The influence of torque and velocity on erector spinae muscle fatigue and its relationship to changes of electromyogram spectrum density

The influence of contraction force and velocity during isokinetic contractions on the development of fatigue in the erector spinae muscle was studied. Seven male subjects performed a series of 250 contractions at 25% and 50% of their isometric maximal voluntary contraction (MVC) at 40 and 80°·s^–1. Fatigue defined as a decrease of the contractile capacity of the muscles was studied by means of a 15-s maximal test-contraction following the exercise. Both the initial force and the force decrement during the test-contraction were studied. Surface electromyogram (EMG) signals of the main tracts of the erector spinae muscle were recorded. The frequency content was studied by calculating the zero-crossing rate for the signals obtained during dynamic contractions and by means of fast Fourier transformation for the test contraction. After the 50% MVC dynamic contractions the initial force during the postexercise test-contraction was significantly lower than after the 25% MVC contractions. No significant influence of contraction velocity on fatigue development was found. The force decrement during the test-contraction did not depend on the experimental conditions. The EMG amplitude indicated that the subjects were better able to relax their muscles during the counter movement (flexion) at high forces and high velocities compared to the other experimental conditions. The frequency content of the EMG signals during the dynamic contractions and the postexercise test-contraction showed only very weak relationships with fatigue. Therefore, spectrum EMG parameters as determined in the present study do not seem suitable as indicators of muscle fatigue as a consequence of dynamic contractions of trunk extensor muscles.     

Upper trapezius muscle mechanomyographic and electromyographic activity in humans during low force fatiguing and non-fatiguing contractions

The purposes of this study were firstly to compare and investigate localised fatigue in the upper trapezius muscle in various arm positions as assessed by mechanomyographic (MMG) and surface electromyographic (EMG) signals and secondly to study the effects of different normalisation methods on MMG and EMG signals during non-fatiguing and fatiguing low level isometric contractions. The MMG, EMG and rate of perceived exertion were recorded from 11 subjects in five arm positions (0° abduction and 0° flexion, 45° and 90° flexion, 45° and 90° abduction) with different bilateral arm loads during 3 s for non-fatiguing (0–0.5–1 kg hand-load) and 3 min for fatiguing contractions (1 kg hand-load). The root mean square (RMS), average rectified value (ARV), mean power frequency (MNF), and median power frequency (MDF) of the MMG and EMG signals were computed and normalised with respect to the initial values obtained in the current arm position or in the reference position (0° abduction and 0° flexion) corresponding to the normal postural activity of the trapezius muscle. For fatiguing contractions, differences in magnitude of the increase in the RMS or ARV and decrease in the MNF or MDF were observed for EMG and MMG. The MMG amplitude and spectral changes followed the subjective sensation of fatigue and were not correlated to their EMG counterparts, suggesting that they may reflect different phenomena. For non-fatiguing contractions, normalisation to the current arm position entailed the loss of dynamic amplitude changes suggesting that a single reference contraction in the middle part of the range of movement is enough for proper normalisation of EMG and MMG signals. For fatiguing contractions, normalisation of the EMG and MMG to some extent can lead to a misleading interpretation. Assessment of the upper trapezius muscle by means of MMG may be valuable in ergonomics. Electronic Publication     

Adaptation of the precentral cortical command to elbow muscle fatigue

The control exerted by individual motor cortical cells on their fatigued target muscles was assessed by analyzing the discharge patterns and electromyographic (EMG) postspike effects of cortical cells in monkeys making repeated forceful, but submaximal, isometric flexions of the elbow to produce fatigue. Two monkeys were trained to perform self-paced isometric contractions (for longer than 2 s) at forces greater than 35% maximal contraction, with three sets of 20 consecutive contractions; the first and last sets were at the same force level. Pairs of EMG electrodes were implanted in the biceps brachii, brachioradialis, and triceps brachii. The cortical cell discharges were modulated with the active and passive movements of the elbow and produced consistent EMG postspike effects during isometric contraction. Muscle fatigue was assessed as a statistically significant (P<0.05) drop in the mean power frequency of the EMG power spectrum in one or both flexors in the last set of contractions. Clear signs of muscular fatigue occurred in 20 different experimental sessions. Before fatigue, cortical cells were classified as phasic-tonic (18), phasicramp (three), or tonic (five). Twenty cells briskly fired to passive elbow extension, and 9 also responded to passive flexion. Only 6 cells showed a decreased discharge to passive extension. A 22–30% increase in the contraction force produced a higher discharge frequency in 13 cells, and a lower frequency in 5 cells. All cells exerted EMG postspike effects in their target muscles: 20 cells facilitated the flexors, and some of these also inhibited (3 cells) or cofacilitated (5 cells) the extensor; the other 6 cells had mixed effects: 5 of them inhibited at least one flexor, and 1 cell only facilitated the extensor. Most cells (24/26) still produced EMG postspike effects in their target muscles during fatigue, and the number of facilitated muscles increased: 21 cells facilitated the flexors, and 12 of them cofacilitated the extensor. Only 3 cells still inhibited the flexors and were tonic cells. The cortical cell firing frequency increased during fatigue in 13 cells and decreased in 8 cells. Increases involved 10 cells excited by passive elbow extension. Fourteen cells showed parallel changes in firing frequency with fatigue and force, and 9 of these cells facilitated both extensors and flexors in fatigue. Increases were found in 8 cells, decreases in 5 cells and no change in 1 cell. As muscle afferents provide substantial information to cortical cells, which in turn establish functional linkages with their target muscles before and during fatigue, the changes in cell firing frequencies during fatigue demonstrate the active participation of the motor cortex in the control of compensation for the peripheral adjustments concomitant with muscle fatigue.     

The influences of muscle fibre proportions and areas upon EMG during maximal dynamic knee extensions

This study is an investigation of the relationship between muscle morphology and surface electromyographic (EMG) parameters [mean frequency of the power spectrum (MNF), signal amplitude (root mean square, RMS) and the signal amplitude ratio (SAR; i.e. the ratio between the RMS level during the passive part of the contraction cycle and the RMS level during the active part of the contraction cycle)] during 100 maximal dynamic knee extensions at 90° · s⁻¹. Each contraction cycle comprised of 1 s of active knee extension and 1 s of passive knee flexion. The surface EMG was recorded from the vastus lateralis muscle. Twenty clinically healthy subjects participated in the study, and muscle biopsy samples of the vastus lateralis were obtained from 19 of those subjects. The relationships between muscle morphology and EMG were investigated at three stages of the test: initially, during the fatigue phase (initial 40 contractions), and at the endurance level (the final 50 contractions). Major findings on correlations are that SAR and MNF tended to correlate positively with the proportion of type 1 fibres, and RMS correlated positively with the proportion of type 2 muscle fibres. The muscle fibre areas showed little correlation with the EMG variables under investigation. The results of the present study showed that the three EMG variables of a dynamic endurance test that were investigated (RMS, MNF and SAR) were clearly correlated with the proportions of the different fibre types, but only to a small extent with fibre areas. These findings contradict some of the theoretical models of the EMG, especially for parameters in the frequency domain. Accepted: 17 June 1999     

Differences in the electromyographic activity of the hamstring muscles during maximal eccentric knee flexion

This study investigated the effects of the knee joint angle and angular velocity on hamstring muscles’ activation patterns during maximum eccentric knee flexion contractions. Ten healthy young males (23.4 ± 1.3 years) performed eccentric knee flexion at constant velocities of 10, 60, 180, and 300 deg/s in random order. The eccentric knee flexion torque and the surface electromyographic (EMG) activity of the biceps femoris (BF), semitendinosus (ST), and semimembranosus (SM) muscles were measured. The results of torque during 10 deg/s were lower than the faster velocities. No significant change was found in eccentric torque output and the EMG amplitude with change in the faster test velocities, although those values showed a decreasing tendency as the knee approached extension. Furthermore, the EMG amplitude of the BF decreased significantly as the knee approached extension, although the EMG activity of the ST and SM remained constant. These results suggest that the neural inhibitory mechanism might be involved in decreasing in maximal voluntary force and hamstring muscles activation toward the knee extension during high-velocity eccentric movement and therefore subjects have difficulties to maintain high eccentric force level throughout the motion. Moreover, the possible mechanism reducing the BF muscle activation as the knee approaches extension was architectural differences in the hamstring muscles, which might reflect each muscle’s function.     

Functional anatomical studies of the elbow movements. I. Electromyographic (EMG) analysis.

Activities of the elbow flexors and extensors during the movement of the elbow flexion and extension were analyzed in six normal human volunteers by electromyography (EMG). In the flexors, the majority of the muscles showed EMG activities during both the flexion and extension phases, although patterns and amplitudes of EMG activities varied from individual to individual. The biceps brachii always became less active when the forearm was in pronation. In the extensors, increase of EMG activities was observed at the period of the maximum elbow extension in the majority of cases, while no EMG activity was shown throughout the movement in some cases. During the elbow movement except at the maximum extension, the triceps brachii was almost inactive and some of their three heads, in particular the long head, often showed no EMG activity. In contrast, the anconeus was usually active, sometimes showing strong EMG activity.     

Experimental muscle pain changes feedforward postural responses of the trunk muscles

Many studies have identified changes in trunk muscle recruitment in clinical low back pain (LBP). However, due to the heterogeneity of the LBP population these changes have been variable and it has been impossible to identify a cause-effect relationship. Several studies have identified a consistent change in the feedforward postural response of transversus abdominis (TrA), the deepest abdominal muscle, in association with arm movements in chronic LBP. This study aimed to determine whether the feedforward recruitment of the trunk muscles in a postural task could be altered by acute experimentally induced LBP. Electromyographic (EMG) recordings of the abdominal and paraspinal muscles were made during arm movements in a control trial, following the injection of isotonic (non-painful) and hypertonic (painful) saline into the longissimus muscle at L4, and during a 1-h follow-up. Movements included rapid arm flexion in response to a light and repetitive arm flexion-extension. Temporal and spatial EMG parameters were measured. The onset and amplitude of EMG of most muscles was changed in a variable manner during the period of experimentally induced pain. However, across movement trials and subjects the activation of TrA was consistently reduced in amplitude or delayed. Analyses in the time and frequency domain were used to confirm these findings. The results suggest that acute experimentally induced pain may affect feedforward postural activity of the trunk muscles. Although the response was variable, pain produced differential changes in the motor control of the trunk muscles, with consistent impairment of TrA activity.     

Signal characteristics of EMG during fatigue

Summary Electromyographic (EMG) activity of m. rectus femoris muscle was registered from young male and female subjects during maintained isometric knee extension at 60% of maximal voluntary contraction. The following EMG parameters were analyzed for the entire fatigue time: integrated EMG (IEMG), averaged motor unit potential (AMUP) and power spectral density function (PSDF). The results indicated a slight but continuous rise of IEMG during the fatigue period. AMUP showed sensitivity to fatigue with increase in amplitude, rise time, and number of spikes counted. PSDF was also easily affected by fatigue so that the total power density curve was shifted towards lower frequencies with a high frequency decay. The mean power frequency decreased linearily as a function of fatigue time. The findings suggest that in addition to natural recruitment of new motor units the fatigue is characterized by marked reduction in the conduction velocities of action potential along the used muscle fibers.     

Rapid slowing of maximal finger movement rate: fatigue of central motor control?

Exploring the limits of the motor system can provide insights into the mechanisms underlying performance deterioration, such as force loss during fatiguing isometric muscle contraction, which has been shown to be due to both peripheral and central factors. However, the role of central factors in performance deterioration during dynamic tasks has received little attention. We studied index finger flexion/extension movement performed at maximum voluntary rate (MVR) in ten healthy subjects, measuring movement rate and amplitude over time, and performed measures of peripheral fatigue. During 20 s finger movements at MVR, there was a decline in movement rate beginning at 7–9 s and continuing until the end of the task, reaching 73% of baseline (P < 0.001), while amplitude remained unchanged. Isometric maximum voluntary contraction force and speed of single ballistic flexion and extension finger movements remained unchanged after the task, indicating a lack of peripheral fatigue. The timing of finger flexor and extensor EMG burst activity changed during the task from an alternating flexion/extension pattern to a less effective co-contraction pattern. Overall, these findings suggest a breakdown of motor control rather than failure of muscle force generation during an MVR task, and therefore that the mechanisms underlying the early decline in movement rate are central in origin.     

Motor patterns of a fast voluntary postural task in man: trunk extension in standing

The co-ordination between muscles controlling the primary movement and associated postural adjustments during fast trunk extension movements was studied in six male subjects. Myoelectrical activity (EMG) was recorded from antagonistic muscle pairs at the trunk, hip, knee and ankle. Horizontal displacements of the upper trunk, hip and knee were recorded with an opto-electronic system (Selspot). A backward displacement of the trunk was accompanied by a forward displacement of the hip and knee. In general, the trunk started to move 30 ms before the hip and knee. Muscle activity first appeared in the ankle extensors (soleus/gastrocnemius) up to 150 ms prior to onset of prime mover muscles (trunk extensor, erector spinae; and hip extensors/knee flexors, hamstrings). This pre-activation was seldom followed by any detectable ankle joint movement. Prime mover muscles were activated simultaneously followed by the hip extensor gluteus maximus. Time to activation of muscles braking the movement (rectus abdominis, rectus femoris and vastus lateralis) was correlated with the amplitude of the primary movement (r = 0.63 0.75, P less than 0.01). Onset of activity in vastus lateralis was highly correlated with the amplitude of the forward displacement of the knee (r = 0.93, P less than 0.01). An associated postural adjustment appeared as an ankle flexion accompanied by activity in the ankle flexor tibialis anterior, and often also in the ankle extensors. This co-activation of antagonistic ankle muscles can under certain conditions have interesting functional implications for the control of posture.     

Immediate changes in feedforward postural adjustments following voluntary motor training

There is limited evidence that preprogrammed feedforward adjustments, which are modified in people with neurological and musculoskeletal conditions, can be trained and whether this depends on the type of training. As previous findings demonstrate consistent delays in feedforward activation of the deep abdominal muscle, transversus abdominis (TrA), in people with recurrent low back pain (LBP), we investigated whether training involving voluntary muscle activation can change feedforward mechanisms, and whether this depends on the manner in which the muscle is trained. Twenty-two volunteers with recurrent LBP were randomly assigned to undertake either training of isolated voluntary activation of TrA or sit-up training to activate TrA in a non-isolated manner to identical amplitude. Subjects performed a trunk perturbation task involving arm movement prior to and after training, and surface and fine-wire electromyography (EMG) recordings were made from trunk and arm muscles. Following a single session of training of isolated voluntary activation of TrA, onset of TrA EMG was earlier during rapid arm flexion and extension, to more closely resemble the responses in pain-free individuals. The magnitude of change in TrA EMG onset was correlated with the quality of isolated training. In contrast, all of the abdominal muscles were recruited earlier during arm flexion after sit-up training, while onset of TrA EMG was further delayed during arm extension. The results provide evidence that training of isolated muscle activation leads to changes in feedforward postural strategies, and the magnitude of the effect is dependent on the type and quality of motor training.     

Co-ordination of a simple voluntary multi-joint movement with postural demands: trunk extension in standing man

The interaction between primary movements and associated postural adjustments was studied during trunk extension movements in standing man. Six healthy male subjects performed one series of six consecutive fast trunk extensions. Movements were recorded with an optoelectronic system (Selspot). Angular displacements, velocities and accelerations at the ankle, knee, hip and trunk were calculated. Trunk inclination was measured as the sum of pelvic tilt and spine flexion. The spatial and temporal coordination patterns chosen by the subjects to complete the task were compared. The self-selected trunk movement amplitude ranged 14-30 degrees for all subjects. The individual amplitude was maintained within 4-7 degrees over the six consecutive trials. Peak velocity of the primary trunk movement was highly correlated with movement amplitude (r = 0.73, P less than 0.01), whereas peak acceleration was not. Trunk extension, was the result of a simultaneous backward pelvic tilt and spine extension, together with hip extension and knee flexion. Knee flexion became increasingly important at larger movement amplitudes since hip extension as well as spine extension was limited. Associated postural adjustments occurred mainly as ankle flexion. In conclusion, individuals can accurately reproduce the multi-joint co-ordination pattern seen during a voluntary trunk extension movement. Movements at the ankle appear to be controlled to counteract the backward shift of the centre of gravity caused by the primary movement. Knee and ankle movements seem to be independently controlled in the preparatory phase, whereas they are closely linked later during the primary movement. It is suggested that the interaction between knee and ankle movements is necessary for optimal equilibrium control during trunk extension movements in standing.     

Dynamic changes in corticospinal excitability during motor imagery

 We investigated temporal changes in the amplitudes of motor-evoked potentials (MEPs) induced by transcranial magnetic stimulation over the left motor cortex during motor imagery. Nine subjects were instructed to imagine repetitive wrist flexion and extension movements at 1 Hz, in which the flexion timing was cued by a tone signal. Electromyographs (EMGs) were recorded from the first dorsal interosseous, flexor carpi radialis and extensor carpi radialis muscles of the right hand, and magnetic stimulation was delivered at 0, 250, 500 and 750 ms after the auditory cue. On average, the evoked EMG responses were larger in the flexor muscle during the phase of imagined flexion than during extension, whilst the opposite was true for the extensor muscle. There were no consistent changes in the amplitudes of MEPs in the intrinsic hand muscle (first dorsal interosseous). The EMG remained relaxed in all muscles and did not show any significant temporal changes during the test. The H-reflex in the flexor muscle was obtained in four subjects. There was no change in its amplitude during motor imagery. These observations lead us to suggest that motor imagery can have dynamic effects on the excitability of motor cortex similar to those seen during actual motor performance. Received: 23 July 1998 / Accepted: 26 October 1998     

Neuromuscular and biomechanical coupling in human cycling

This study exploited the alterations in pedal speed and joints kinematics elicited by changing crank length (CL) to test how altered task mechanics during cycling will modulate the muscle activation characteristics in human rectus femoris (RF), biceps femoris long head (BF), soleus (SOL) and tibialis anterior (TA). Kinetic (torque), kinematic (joint angle) and muscle activity (EMG) data were recorded simultaneously from both legs of 10 healthy adults (aged 20–38 years) during steady-state cycling at ~60 rpm and 90–100 W with three symmetrical CLs (155 mm, 175 mm and 195 mm). The CL elongation (CL) resulted in similar increases in the knee joint angles and angular velocities during extension and flexion, whilst the ankle joint kinematics was significantly influenced only during extension. CL resulted in significantly reduced amplitude and prolonged duration of BF EMG, increased mean SOL and TA EMG amplitudes, and shortened SOL activity time. RF activation parameters and TA activity duration were not significantly affected by CL. Thus total SOL and RF EMG activities were similar with different CLs, presumably enabling steady power output during extension. Higher pedal speeds demand an increased total TA EMG activity and decreased total BF activity to propel the leg through flexion into extension with a greater degree of control over joint stability. We concluded that the proprioceptive information about the changes in the cycling kinematics is used by central neural structures to adapt the activation parameters of the individual muscles to the kinetic demands of the ongoing movement, depending on their biomechanical function.     

Trunk extensor endurance and its relationship to electromyogram parameters

Summary The present study was designed to investigate the relationship between muscle performance and electromyogram (EMG) parameters of the trunk extensor muscles in the development of fatigue. Nine subjects performed continuous isometric trunk extensions at 25% and 40% maximal voluntary contraction. The EMG signals of the longissimus thoracis, iliocostalis lumborum, multifidus and latissimus dorsi muscles were recorded. The EMG amplitude (RA-EMG) appeared to increase consistently during the contractions in all muscles, whereas the mean power frequency (MPF) showed a fairly consistent decrease during the contractions. The time constants of the exponential change of the RA-EMG and of the MPF were related to the endurance time. The prediction of endurance based on both EMG parameters appeared to yield better results than the prediction based on the relative force. In particular the time constants of the MPF changes of the multifidus and longissimus muscles appeared to be good predictors of endurance time. The consistency of the spectrum shift of EMG appeared to coincide with a reduced variability of the activation of the muscle involved.     

Intra-abdominal pressure increases during exhausting back extension in humans

The aim of this study was to investigate in humans the effect of maintained spinal load on the intra-abdominal pressure (IAP), low back kinematics and trunk muscle activity. This study consisted of two endurance tests for the low back muscles performed 3 weeks apart. Nine healthy subjects participated in the study. In the first test (upright-test), the subject had to pull with the back muscles at a constant force in an upright position, and in the second test (incline-test), the subject had to resist a constant forward pulling force while standing with a 45° inclination of the back in relation to vertical. The IAP, rate of perceived exertion and electromyogram (EMG) from the erector spinae, iliocostalis lumborum, rectus abdominus and the external and internal oblique muscles were measured using surface electrodes. There was no significant difference in endurance time between the two tests. Both tests showed a significant increase in EMG amplitude with time for all muscles except the erector spinae muscle. A decrease in the median frequency for the erector spinae muscle was found indicating fatigue, and since no increase in EMG amplitude was found a decrease in force output from the muscle must be assumed. The IAP increased significantly during both tests. There was a considerable variation between the subjects in the size of this development, but the pattern was the same for all subjects. In conclusion it was found that trunk extension until exhaustion initiates an increase in the activity of the abdominal muscles and an increase in the IAP as the low back muscles become fatigued. Electronic Publication     

Effect of antagonist muscle fatigue on knee extension torque

The effect of hamstring fatigue on knee extension torque was examined at different knee angles for seven male subjects. Before and after a dynamic flexion fatigue protocol (180° s^–1, until dynamic torque had declined by 50%), maximal voluntary contraction extension torque was measured at four knee flexion angles (90°, 70°, 50° and 30°). Maximal torque generating capacity and voluntary activation of the quadriceps muscle were determined using electrical stimulation. Average rectified EMG of the biceps femoris was determined. Mean dynamic flexion torque declined by 48±11%. Extensor maximal voluntary contraction torque, maximal torque generating capacity, voluntary activation and average rectified EMG at the four knee angles were unaffected by the hamstring fatigue protocol. Only at 50° knee angle was voluntary activation significantly lower (15.7%) after fatigue (P<0.05). In addition, average rectified EMG before fatigue was not significantly influenced by knee angle. It was concluded that a fatigued hamstring muscle did not increase the maximal voluntary contraction extension torque and knee angle did not change coactivation. Three possible mechanisms may explain the results: a potential difference in recruited fibre populations in antagonist activity compared with the fibres which were fatigued in the protocol, a smaller loss in isometric torque generating capacity of the hamstring muscle than was expected from the dynamic measurements and/or a reduction in voluntary activation.     

Effects of gravitational forces on single joint arm movements in humans

We have examined the kinematics and muscle activation patterns of single joint elbow movements made in the vertical plane. Movements of different amplitudes were performed during a visual, step-tracking task. By adjusting shoulder position, both elbow flexion and extension movements were made under three conditions: (a) in the horizontal plane, (b) in the vertical plane against gravity, and (c) in the vertical plane with gravity. Regardless of the gravitational load, all movements were characterized by time symmetric velocity profiles. In addition, no differences were found in the relationships between movement duration, peak velocity, and movement amplitude in movements with or against gravity. The pattern of muscle activation was influenced however, by the gravitational load. Both flexion and extension movements made with gravity were characterized by a reciprocally organized pattern of muscle activity in which phasic agonist activity was followed by phasic antagonist activity. Flexion and extension movements made against gravity were characterized by early phasic antagonist activity occurring at about the same time as the initial agonist burst. These findings suggest that EMG patterns are modified in order to preserve a common temporal structure in the face of different gravitational loads.     

Vibration-induced changes in movement-related EMG activity in humans

The effect of muscle tendon vibration during voluntary arm movement was studied in normal humans. Subjects made alternating step flexion and extension movements about the elbow. A small vibrator was mounted over either the biceps or the triceps muscle and vibration was applied during flexion or extension movements. The vibrator was turned off between movements. After a period of practice, subjects learned the required movements and were able to make them with their eyes closed. Application of vibration to the muscle antagonist to the movement being performed produced an undershoot of the required end-movement position. The undershoot was 20-30% of the total movement amplitude. In contrast, vibration of the muscle agonist to the movement resulted in no change in movement end position. The vibration-induced undershoot was associated with an increase in the EMG activity of the vibrated (antagonist) muscle and a resultant increase in the ratio of the antagonist to agonist EMG activity. The increase in antagonist EMG produced by the vibration occurred with a latency of approximately 60 ms from vibration onset. The observed results are consistent with vibration-induced activation of muscle spindle receptors in the lengthening muscle during movement. It is suggested that, during movement, the sensitivity of the spindle receptors in the shortening muscle is decreased and the information concerning limb position during movement comes primarily from the lengthening muscle.     

静态姿势下均匀负重和非负重时躯干和表面肌电活动

目的观察躯干均匀负重和非负重状态下静态前屈和后伸时腰背部、腹部及臀中肌的肌电活动规律和运动学特征。方法 6位正常健康的男性受试者直腿站立于特殊设计的试验框架和平台内做静态前屈和后伸的负重和非负重动作,每次试验持续4 s,重复3次。动作时,记录双侧腰臀部的10块肌肉:腹直肌、腹外斜肌、竖脊肌、多裂肌、臀中肌的肌电活动,以及三维角度运动轨迹和足底力系。计算标准化肌电、腰部躯干角和足底中心压力的位移。将所得数据作常规的统计分析。结果负重和非负重前屈时,都是背侧肌活动较大(10.47～16.94)。非负重后伸时,腹侧肌活动也较大;负重后,背侧肌(3.70～17.95)和臀中肌(6.64～11.52)活动增加,腹肌活动减少(10.66～4.18)。后伸时,躯干的闪动次数随负重增加而增多,在3D角上增加1.55次;负重后,足底中心压力向前后的移动多于侧方移动,前屈的前后方移动(14.60)多于后伸的移动(7.65)。结论后伸状态增加了背侧肌的活动,而且多裂肌更明显;同时,腰部角位移度和闪动次数增加,特别在后伸提重时更为显著。