Indirect vibration stimulation, i.e., whole body vibration or upper limb vibration, has been investigated increasingly as an exercise intervention for rehabilitation applications. However, there is a lack of evidence regarding the effects of graded isometric contractions superimposed on whole body vibration stimulation. Hence, the objective of this study was to quantify and analyse the effects of variations in the vibration parameters and contraction levels on the neuromuscular responses to isometric exercise superimposed on whole body vibration stimulation. Methods: In this study, we assessed the 'neuromuscular effects' of graded isometric contractions, of 20%, 40%, 60%, 80% and 100% of maximum voluntary contraction, superimposed on whole body vibration stimulation (V) and control (C), i.e., no-vibration in 12 healthy volunteers. Vibration stimuli tested were 30 Hz and 50 Hz frequencies and 0.5 mm and 1.5 mm amplitude. Surface electromyographic activity of the vastus lateralis, vastus medialis and biceps femoris were measured during V and C conditions with electromyographic root mean square and electromyographic mean frequency values used to quantify muscle activity and their fatigue levels, respectively. Results: Both the prime mover (vastus lateralis) and the antagonist (biceps femoris) displayed significantly higher (P < 0.05) electromyographic activity with the V than the C condition with varying percentage increases in EMG root-mean-square (EMGrms) values ranging from 20% to 200%. For both the vastus lateralis and biceps femoris, the increase in mean EMGrms values depended on the frequency, amplitude and muscle contraction level with 50 Hz-0.5 mm stimulation inducing the largest neuromuscular activity. Conclusions: These results show that the isometric contraction superimposed on vibration stimulation leads to higher neuromuscular activity compared to isometric contraction alone in the lower limbs. The combination of the vibration frequency with the amplitude and the muscle tension together grades the final neuromuscular output.Peer reviewe

A neurophysiological examination of voluntary isometric contractions: modulations in sensorimotor oscillatory dynamics with contraction force and physical fatigue, and peripheral contributions to maximal force production

Human motor control is a complex process involving both central and peripheral components of the nervous system. Type Ia afferent input contributes to both motor unit recruitment and firing frequency, however, whether maximal force production is dependent on this input is unclear. Therefore, chapter 2 examined maximal and explosive force production of the knee extensors following prolonged infrapatellar tendon vibration; designed to attenuate the efficacy of the homonymous Ia afferent-α-motoneuron pathway. Despite a marked decrease in H-reflex amplitude, indicating an attenuated efficacy of the Ia afferent-α-motoneuron pathway, both maximal and explosive force production were unaffected after vibration. This suggested that maximal and explosive isometric quadriceps force production was not dependent upon Ia afferent input to the homonymous motor unit pool. Voluntary movements are linked with various modulations in ongoing neural oscillations within the supraspinal sensorimotor system. Despite considerable interest in the oscillatory responses to movements per se, the influence of the motor parameters that define these movements is poorly understood. Subsequently, chapters 3 and 4 investigated how the motor parameters of voluntary contractions modulated the oscillatory amplitude. Chapter 3 recorded electroencephalography from the leg area of the primary sensorimotor cortex in order to investigate the oscillatory responses to isometric unilateral contractions of the knee-extensors at four torque levels (15, 30, 45 and 60% max.). An increase in movement-related gamma (30-50 Hz) activity was observed with increments in knee-extension torque, whereas oscillatory power within the delta (0.5-3 Hz), theta (3-7 Hz), alpha (7-13 Hz) and beta (13-30 Hz) bands were unaffected. Chapter 4 examined the link between the motor parameters of voluntary contraction and modulations in beta (15-30 Hz) oscillations; specifically, movement-related beta decrease (MRBD) and post-movement beta rebound (PMBR). Magnetoencephalography (MEG) was recorded during isometric ramp and constant-force wrist-flexor contractions at distinct rates of force development (10.4, 28.9 and 86.7% max./s) and force output (5, 15, 35 and 60%max.), respectively. MRBD was unaffected by RFD or force output, whereas systematic modulation of PMBR by both contraction force and RFD was identified for the first time. Specifically, increments in isometric contraction force increased PMBR amplitude, and increments in RFD increased PMBR amplitude but decreased PMBR duration. Physical fatigue arises not only from peripheral processes within the active skeletal muscles but also from supraspinal mechanisms within the brain. However, exactly how cortical activity is modulated during fatigue has received a paucity of attention. Chapter 5 investigated whether oscillatory activity within the primary sensorimotor cortex was modulated when contractions were performed in a state of physical fatigue. MEG was recorded during submaximal isometric contractions of the wrist-flexors performed both before and after a fatiguing series of isometric wrist-flexions or a time matched control intervention. Physical fatigue offset the attenuation in MRBD observed during the control trial, whereas PMBR was increased when submaximal contractions were performed in a fatigued state

The perception of effort during muscular fatigue and recovery

This study investigated how sense of effort is altered during fatigue in nine normal subjects. A contralateral limb matching paradigm was used in which the subjects nondominant (reference) arm was held at 20% MVC with force production matched at one minute intervals by the dominant (marching arm). It was found that matching force increased in a linear fashion with fatigue. It was also observed that EMG amplitude increased in the reference and matching arm and remained elevated during a 15 minute recovery period. As in previous studies strong correlation (r = 0.85) between rmsEMG in the reference arm and matching force was recorded. It was found that a subject was able to estimate force accurately a short time (in 10 minutes) after the fatiguing influence was removed although strength had not fully recovered. As with previous studies it was concluded that judgements of force production were based on the subjects internally generated perception of effort and not on the absolute force being generated

Unilateral hamstring foam rolling does not impair strength but the rate of force development of the contralateral muscle

Background Self-administered foam rolling (SAFR) is an effective massage technique often used in sport and rehabilitation settings to improve range of motion (ROM) without impairing the strength performance. However, the effects of unilateral SAFR on contralateral non-intervened muscle’s rate of force development (RFD) are unknown. Therefore, the purpose of this investigation was to examine the acute effects of unilateral hamstrings SAFR on the contralateral limb flexibility, the isometric strength, and the RFD parameters. Methods Thirty-four subjects (21 women) completed two separate randomly sequenced experimental visits, during which the control (rested for 10 min) or ten, 30-second SAFR were performed with the dominant hamstring muscle group. Before (Pre) and after (Post) the interventions, the contralateral hip flexion passive ROM, the maximal explosive isometric strength of the contralateral knee flexors with the corresponding prime mover muscles’ surface electromyographic (EMG) amplitude were measured. Separate two-way (time ×intervention) repeated measures analyses of variance (ANOVAs) were used to examine the potential changes of the dependent variables. Results The SAFR significantly improved the contralateral limb ROM (Pre vs. Post: 68.3 ± 21.0 vs. 73.2 ± 23.2 degrees, p < 0.001; d = 0.22). No change was found for the contralateral isometric strength or the maximal EMG amplitude. For the RFD parameters, the percent changes of the RFDs for the first 50, 100, and 200 ms of the maximal explosive isometric contraction were −31.2%, −16.8%, and −10.1%, respectively, following the unilateral SAFR, relative to the control condition. In addition, the decrement of the first 50-ms RFD reached statistical significance (p = 0.007; Cohen’s d = 0.44). Conclusion Ten sets of 30-second unilateral hamstring SAFR improved the ROM of the non-intervened contralateral limb, but decreased its ability to generate force, especially during the early phase (e.g., 50 ms) of the maximal explosive contraction

Neuromuscular adaptations during long-term bed rest

The weightless environment encountered during human spaceflight virtually eliminates the mechanical loading of the human body. The accompanying physical inactivity sets in motion a cascade of changes that affects practically every physiological system in the human body. Of particular medical and operational concern are the decrements in skeletal muscle strength (force, power and endurance capacity) of the legs and the demineralization of weight- bearing bones. By now, it is acknowledged that these functional impairments may be prevented by adequate muscle exercise. This thesis addresses the study into the neuromuscular adaptations in the quadriceps femoris muscle as a consequence of bed rest-induced physical inactivity. Bed rest is hereby used as a simulation model of human spaceflight. The nature and progression of the adaptations, but also the preventative effect of physical were studied. In the Berlin bed rest study, the used training paradigm comprised combined resistance and vibration exercise, i.e. strength training exercises were performed against a mechanically vibrated platform. We were interested in the changes that occur during – particularly in the early stage of – bed rest. Hence, besides conducting experiments pre- and post bed rest, we also performed seven experiments – with an increasing time interval between experiments - during the eight weeks of bed rest. To disentangle the neural activation of the muscle from more intrinsic muscle characteristics both voluntary and electrically stimulated contractions were assessed. Voluntary motor control was assessed using two procedures: by means of superimposed electrical stimulation and by means of recording the electrophysiological properties of the quadriceps muscle by means of a sophisticated high-density surface electromyography methodology (HD-sEMG). The results of the studies described in this thesis have contributed to a better understanding of the underlying mechanisms and the time course in which they contribute to the various manifestations of muscle weakness as a result of physical inactivity imposed by strict bed rest. Most notably were the findings of a linear reduction in voluntary isometric knee extension strength, and an increase in relative muscle fatigability. These adaptations were predominantly the result of a linear decay in the cross- sectional area of the quadriceps femoris muscle and a reduced blood flow as a consequence of bed rest. Changes in the intrinsic contractile characteristics of the quadriceps femoris towards a faster muscle also progressed linearly in time. An unexpected finding across experiments was that the adopted longitudinal study fully prevented neural deconditioning. Vigorous resistive vibration exercise training during bed rest appeared a suitable gravity-independent countermeasure that offset or substantially mitigated most of the adaptive changes in quadriceps femoris muscle that evolved during bed rest in the absence of this countermeasure.Haan, A. [Promotor]de Stegeman, D.F. [Promotor]Gerrits, H.L. [Copromotor

Investigation of the feasibility of using focal vibratory stimulation with robotic aided therapy for spasticity rehabilitation in spinal cord injury

The occurrence of a traumatic spinal cord injury is in hundreds of thousands of people every year. Survivors are left with loss of many bodily functions, loss of sensation below the point of injury and many more painful and uncomfortable repercussions which interfere with activities of daily living. Over 70% of people with SCI develop spasticity: abnormally increased muscle tone and connected joint stiffness that interfere with residual volitional control of the limbs. Treatments for spasticity include many pharmacological and non-pharmacological techniques, however many of them have severe sideeffects. Evidence suggest the use of vibratory stimulation to relieve repercussions of spasticity, despite not agreeing on the most advantageous protocol. This thesis evaluated effects that focal vibratory stimulation have on the muscle performance. Within two studies, focal muscle vibration is compared against different application conditions such as timing and location. The results suggests that if focal vibrations are applied to the relaxed muscle, the increase in muscle's force is observed. Analysis of the cortical waves indicates minimal cortical involvement in vibratory stimulation modulation. On the other hand, FV applied of the connected tendon/bone imposed to a contraction seems to have a potential to increase muscle's activation. There is evidence that motor cortex is responding to this stimulation to stabilise the muscle in order to perform the contraction. Within clinical trial, focal muscle vibratory stimulation is employed in total of 6 interventional sessions while a joint's spastic exor and extensor muscles were relaxed. Spasticity appears to be reduced as a consequence of the stimulation. Moreover, engaging the joint into robotic-aided therapy increase volitional control of the wrist, according to the analysis of the active range of motion, joint stiffness and kinematic parameters associated to the movement. The measurement and movement facilitation device used in the clinical trial was designed and developed in accordance to the spasticity and spinal cord injury repercussions consideration. The studies conducted for this thesis demonstrated feasibility and potential for the use of focal muscle vibratory stimulation to enhance muscle power in healthy muscles but also relieve consequences of spasticity. Vibrations combined with movement robotic-aided therapy have a prospects to enhance motor control