Adaptations in coactivation after isometric resistance training.

Twenty sedentary male university students were randomly assigned to an experimental or a control group. The experimental group trained the knee extensors of one leg by producing 30 isometric extension maximal voluntary contractions (MVC) per day, three times per week for 8 wk. After 8 wk of training, extensor MVC in the trained leg increased 32.8% (P less than 0.05), but there was no change in vastus lateralis maximal integrated electromyographic activity (IEMGmax). The most important finding was that the degree of hamstring coactivation during extension MVC decreased by approximately 20% (P less than 0.05) after the 1st wk of training. Less pronounced adaptations occurred in the untrained leg: extension MVC force increased 16.2% (P less than 0.05), hamstring coactivity decreased 13% (P less than 0.05) after 2 wk of training, and vastus lateralis IEMGmax was unchanged. The same measures in legs of the control group were not changed during the study. There were no changes in flexion MVC, biceps femoris IEMGmax, or the degree of quadriceps coactivity during flexion MVC in either leg of the control or experimental group. A reduction in hamstring coactivity in the trained and untrained legs indicates that these muscles provide less opposing force to the contracting quadriceps. We conclude that this small but significant decrease in hamstring coactivation that occurs during the early stages of training is a nonhypertrophic adaptation of the neuromuscular system in response to static resistance training of this type.     

Decreased rate of leg extensor force development in independently ambulant patients with acute stroke with mild paresis

We aimed to examine the rate of force development (RFD) of knee extensors on both sides in independently ambulant patients with acute stroke with mild paresis compared with that in age-matched healthy adults. A total 31 patients with acute stroke history (patient group: 67 ± 12 years) and 54 age-matched healthy, community-dwelling adults (control group: 67 ± 8 years) were included. Maximum voluntary contraction (MVC) and RFD were assessed <1 month post-stroke during isometric knee extension (sitting position; 90° knee flexion) using a hand-held dynamometer. RFD was measured as the average slope of the torque–time curve over time intervals of 0–50 ms and 0–200 ms from contraction onset. In the patient group, MVC and RFD for 0–50 ms were significantly lower on the affected side than on the unaffected side (p < 0.01). RFD was significantly decreased in the patient group, to 32%–38% and 62%–71% of that in the control group, over 0–50 ms and 0–200 ms, respectively, regardless of the affected side (p < 0.01). No significant differences in MVC between patient and control groups were observed for either side. RFD of the knee extensors significantly decreased without MVC reduction in patients with acute stroke history compared with that in age-matched healthy adults in both the affected and unaffected sides. These results suggest that decrease in RFD was initiated from the acute phase of stroke, even in patients with stroke who had good motor function.     

Electromechanical delay in isometric muscle contractions evoked by voluntary, reflex and electrical stimulation

Electromechanical delay (EMD) in isometric contractions of knee extensors evoked by voluntary, tendon reflex (TR) and electrical stimulation (ES) was investigated in 21 healthy young subjects. The subject performed voluntary knee extensions with maximum effort (maximal voluntary contraction, MVC), and at 30%, 60% and 80% MVC. Patellar tendon reflexes were evoked with the reflex hammer being dropped from 60°, 75° and 90° positions. In the percutaneous ES evoked contractions, single switches were triggered with pulses of duration 1.0 ms and of intensities 90, 120 and 150 V. Electromyograms of the vastus lateralis and rectus femoris muscles were recorded using surface electrodes. The isometric knee extension force was recorded using a load cell force transducer connected to the subject's lower leg. The major finding of this study was that EMD of the involuntary contractions [e.g. mean 22.1 (SEM 1.32) ms in TR 90°; mean 17.2 (SEM 0.62) ms in ES 150 V] was significantly shorter than that of the voluntary contractions [e.g. mean 38.7 (SEM 1.18) ms in MVC,P < 0.05]. The relationships between EMD, muscle contractile properties and muscle fibre conduction velocity were also investigated. Further study is needed to explain fully the EMD differences found between the voluntary and involuntary contractions.     

Age and contraction type influence motor output variability in rapid discrete tasks.

The purpose of this study was to examine the ability to control knee-extension force during discrete isometric (IC), concentric (CC), and eccentric contractions (EC) in 24 young (mean age +/- SD = 25.3 +/- 2.8 yr) and 24 old (mean age +/- SD = 73.3 +/- 5.5 yr) healthy and active individuals. Subjects were to match a parabola with a time to peak force of 200 ms during IC, CC, and EC at six target levels of force [20, 35, 50, 65, 80, and 90% of the maximum voluntary contraction (MVC)]. ICs were performed at 90 degrees of knee flexion, whereas CCs and ECs ranged from 90 to 80 degrees of knee flexion (0 degrees is full extension) at a slow velocity (25 degrees /s). Results showed that subjects produced similar MVC forces for the three types of contractions. Young subjects produced greater MVC forces than old subjects, and within each age group, men produced greater force than women. The variability (standard deviation) of peak force and impulse in absolute values was greater for young compared with old subjects. When variability was normalized to the force produced [coefficient of variation (CV)], however, old subjects exhibited greater CV than young subjects for peak force and impulse. Both the standard deviation and CV of time to peak force and impulse duration were greater for the old adults. In general, ECs were more variable than ICs and CCs, and old adults exhibited greater CV compared with young adults during rapid, discrete ICs, CCs, and particularly ECs of the quadriceps.     

The combined effect of the cold pressor test and isometric exercise on heart rate and blood pressure

The purpose of this study was to determine if the cold pressor test during isometric knee extension [15% of maximal voluntary contraction (MVC)] could have an additive effect on cardiovascular responses. Systolic and diastolic blood pressures, heart rate and pressure rate product were measured in eight healthy male subjects. The subjects performed the cold pressor tests and isometric leg extensions singly and in combination. The increases of systolic and diastolic blood pressure during isometric exercise were of almost the same magnitude as those during the cold pressor test. The responses of arterial blood pressure, and heart rate to a combination of the cold pressor test and isometric knee extension were greater than for each test separately. It is suggested that this additional effect of cold immersion of one hand during isometric exercise may have been due to vasoconstriction effects in the contralateral unstressed limb. In summary, the circulatory effects of the local application of cold during static exercise at 15% MVC were additive.     

Potentiation of knee extensor contraction by antagonist conditioning contraction at several intensities

The purpose of this study was to examine the effect of graded conditioning contractions of the antagonist knee flexor muscles on the output characteristics of knee extensor muscles in healthy humans. Eight male university students performed maximum isometric contractions of knee extensors, preceded by isometric conditioning contractions of the antagonist knee flexors. The developed force and electromyographic (EMG) amplitudes of the knee extensors after the conditioning contraction were measured and compared with those of simple knee extension without conditioning. The forces of the conditioning flexor contraction were set at three levels: low (20% of maximum voluntary contraction: MVC), moderate (60% of MVC), and high (100% of MVC). The EMG amplitudes of the vastus medialis, vastus lateralis, and rectus femoris muscle were recorded and the root mean square amplitudes were calculated. The strongest enhancement of the extension force was obtained by moderate intensity conditioning contraction (108.95+/-1.87% of simple knee extension), although high intensity conditioning also induced a significant increase (105.41+/-2.69%). Low intensity conditioning did not cause a significant enhancement of the contraction force (103.17+/-2.99%). Similarly, the EMG amplitudes were significantly increased by moderate and/or high conditioning. These results suggest that antagonist conditioning contraction of moderate intensities is sufficient and may be optimal to potentiate knee extensor contraction.     

Dependence of average muscle fibre conduction velocity on voluntary contraction force

Average muscle fibre conduction velocity (CV) measured with multichannel surface electrodes decreases with time during sustained isometric contraction. Based on this property, CV is considered a candidate for an objective index to localized muscular fatigue. CV, however, also depends on many other factors that include muscle temperature and voluntary contraction force. In this paper, the effect of contraction force on CV was studied by defining not only the target force level but also the whole force trajectory. The contraction was isometric and lasted 14 s. The target force was set at four levels from 30% to 90% of the maximal voluntary contraction (MVC). Three typical muscles were studied in seven healthy male subjects. In the vastus lateralis, CV increased with contraction force in many cases. In the biceps brachii, CV decreased rapidly with time before the contraction force reached the target levels of 70% or 90% MVC. At these force levels, CV was smaller than that at 50% MVC. CV in the biceps consequently showed no apparent dependence on the contraction force. The tibialis anterior showed intermediate change in CV between the vastus lateralis and the biceps brachii. These results indicate that CV basically increases with contraction force, but this relationship becomes unclear when CV decreases rapidly with time.     

Association between knee extensor force steadiness and postural stability against mechanical perturbation in patients with Parkinson’s disease

ObjectiveLower extremity force steadiness has been shown to decrease with aging and neuromotor dysfunction and to be associated with physical function and fall. Although patients with Parkinson’s disease (PD) experience decreased force steadiness, whether the extent of force steadiness differs according to target force or whether this steadiness is associated with postural control remain unclear. Therefore, this study aimed to compare the force steadiness while steadily exerting low and moderate levels of knee extensor force between individuals with and without PD and to examine the association between force steadiness and postural instability against mechanical perturbation in PD.MethodsA total of 33 patients with PD (mean age, 71.7 years) and 33 healthy controls (72.2 years) participated in this study. Participants with PD were classified into postural stability or instability groups based on the Movement Disorder Society-sponsored revision of the Unified Parkinson’s Disease Rating Scale motor exam item 12. Participants performed steady task of the knee isometric extension at two levels (10% and 50% of maximal voluntary contraction [MVC]).ResultsForce steadiness at 10% MVC was lower in postural instability group than that in the control and postural stability groups (P < 0.05) after adjusting for age, sex, and body mass index, whereas it was not significantly different at 50% MVC among the three groups.DiscussionThese results suggest that the knee extensor force steadiness is affected in patients with PD having postural instability against mechanical perturbation during low intensity force exertion and is not affected regardless of the presence of postural instability during moderate intensity force exertion.     

Gender differences in effects of 20 days horizontal bed rest on muscle strength in young subjects

Gender differences in the effect of 20 days bed rest (BR) on muscle strength were evaluated in voluntary 11 male and 7 female students. Maximum Isometric Voluntary Contractions (MVC) of 4 right arm muscles (RAM), 5 right leg muscles (RLM), and 2 body trunk muscles were measured with an isometric dynamometer, respectively. Muscle masses (MM) of right arm and leg and body trunk were determined by dual energy X-ray absorptiometry, respectively. The maximum cross sectional area (CSAmax) of right m. quadriceps femoris was measured by magnetic resonance imaging. Elbow flexion MVC in males and all MVC of RLM except knee flexion in both males and females were decreased (p<0.05), but elbow extension MVC in females was increased (p<0.05), while all of other MVC only tended to decrease. However, the decrements in leg MVC were not correlated to the leg MM, and also the decrement in knee extension was not correlated to the CSAmax of m. quadriceps. The reduction of MVC of antigravity muscles might be caused not only by a decrease in MM but also by other factors. The greater decrements of leg MVC during BR were the higher initial level in males, but the inverse was observed in females. However, this discrepancy between males and females cannot be explained in the present study.     

Specific strength and voluntary muscle activation in young and elderly women and men.

The extents to which decreased muscle size or activation are responsible for the decrease in strength commonly observed with aging remain unclear. Our purpose was to compare muscle isometric strength [maximum voluntary contraction (MVC)], cross-sectional area (CSA), specific strength (MVC/CSA), and voluntary activation in the ankle dorsiflexor muscles of 24 young (32 +/- 1 yr) and 24 elderly (72 +/- 1 yr) healthy men and women of similar physical activity level. Three measures of voluntary muscle activation were used: the central activation ratio [MVC/(MVC + superimposed force)], the maximal rate of voluntary isometric force development, and foot tap speed. Men had higher MVC and CSA than did women. Young men had higher MVC compared with elderly men [262 +/- 19 (SE) vs. 197 +/- 22 N, respectively], whereas MVC was similar in young and elderly women (136 +/- 15 vs. 149 +/- 16 N, respectively). CSA was greater in young compared with elderly subjects. There was no age-related impairment of specific strength, central activation ratio, or the rate of voluntary force development. Foot tap speed was reduced in elderly (34 +/- 1 taps/10 s) compared with young subjects (47 +/- 1 taps/10 s). These results suggest that isometric specific strength and the ability to fully and rapidly activate the dorsiflexor muscles during a single isometric contraction were unimpaired by aging. However, there was an age-related deficit in the ability to perform rapid repetitive dynamic contractions.     

Influences of repetitive muscle contractions with different modes on tendon elasticity in vivo.

The present study aimed to investigate the effects of repetitive muscle contractions on the elasticity of human tendon structures in vivo. Before and after each endurance test, the elongation of the tendon and aponeurosis of vastus lateralis muscle (L) was directly measured by ultrasonography while the subjects performed ramp isometric knee extension up to maximal voluntary isometric contraction (MVC). Six male subjects performed muscle endurance tests that consisted of knee extension tasks with four different contraction modes: 1) 50 repetitions of maximal voluntary eccentric action for 3 s with 3 s of relaxation (ET1), 2) three sets of 50 repetitions of MVC for 1 s with 3 s of relaxation (ET2), 3) 50 repetitions of MVC for 3 s with 3 s of relaxation (ET3), and 4) 50 repetitions of 50% MVC for 6 s with 6 s of relaxation (ET4). In ET1 and ET2, there were no significant differences in L values at any force production levels between before and after endurance tests. In the cases of ET3 and ET4, however, the extent of elongation after the completion of the tests tended to be greater. The L values above 330 N in ET3 and 440 N in ET4, respectively, were significantly greater after endurance tests than before. These results suggested that the repeated longer duration contractions would make the tendon structures more compliant and that the changes in the elasticity might be not be affected by either muscle action mode or force production level but by the duration of action.     

Yoga as steadiness training: effects on motor variability in young adults

Exercise training programs can increase strength and improve submaximal force control, but the effects of yoga as an alternative form of steadiness training are not well described. The purpose was to explore the effect of a popular type of yoga (Bikram) on strength, steadiness, and balance. Young adults performed yoga training (n = 10, 29 +/- 6 years, 24 yoga sessions in 8 weeks) or served as controls (n = 11, 26 +/- 7 years). Yoga sessions consisted of 1.5 hours of supervised, standardized postures. Measures before and after training included maximum voluntary contraction (MVC) force of the elbow flexors (EF) and knee extensors (KE), steadiness of isometric EF and KE contractions, steadiness of concentric (CON) and eccentric (ECC) KE contractions, and timed balance. The standard deviation (SD) and coefficient of variation (CV, SD/mean force) of isometric force and the SD of acceleration during CON and ECC contractions were measured. After yoga training, MVC force increased 14% for KE (479 +/- 175 to 544 +/- 187 N, p < 0.05) and was unchanged for the EF muscles (219 +/- 85 to 230 +/- 72 N, p > 0.05). The CV of force was unchanged for EF (1.68 to 1.73%, p > 0.05) but was reduced in the KE muscles similarly for yoga and control groups (2.04 to 1.55%, p < 0.05). The variability of CON and ECC contractions was unchanged. For the yoga group, improvement in KE steadiness was correlated with pretraining steadiness (r = -0.62 to -0.84, p < 0.05); subjects with the greatest KE force fluctuations before training experienced the greatest reductions with training. Percent change in balance time for individual yoga subjects averaged +228% (19.5 +/- 14 to 34.3 +/- 18 seconds, p < 0.05), with no change in controls. For young adults, a short-term yoga program of this type can improve balance substantially, produce modest improvements in leg strength, and improve leg muscle control for less-steady subjects.     

Is the coactivation of biceps femoris during isometric knee extension affected by adiposity in healthy young humans?

This study aimed to verify if the level of biceps femoris antagonist activity measured during isometric knee extension was affected by the individual degree of adiposity in 14 young healthy subjects of both genders aged between 18 and 24. Surface EMG signals were recorded from the biceps femoris muscle of the dominant leg during isometric knee extension at three levels of voluntary contraction: maximum (MVC), 80% MVC and 200 N, respectively. In addition, whole-body percentage of fat, volume of the thigh and skinfold thickness below the electrodes were achieved. Biceps femoris coactivation values were: 28.5 +/- 17.9%, 30.9 +/- 17.7% and 25.3 +/- 17.5% for MVC, 80% MVC and 200 N trials, respectively (NS). Neither the whole-body percentage of fat nor the skinfold thickness influenced percentage coactivation, irrespective of the intensity of contraction. However, an increase in the whole-body percentage of fat showed a tendency to augment the biceps femoris coactivation (P(I)=0.079; P(II)=0.575). No differences in coactivation were observed between genders. In addition, the duration of contraction did not affect the level of coactivation.     

Heart rate and blood pressure responses to isometric exercise in young and older men

The aim of this study was to examine the isometric endurance response and the heart rate and blood pressure responses to isometric exercise in two muscle groups in ten young (age 23–29 years) and seven older (age 54–59 years) physically active men with similar estimated forearm and thigh muscle masses. Isometric contractions were held until fatigue using the finger flexor muscles (handgrip) and with the quadriceps muscle (one-legged knee extension) at 20%, 40%, and 60% of the maximal voluntary contraction (MVC). Heart rate and arterial pressure were related to the the individual's contraction times. The isometric endurance response was longer with handgrip than with one-legged knee extension, but no significant difference was observed between the age groups. The isometric endurance response averaged 542 (SEM 57), 153 (SEM 14), and 59 (SEM 5) s for the handgrip, and 276 (SEM 35), 94 (SEM 10) and 48 (SEM 5) s for the knee extension at the three MVC levels, respectively. Heart rate and blood pressure became higher during one-legged knee extension than during handgrip, and with increasing level of contraction. The older subjects had a lower heart rate and a higher blood pressure response than their younger counterparts, and the differences were more apparent at a higher force level. The results would indicate that increasing age is associated with an altered heart rate and blood pressure response to isometric exercise although it does not affect isometric endurance. Accepted: 23 October 1997     

Repeatability of surface EMG variables during voluntary isometric contractions of the biceps brachii muscle.

The repeatability of initial value and rate of change of mean spectral frequency (MNF), average rectified values (ARV) and muscle fiber conduction velocity (CV) was investigated in the dominant biceps brachii of ten normal subjects during sustained isometric voluntary contractions. Four levels of contraction were studied: 10%, 30%, 50% and 70% of the maximal voluntary contraction level (MVC). Each contraction was repeated three times in each of three different days for a total of nine contractions/level/subject and 90 contractions per level across the ten subjects. Repeatability was investigated using the Intraclass Correlation Coefficient (ICC) and the standard error of the mean (SEM) of the estimates for each subject. Contrary to observations in other muscles, CV estimates appeared to be very repeatable both within and between subjects. CV showed a small but significant increase when contraction force increased from 10% to 50% MVC but no change for further increase of force. As force increased, MNF showed a slight decrease possibly related to a wider spreading of the CV values. The rate of time decrement of MNF and CV increased with the level of contraction. The normalized decrement (% of initial value per second) was in general higher for MNF than for CV and was more repeatable between subjects at 10% MVC than at 70% MVC. A final observation is that a resting time of 5 minutes may not be sufficient after a contraction at 50% or 70% MVC.     

Voluntary stepping behavior under single- and dual-task conditions in chronic stroke survivors: A comparison between the involved and uninvolved legs

ObjectiveIf balance is lost, quick step execution can prevent falls. Research has shown that speed of voluntary stepping was able to predict future falls in old adults. The aim of the study was to investigate voluntary stepping behavior, as well as to compare timing and leg push-off force–time relation parameters of involved and uninvolved legs in stroke survivors during single- and dual-task conditions. We also aimed to compare timing and leg push-off force–time relation parameters between stroke survivors and healthy individuals in both task conditions.MethodsTen stroke survivors performed a voluntary step execution test with their involved and uninvolved legs under two conditions: while focusing only on the stepping task and while a separate attention-demanding task was performed simultaneously. Temporal parameters related to the step time were measured including the duration of the step initiation phase, the preparatory phase, the swing phase, and the total step time. In addition, force–time parameters representing the push-off power during stepping were calculated from ground reaction data and compared with 10 healthy controls.ResultsThe involved legs of stroke survivors had a significantly slower stepping time than uninvolved legs due to increased swing phase duration during both single- and dual-task conditions. For dual compared to single task, the stepping time increased significantly due to a significant increase in the duration of step initiation. In general, the force time parameters were significantly different in both legs of stroke survivors as compared to healthy controls, with no significant effect of dual compared with single-task conditions in both groups.ConclusionsThe inability of stroke survivors to swing the involved leg quickly may be the most significant factor contributing to the large number of falls to the paretic side. The results suggest that stroke survivors were unable to rapidly produce muscle force in fast actions. This may be the mechanism of delayed execution of a fast step when balance is lost, thus increasing the likelihood of falls in stroke survivors.     

Older adults are less steady during submaximal isometric contractions with the knee extensor muscles.

This study compared the steadiness of submaximal contractions with the knee extensor muscles in young and old adults. Twenty young and twenty old subjects underwent assessment of isometric maximum voluntary contraction (MVC), one-repetition maximum (1-RM) strength, and steadiness during isometric, concentric, and eccentric contractions with the knee extensor muscles. The old adults displayed 33% lower MVC force and a 41% lower 1-RM load. The coefficient of variation for force was significantly greater for the old adults during isometric contractions at 2, 5, and 10% of MVC but not at 50% MVC. The decline in steadiness at low forces experienced by the men was marginally greater than that experienced by the women. The steadiness of concentric and eccentric contractions was similar in young and old adults at 5, 10, and 50% of 1-RM load. Old subjects exhibited greater coactivation of an antagonist muscle compared with young subjects during the submaximal isometric and anisometric contractions. These results indicate that, whereas the ability to exert steady submaximal forces with the knee extensor muscles was reduced in old adults, fluctuations in knee joint angle during slow movements were similar for young and old adults.     

Effects of low-load resistance training with vascular occlusion on the mechanical properties of muscle and tendon

The present study aimed to investigate the effects of low-load resistance training with vascular occlusion on the specific tension and tendon properties by comparing with those of high-load training. Nine participants completed 12 weeks (3 days/week) of a unilateral isotonic training program on knee extensors. One leg was trained using low load (20% of 1 RM) with vascular occlusion (LLO) and other leg using high load (80% of 1 RM) without vascular occlusion (HL). Before and after training, maximal isometric knee extension torque (MVC) and muscle volume were measured. Specific tension of vastus lateralis muscle (VL) was calculated from MVC, muscle volume, and muscle architecture measurements. Stiffness of tendon-aponeurosis complex in VL was measured using ultrasonography during isometric knee extension. Both protocols significantly increased MVC and muscle volume of quadriceps femoris muscle. Specific tension of VL increased significantly 5.5% for HL, but not for LLO. The LLO protocol did not alter the stiffness of tendon-aponeurosis complex in knee extensors, while the HL protocol increased it significantly. The present study demonstrated that the specific tension and tendon properties were found to remain following low-load resistance training with vascular occlusion, whereas they increased significantly after high-load training.     

Strength-velocity relationship and fatiguability of unilateral versus bilateral arm extension

Strength-velocity relations and fatigue resistance in an arm bench press manoeuvre were compared between conditions of bilateral (BL, both arms acting together) and unilateral muscle contraction in 9 young men. BL and UL (sum of the 2 arms acting singly) strength was similar for isometric and slow isokinetic maximal voluntary contractions (MVC); at high velocities BL MVC declined more than UL. In both types of contractions a curvilinear relation was observed between strength and velocity, with significantly higher peak torques (PT) being produced under isometric conditions than for slow velocity efforts (p less than 0.01). Mean declines in PT during 100 repetitive MVCs of approximately 70s were to 25% of initial values for the BL fatigue test and to 37% for UL (p less than 0.01). In contrast to results of a similar investigation of leg extension in the same subjects, the arms showed no BL deficit of strength in the initial part of the strength-velocity curve and approximately twice as much fatigue in repetitive contractions. These physiological differences may stem from the varying habitual activity patterns of the arms and legs.     

Effects of prolonged vibration to vastus intermedius muscle on force steadiness of knee extensor muscles during isometric force-matching task

Afferent inputs from Ia fibers in muscle spindles are essential for the control of force and prolonged vibration has been applied to muscle-tendon units to manipulate the synaptic input from Ia afferents onto α-motor neurons. The vastus intermedius (VI) reportedly provides the highest contribution to the low-level knee extension torque among the individual synergists of quadriceps femoris (QF). The purpose of the present study was to examine the effect of prolonged vibration to the VI on force steadiness of the QF. Nine healthy men (25.1 ± 4.3 years) performed submaximal force-matching task of isometric knee extension for 15 s before and after mechanical vibration to the superficial region of VI for 30 min. Target forces were 2.5%, 10%, and 30% of maximal voluntary contraction (MVC), and force steadiness was determined by the coefficient of variation (CV) of force. After the prolonged VI vibration, the CV of force at 2.5%MVC was significantly increased, but CVs at 10% and 30%MVCs were not significantly changed. The present study concluded that application of prolonged vibration to the VI increased force fluctuations of the QF during a very low-level force-matching task.