Influence of vibration on delayed onset of muscle soreness after eccentric exercise
Power Plate Studies
The first detailed description of delayed onset muscle soreness (DOMS) was given by Hough in 1902.1 DOMS is often precipitated predominantly by eccentric exercise, such as downhill running, plyometrics and resistance training. It has been explained2 3 that the injury itself is a result of eccentric exercise causing damage to the muscle cell membrane, which sets off an inflammatory response. In other words, shocking the muscles during the eccentric range ofmotion is probably the leading factor in producing DOMS. It has been claimed that the type of force development during eccentric exercise may cause sarcoma disruption and consequently an inflammatory response within the muscle.4–6 DOMS is typically experienced by all individuals regardless of fitness level, and is a normal physiological response to increased exertion and the introduction of unfamiliar physical activities.7 The pain and discomfort associated with DOMS typically peaks 24–48 hours after an exercise bout, and resolves within 96 hours.8 Generally, an increased perception of soreness occurs with greater intensity and a higher degree of unfamiliar activities.9 Other factors that play a role in DOMS are muscle stiffness, contraction velocity, fatigue, and angle of contraction.2 Due to the sensation of pain and discomfort, which can impair physical training and performance, prevention and treatment of DOMS is of great concern to coaches, trainers and therapists.10 Although DOMS is experienced widely, science has not established a sound and consistent treatment for it.

Recent research has shown that vibration training (VT) may improve muscle performance.11 12 By considering this mechanism, we designed this study to find out if VT before eccentric exercise may prevent DOMS by improving muscular strength and power development strategy,13–17 improving kinesthetic awareness,18 and providing insights into the effects of fatigue, 19 20 within the vibrated muscles.

MATERIALS AND METHODS
 
The study was approved by the ethical committee of Semnan University of Medical Sciences. Fifty healthy non-athletic volunteers (25 females, mean (SD) age 21.1 (0.2) years and 25 males mean (SD) age 20.1 (0.5) years) were assigned randomly into two experimental, VT and non-VT groups. Exclusion criteria included a history of cardiac and neuromuscular diseases, undertaking severe sport activity or having received an intramuscular injection during the last week.
 
A computer generated randomisation list was drawn up by the statistician for each group. It was given to the physiotherapy department in sealed numbered envelopes. When the subjects qualified to enter the study and had signed their informed consent forms, the appropriate numbered envelope was opened at the reception; the card inside indicated the subject’s allocation to one of the VT or non-VT groups. This information was then given to the physiotherapist to administer the appropriate intervention.
 
Intervention
 
Both experimental groups walked downhill on a 10° declined treadmill at a speed of 4 km per hour for 30 min. In the VT group, a 50 Hz vibrator apparatus (model VR-7N, ITO, Tokyo, Japan) was used to apply vibration on the middle line of each of the left and right quadriceps, hamstring and calf muscles for 1 min before downhill treadmill walking, while the subjects in the non-VT group did not receive any vibration before downhill treadmill walking.
 
Measurements
 
Measurements were performed before and 24 h after treadmill walking and included isometric maximum voluntary contraction. Figure 1 Mean change of maximum isometric voluntary contraction force of quadricepsThe IMVC force of the left and right quadriceps muscles in 100°of knee flexion was measured in the sitting position. The subject was asked to sit on a back-supported quadriceps table and a load cell was connected to the distal end of her/his leg by means of a tight sling. The output of the load cell was connected to a digital monitor so it was possible to record and save the maximum tension on the load cell. After bringing to zero the output of the load cell, the subject was encouraged to perform IMVC by pulling the sling tight as hard as she/he could, three times, with 30-second intervals between each pull. The best attempt was recorded and considered as the quadriceps IMVC force in newtons.
 
Pressure pain threshold (PPT) on the 5, 10 and 15 cm above the left and right patellae and also on the middle line of calf muscles was measured by a 20 ml syringe with a spring inside which was scaled from 0 to 10. The rounded tip of syringe was placed at the above points in a vertical position and the piston was pressed down. The subject was asked to announce any unpleasant sensation (pain), and then the indicating number on the syringe was recorded as the PPT.
 
The level of muscle soreness was evaluated by mean of a Visual Analogue Scale (VAS). The day after treadmill walking, the subject was asked to indicate her/his feel of the level of muscle soreness in each lower limb along a 10 cm line ranging from 0 (‘‘no muscle soreness at all’’) to 10 (‘‘the most severe muscle soreness that I can imagine).
 
The serum level of creatine kinase (CK) was measured 24 hours after treadmill walking by taking 3 cc blood samples from the brachial artery in the front of the elbow and then the level of CK enzyme was measured in the laboratory.
 
Statistics
 
To compare the possible effect of VT on DOMS, an intention to treat analysis was used which involved all subjects who were randomly assigned to their group. Student’s t tests were used to compare the mean changes in the IMVC force, PPT values and the mean of CK level and muscle soreness between the experimental groups.
 
RESULTS
 
Fifty healthy subjects were randomly assigned into two experimental VT (n=25, 12 male and 13 female) and non-VT (n=25, 13 male and 12 female,) groups and the study was then completed. The mean age was 20.6 (1.9) years (mean (SD)) for the VT group, and 20.6 (2.1) years (mean (SD)) for non-VT group, without any significant differences between the two groups.
 
Isometric maximum voluntary contraction force
 
A comparison of the mean change in the IMVC force in the right quadriceps showed a significantly higher decrease (P=0.001) in the non-VT group (-39.6 (46.6) years, mean (SD)) compared with the VT group (37.2 (100.1) years, mean (SD)). This reduction was also seen in the left quadriceps (non- VT group -16.5 (77.6) years vs VT group 56.8 (100.9) years, P=0.006), fig 1.

Pain pressure threshold
 
Table 1 shows the mean changes in PPT at 5, 10 and 15 cm above the right and left patella. Comparison of these values from non-VT and VT groups showed significant reduction of PPT in the non-VT group (P=0.0001). The same significant reduction (P=0.0001) of PPT was seen in the calf muscles of the non-VT group (right -1.1 (1.3) and left -1.3 (1.4)) compared with the VT group (right 0.4 (0.8) and left 0.4 (1.2)).
 
Level of muscle soreness
 
The comparison of mean level of muscle soreness recorded the day after treadmill walking showed higher soreness in the non- VT group (right 2.3 (1.9) and left 2.3 (2.1)) vs VT group (right 0.4 (1.1) and left 0.5 (1.1)). These differences were significant in both lower limbs (P=0.0001), fig 2.
 
Level of CK enzyme
 
The higher mean of the CK enzyme was found in the non-VT group (195.2 (109.2)) compared with the VT group (116.1 (27.8)) which was statistically significant (P=0.001), fig 3.

 
Figure 2 Mean change of delayed onset muscle soreness at 24 h after eccentric exercises.

 
Mean changes in the pain pressure threshold of the right and left quadriceps at 5, 10 and 15 cm above the patella
DISCUSSION
 
Delayed onset muscle soreness and reduced muscle strength after eccentric exercise5 21 may decrease functional activities in athletics.7 22 Because of this, different methods have been investigated and recommended to prevent these symptoms.2 This study was designed to find the possible effects of VT to control and prevent DOMS after eccentric exercise.
 
A previous study showed muscle strength reductionFigure 3 Mean serum level of creatine kinase enzyme at 24 h after eccentric exercise. after eccentric activities,9 while our findings showed no muscle strength reduction in the IMVC force of quadriceps in the VT group, which may be due to the establishment of optimum neuromuscular function in the quadriceps muscles by applying VT. This has been reported by other researchers,12 15 who showed better muscle performance after vibratory stimulation. Thompson and Belanger (2002) also showed that VT may increase muscle spindle activities and establish motor unit activity synchronisation that may optimise neuromuscular function.23 By contrast, it has been shown that muscle spindle stimulation by vibration may increase the afferent activities of muscle spindles which may increase background tension in the vibrated muscles.24 25 This increased background tension and motor unit activity synchronisation in the vibrated muscle may prevent sarcoma disruption or damage to excitation–contraction coupling, which may happen due to tension development during eccentric exercise.4 Therefore, this optimised muscle performance may control and prevent muscle damage and so reduce DOMS. This reduction in DOMS was seen in our study, as we found increased PPT in the right and left quadriceps and calf muscles, lower muscle soreness, and lower levels of CK enzyme in the VT group compared with the non-VT group.
 
The CK enzyme has been defined as an index for muscle damage and its level will be increased within 24 to 48 hours after eccentric activities,5 22 which is a sign of eccentric muscle damage. However this increase was seen only in the non-VT group, and not in the VT group. In fact, the lower CK level in the VT group may indicate lower muscle damage in this group, while the non-VT group showed a higher CK level and so higher muscle damage, which was accompanied by higher muscle soreness.
 
These findings may indicate that vibration training before eccentric exercise may help the muscles to build up a background tension and optimal neuromuscular activity to overcome the increased passive tension within the exercised muscles during eccentric activities. Thus, vibration training could be used before eccentric activities to control and prevent delayed onset muscle soreness and it might be a useful method for athletes to prevent any DOMS in their sports activities.
 
CONCLUSION
 
DOMS is a major complication faced by athletes after eccentric activities, which may compel them to postpone their sports activities, thus prevention and treatment of DOMS is of great concern to coaches, trainers, and therapists. In this study, we investigated the effect of vibration on muscle before downhill treadmill walking and our results showed that applying vibration before eccentric activities may prevent DOMS and so it may help non-athletic people to follow and complete their sport activities without any delay. Further studies are needed to investigate these results to find the possible application in athletics.