Reliability of assessing trunk motor control using position and force tracking and stabilization tasks

System-based methods have been applied to assess trunk motor control in people with and without back pain, although the reliability of these methods has yet to be established. Therefore, the goal of this study was to quantify within- and between-day reliability using systems-based methods involving position and force tracking and stabilization tasks. Ten healthy subjects performed six tasks, involving tracking and stabilizing of trunk angular position in the sagittal plane, and trunk flexion and extension force. Tracking tasks involved following a one-dimensional, time-varying input signal displayed on a screen by changing trunk position (position tracking) or trunk force (force tracking). Stabilization tasks involved maintaining a constant trunk position (position stabilization) or constant trunk force (force stabilization) while a sagittal plane disturbance input was applied to the pelvis using a robotic platform. Time and frequency domain assessments of error (root mean square and H₂ norm, respectively) were computed for each task on two separate days. Intra-class correlation coefficients (ICC) for error and coefficients of multiple correlations (CMC) for frequency response curves were used to quantify reliability of each task. Reliability for all tasks was excellent (between-day ICC≥0.8 and CMC>0.75, within-day CMC>0.85). Therefore, position and force control tasks used to assess trunk motor control can be deemed reliable.     

Within- and between-day reliability of trunk mechanical behaviors estimated using position-controlled perturbations

Recent applications of position-controlled perturbation techniques to the human trunk have allowed separate estimation of intrinsic and reflexive trunk mechanical behaviors. These mechanical behaviors play an important role in spinal stability and have been associated with low back pain risk, yet the reliability of these measures remains unknown. Therefore, the objective of the current study was to assess within- and between-day reliability of several measures of trunk mechanical behaviors obtained from position-controlled trunk perturbations. A secondary objective was to assess if different harness designs, used to connect a participant to the perturbing device, influenced reliability. Data were analyzed from baseline measurements obtained from two previously published studies, and a third unpublished study. The total combined subject pool included 33 healthy young adults (17 M, 16 F). Relative and absolute reliability was quantified using intraclass correlation coefficients (ICCs) and standard errors of measurement (SEM), respectively. Within-day ICCs of intrinsic trunk stiffness (0.84-0.90) and effective mass (0.91-95) were excellent, and were generally higher than ICCs for reflex gain (0.55-0.85), maximum reflex force (0.65-0.85), and timing of maximum reflex force (0.48-0.86). Within-day ICCs (0.48-0.95) were consistently superior to between-day values (0.19-0.72). Improvements in harness design increased both within- and between-day reliability and reduced SEMs for most measures.     

Trunk response to sudden forward perturbations – Effects of preload and sudden load magnitudes,posture and abdominal antagonistic activation

Unexpected loading of the spine is a risk factor for low back pain. The trunk neuromuscular and kinematics responses are likely influenced by the perturbation itself as well as initial trunk conditions. The effect of four parameters (preload, sudden load, initial trunk flexed posture, initial abdominal antagonistic activity) on trunk kinematics and back muscles reflex response were evaluated. Twelve asymptomatic subjects participated in sudden forward perturbation tests under six distinct conditions. Preload did not change the reflexive response of back muscles and the trunk displacement; while peak trunk velocity and acceleration as well as the relative load peak decreased. Sudden load increased reflex response of muscles, trunk kinematics and loading variables. When the trunk was initially flexed, back muscles latency was delayed, trunk velocity and acceleration increased; however, reflex amplitude and relative trunk displacement remained unchanged. Abdominal antagonistic preactivation increased reflexive response of muscles but kinematics variables were not affected. Preload, initial flexed posture and abdominal muscles preactivation increased back muscles preactivity. Both velocity and acceleration peaks of the trunk movement decreased with preload despite greater total load. In contrast, they increased in the initial flexed posture and to some extent when abdominal muscles were preactivated demonstrating the distinct effects of pre-perturbation variables on trunk kinematics and risk of injury.     

Changes in the mechanical properties of the trunk in low back pain may be associated with recurrence

Exercise is one of the few effective treatments for LBP. Although exercise is often based on the premise of reduced spinal stiffness, trunk muscle adaptation may increase stiffness. This study developed and validated a method to assess trunk stiffness and damping, and tested these parameters in 14 people with recurring LBP and 17 pain-free individuals. Effective trunk stiffness, mass and damping were estimated with the trunk modeled as a linear second-order system following trunk perturbation. Equal weights (12–15% body weight) were attached to the front and back of the trunk via pulleys such that the trunk could move freely and no muscle activity was required to hold the weights. The trunk was perturbed by the unexpected release of one of the weights. Trunk kinematics and cable force were used to estimate system properties. Reliability was assessed in 10 subjects. Trunk stiffness was greater in recurrent LBP patients (forward perturbation only), but damping was lower (both directions) than healthy controls. Estimates were reliable and validated by accurately estimated mass. Contrary to clinical belief, trunk stiffness was increased, not reduced, in recurrent LBP, most likely due to augmented trunk muscle activity and changes in reflex control of trunk muscles. Although increased stiffness may aid in the protection of spinal structures, this may have long-term consequences for spinal health and LBP recurrence due to compromised trunk dynamics (decreased damping).     

The intrinsic stiffness of the in vivo lumbar spine in response to quick releases: Implications for reflexive requirements

Torso muscles contribute both intrinsic and reflexive stiffness to the spine; recent modeling studies indicate that intrinsic stiffness alone is sometimes insufficient to maintain stability in dynamic situations. The purpose of this study was to experimentally test this idea by limiting muscular reflexive responses to sudden trunk perturbations. Nine healthy males lay on a near-frictionless apparatus and were subjected to quick trunk releases from the neutral position into flexion or right-side lateral bend. Different magnitudes of moment release were accomplished by having participants contract their musculature to create a range of moment levels. EMG was recorded from 12 torso muscles and three-dimensional lumbar spine rotations were monitored. A second-order linear model of the trunk was employed to estimate trunk stiffness and damping during each quick release. Participants displayed very limited reflex responses to the quick load release paradigms, and consequently underwent substantial trunk displacements (>50% flexion range of motion and >70% lateral bend range of motion in the maximum moment trials). Trunk stiffness increased significantly with significant increases in muscle activation, but was still unable to prevent the largest trunk displacements in the absence of reflexes. Thus, it was concluded that the intrinsic stiffness of the trunk was insufficient to adequately prevent the spine from undergoing potentially harmful rotational displacements. Voluntary muscular responses were more apparent than reflexive responses, but occurred too late and of too low magnitude to sufficiently make up for the limited reflexes.     

Reliability of EMG measurements for trunk muscles during maximal and sub-maximal voluntary isometric contractions in healthy controls and CLBP patients.

The purpose of this study was to compare the reliability of trunk muscle activity measured by means of surface electromyography (EMG) during maximal and sub-maximal voluntary isometric contractions (MVC/sub-MVC) over repeated trials within-day and between-days in healthy controls and patients with chronic low back pain (CLBP). Eleven volunteers (six controls and five CLBP patients) were assessed twice with a 1-week interval. Surface EMG signals were recorded bilaterally from six trunk muscles. Intra-class correlation coefficients (ICC) and standard error of measurement as a percentage of the grand mean (%SEM) were calculated. MVC and sub-MVC showed excellent within-day reliability in both healthy controls and CLBP patients (ICC mean 0.91; range 0.75-0.98; %SEM mean 4%; range 1-12%). Sub-MVC for both groups between-days showed excellent reliability (ICC mean 0.88; range 0.78-0.97; %SEM mean 7%; range 3-11%). The between-days MVC for both groups showed trends towards lower levels of reliability (ICC mean 0.70; range 0.19-0.99; %SEM mean 17%; range 4-36%) when compared to sub-MVC. Findings of the study provide evidence that sub-MVC are preferable for amplitude normalisation when assessing EMG signals of trunk muscles between-days.     

Surface electromyography assessment of back muscle intrinsic properties.

The purpose of this study was to assess (1) the reliability and (2) the sensitivity to low back pain status and gender of different EMG indices developed for the assessment of back muscle weakness, muscle fiber composition and fatigability. Healthy subjects (men and women) and chronic low back pain patients (men only) performed, in a static dynamometer, maximal and submaximal static trunk extension tasks (short and long duration) to assess weakness, fiber composition and fatigue. Surface EMG signals were recorded from four (bilateral) pairs of back muscles and three pairs of abdominal muscles. To assess reliability of the different EMG parameters, 40 male volunteers (20 controls and 20 chronic low back pain patients) were assessed on three occasions. Reliable EMG indices were achieved for both healthy and chronic low back pain subjects when specific measurement strategies were applied. The EMG parameters used to quantify weakness and fiber composition were insensitive to low back status and gender. The EMG fatigue parameters did not detect differences between genders but unexpectedly, healthy men showed higher fatigability than back pain patients. This result was attributed to the smaller absolute load that was attributed to the patients, a load that was defined relative to their maximal strength, a problematic measure with this population. An attempt was made to predict maximal back strength from anthropometric measurements but this prediction was prone to errors. The main difficulties and some potential solutions related to the assessment of back muscle intrinsic properties were discussed.     

Evaluation of measurement strategies to increase the reliability of EMG indices to assess back muscle fatigue and recovery.

The purpose of this study was to assess different measurement strategies to increase the reliability of different electromyographic (EMG) indices developed for the assessment of back muscle impairments. Forty male volunteers (20 controls and 20 chronic low back pain patients) were assessed on three sessions at least 2 days apart within 2 weeks. Surface EMG signals were recorded from four pairs (bilaterally) of back muscles (multifidus at the L5 level, iliocostalis lumborum at L3, and longissimus at L1 and T10) while the subjects performed, in a static dynamometer, two static trunk extension tasks at 75% of the maximal voluntary contraction separated by a 60 s rest period: (1) a 30 s fatigue task and (2) a 5 s recovery task. Different EMG indices (based on individual muscles or averaged across bilateral homologous muscles or across all muscles) were computed to evaluate muscular fatigue and recovery. Intra-class correlation coefficient (ICC) and standard error of measurement (SEM) in percentage of the grand mean were calculated for each EMG variable. Reliable EMG indices are achieved for both healthy and chronic low back pain subjects when (1) electrodes are positioned on medial back muscles (multifidus at the L5 level and longissimus at L1) and (2) measures are averaged across bilateral muscles and/or across two fatigue tests performed within a session. The most reliable EMG indices were the bilateral average of medial back muscles (ICC range: 0.68-0.91; SEM range: 5-35%) and the average of all back muscles (ICC range: 0.77-0.91; SEM range: 5-30%). The averaging of measures across two fatigue tests is predicted to increase the reliability by about 13%. With regards to EMG indices of fatigue, the identification of the most fatigable muscle also lead to satisfactory results (ICC range: 0.74-0.79; SEM range: 21-26%). The assessment of back muscle impairments through EMG analysis necessitates the use of multiple electrodes to achieve reliable results.     

Ankle intrinsic stiffness changes with postural sway

In standing, the human body is inherently unstable and its stabilization requires constant regulation of ankle torque, generated by a combination of ankle intrinsic properties, peripheral reflexes, and central contributions. Ankle intrinsic stiffness, which quantifies the joint intrinsic properties, has been usually assumed constant in standing; however, there is strong evidence that it is highly dependent on the joint torque, which changes significantly with sway in stance. In this study, we examined how ankle intrinsic stiffness changes with postural sway during standing. Ten subjects stood on a standing apparatus, while subjected to pulse perturbations of ankle position. The mean torque of a short period before the start of each pulse was used as a measure of background torque. Responses with similar background torques were grouped together and used to estimate the parameters of an intrinsic stiffness model. Stiffness estimates were normalized to the critical stiffness and the background torque was transformed to the center of pressure location. We found that in most subjects, the normalized stiffness increased linearly with the movement of center of pressure towards the toes, with an average slope of 2.11 ± 0.80 1/m·rad. This modulation of ankle intrinsic stiffness seems functionally appropriate, since the intrinsic stiffness increases quickly, as the center of pressure moves toward the toes and the limits of stability. These large changes of ankle intrinsic stiffness with postural sway must be incorporated in any model of stance control.     

Between-days reliability of subjective and objective assessments of back extensor muscle fatigue in subjects without lower-back pain.

BACKGROUND: It is important to evaluate the reliability of common used methods of examining muscle fatigue from the lower back since the methods are used in patient evaluation. METHODS: To establish between-days reliability, ten subjects without lower-back pain performed a Sorensen test, a prone test for back extensor muscles against gravity, on three separate days. EMG was recorded from the L1 and L5 of the back extensor muscles. Fatigue was subjectively rated using a Borg CR-10 scale. Intraclass correlation coefficient, standard error of measurement and coefficient of variation were calculated from a one-way ANOVA. Percent agreement was also calculated. RESULTS: The study revealed good reliability for the slope for the total time (ICC 0.65-0.90), the initial and end median frequency (ICC 0.75-0.89), median frequencies at Borg ratings of three (ICC 0.63-0.88), five (ICC 0.62-0.84) and seven (ICC 0.67-0. 87), endurance time (ICC 0.89). The Borg ratings of the first minute agreed better than those of the second and the third. The Borg ratings at the second and the third test agreed to 40-80%, indicating a need for a practice session. CONCLUSION: The protocol used for assessing fatigue in the back extensor muscles proved to be reliable and is recommended for further use.     

Trunk muscle contributions of to L4–5 joint rotational stiffness following sudden trunk lateral bend perturbations

The purpose of this research was to investigate the contributions of individual muscles to joint rotational stiffness and total joint rotational stiffness about the lumbar spine’s L_4–5 joint prior to, and following, sudden dynamic lateral perturbations to the trunk. Kinematic and surface EMG data were collected while subjects maintained a kneeling posture on a robotic platform, while restrained so that motions caused by the perturbation were transferred to the pelvis, causing motion of the trunk and head. The robotic platform caused sudden inertial trunk lateral perturbations to the right or left, with or without timing and direction knowledge. An EMG-driven model of the lumbar spine was used to calculate the muscle forces and contributions to joint rotational stiffness during the perturbations. Data showed 95% and 106% increases in total joint rotational stiffness_, about the lateral bend and axial twist axes, when subjects had knowledge of the timing of the perturbation. Also, the contralateral muscles exhibited a significantly larger total joint rotational stiffness about the lateral bend axis, and earlier surface EMG responses, than the ipsilateral muscles. The results indicate that, when the timing of the perturbation was unknown, subjects relied more on delayed muscle forces following the perturbation to stiffen the L_4–5 joint.     

Reliability of assessing postural control during seated balancing using a physical human-robot interaction

This study evaluated the within- and between-visit reliability of a seated balance test for quantifying trunk motor control using input–output data. Thirty healthy subjects performed a seated balance test under three conditions: eyes open (EO), eyes closed (EC), and eyes closed with vibration to the lumbar muscles (VIB). Each subject performed three trials of each condition on three different visits. The seated balance test utilized a torque-controlled robotic seat, which together with a sitting subject resulted in a physical human-robot interaction (pHRI) (two degrees-of-freedom with upper and lower body rotations). Subjects balanced the pHRI by controlling trunk rotation in response to pseudorandom torque perturbations applied to the seat in the coronal plane. Performance error was expressed as the root mean square (RMSE) of deviations from the upright position in the time domain and as the mean bandpass signal energy (E_mb) in the frequency domain. Intra-class correlation coefficients (ICC) quantified the between-visit reliability of both RMSE and E_mb. The empirical transfer function estimates (ETFE) from the perturbation input to each of the two rotational outputs were calculated. Coefficients of multiple correlation (CMC) quantified the within- and between-visit reliability of the averaged ETFE. ICCs of RMSE and E_mb for all conditions were ≥0.84. The mean within- and between-visit CMCs were all ≥0.96 for the lower body rotation and ≥0.89 for the upper body rotation. Therefore, our seated balance test consisting of pHRI to assess coronal plane trunk motor control is reliable.     

A biomechanical model to simulate the effect of a high vertical loading on trunk flexural stiffness

A human trunk model was developed to simulate the effect of a high vertical loading on trunk flexural stiffness. A force–length relationship is attributed to each muscle of the multi-body model. Trunk stiffness and muscle forces were evaluated experimentally and numerically for various applied loads. Experimental evaluation of trunk stiffness was carried out by measuring changes in reaction force following a sudden horizontal displacement at the T10 level prior to paraspinal reflexes induction. Results showed that the trunk stiffness increases under small applied loads, peaks when the loads were further increased and decreases when higher loads are applied. A sensitivity analysis to muscle force–length relationship is provided to determine the model's limitations. This model pointed out the importance of taking into account the changes in muscle length to evaluate the effect of spinal loads beyond the safe limit that cannot be evaluated experimentally and to predict the trunk instability under vertical load.     

Lumbar loading during lifting: a comparative study of three measurement techniques.

Low back loading during occupational lifting is thought to be an important causative factor in the development of low back pain. In order to regulate spinal loading in the workplace, it is necessary to measure it accurately. Various methods have been developed to do this, but each has its own limitations, and none can be considered a "gold standard". The purpose of the current study was to compare the results of three contrasting techniques in order to gain insight into possible sources of error to which each is susceptible. The three techniques were a linked segment model (LSM), an electromyographic (EMG)-based model, and a neural network (NN) that used both EMG and inertial sensing techniques. All three techniques were applied simultaneously to calculate spinal loading when eight volunteers performed a total of eight lifts in a laboratory setting. Averaged results showed that, in comparison with the LSM, the EMG technique calculated a 25.5+/-33.4% higher peak torque and the NN technique a 17.3+/-10.5% lower peak torque. Differences between the techniques varied with lifting speed and method of lifting, and could be attributed to differences in anthropometric assumptions, antagonistic muscle activity, damping of transient force peaks by body tissues, and, specific to the NN, underestimation of trunk flexion. The results of the current study urge to reconsider the validity of other models by independent comparisons.     

Is the trunk movement more perturbed after an asymmetric than after a symmetric perturbation during lifting?

Low back injury is associated with sudden movements and loading. Trunk motion after sudden loading depends on the stability of the spine prior to loading and on the trunk muscle activity in response to the loading. Both factors are not axis-symmetric. Therefore, it was hypothesized that the effects on trunk dynamics would be larger after an asymmetric than after a symmetric perturbation. Ten subjects lifted a crate in which, prior to lifting, a mass was displaced to the front or to the side without the subjects being aware of this. Crate and subject movements, crate reaction forces and muscle activity were recorded. From this, the stability prior to the perturbation was estimated, and the trunk angular kinematics and moments at the lumbo-sacral joint were calculated. Both perturbations only minimally affected the trunk kinematics, although the stability of the spine prior to the lifting movement was higher in the sagittal plane than in the frontal plane. In both conditions the stability appeared to be sufficient to absorb the applied perturbation.     

Effects of trunk muscle fatigue and load timing on spinal responses during sudden hand loading

The purpose of this study was to investigate the responses of the spine during sudden loading in the presence of back and abdominal muscle fatigue, with a primary focus on the implications for spinal stability. Fifteen females were studied and each received sudden loads to the hands, at both known and unknown times. Participants received these loading trials (a) while rested, (b) with back muscle fatigue, and (c) with a combination of back and abdominal muscle fatigue. Measures were taken on the EMG activity of two trunk extensor and two abdominal muscles, and on the trunk angle and centre of pressure. A 3 × 2 Repeated Measures ANOVA was also performed. There were no preparations made prior to the perturbation even when it could be anticipated. However, the peak responses that followed were greater in the unexpected versus the expected condition. In addition, trunk muscle fatigue led to an increase in the baseline activity of the trunk muscles but no additional increase in activity just prior to loading. There was increased activation of both (opposing) muscle groups when only one muscle group was fatigued. Because the peak responses following the perturbation were enhanced in the unknown timing condition, preparations must have taken place prior to the anticipated perturbations, perhaps in other segments of the body that were not measured. Also, the load impact may not have been great enough to elicit large preparations. The heightened baseline activity with fatigue suggests that there may have been increased spinal stiffness whenever the spine was fatigued, and not just immediately prior to an impending perturbation. The increased activation of opposing muscle groups is evidence of increased cocontraction in response to fatigue, possibly to maintain stability with decreasing coordination.     

Role of reflex gain and reflex delay in spinal stability--a dynamic simulation

The goal of this study was to investigate the role of reflex and reflex time delay in muscle recruitment and spinal stability. A dynamic biomechanical model of the musculoskeletal spine with reflex response was implemented to investigate the relationship between reflex gain, co-contraction, and stability in the spine. The first aim of the study was to investigate how reflex gain affected co-contraction predicted in the model. It was found that reflexes allowed the model to stabilize with less antagonistic co-contraction and hence lower metabolic power than when limited to intrinsic stiffness alone. In fact, without reflexes there was no feasible recruitment pattern that could maintain spinal stability when the torso was loaded with 200N external load. Reflex delay is manifest in the paraspinal muscles and represents the time from a perturbation to the onset of reflex activation. The second aim of the study was to investigate the relationship between reflex delay and the maximum tolerable reflex gain. The maximum acceptable upper bound on reflex gain decreased logarithmically with reflex delay. Thus, increased reflex delay and reduced reflex gain requires greater antagonistic co-contraction to maintain spinal stability. Results of this study may help understanding of how patients with retarded reflex delay utilize reflex for stability, and may explain why some patients preferentially recruit more intrinsic stiffness than healthy subjects.     

Individuals with non-specific low back pain use a trunk stiffening strategy to maintain upright posture

There is increasing evidence that individuals with non-specific low back pain (LBP) have altered movement coordination. However, the relationship of this neuromotor impairment to recurrent pain episodes is unknown. To assess coordination while minimizing the confounding influences of pain we characterized automatic postural responses to multi-directional support surface translations in individuals with a history of LBP who were not in an active episode of their pain. Twenty subjects with and 21 subjects without non-specific LBP stood on a platform that was translated unexpectedly in 12 directions. Net joint torques of the ankles, knees, hips, and trunk in the frontal and sagittal planes as well as surface electromyographs of 12 lower leg and trunk muscles were compared across perturbation directions to determine if individuals with LBP responded using a trunk stiffening strategy. Individuals with LBP demonstrated reduced peak trunk torques, and enhanced activation of the trunk and ankle muscle responses following perturbations. These results suggest that individuals with LBP use a strategy of trunk stiffening achieved through co-activation of trunk musculature, aided by enhanced distal responses, to respond to unexpected support surface perturbations. Notably, these neuromotor alterations persisted between active pain periods and could represent either movement patterns that have developed in response to pain or could reflect underlying impairments that may contribute to recurrent episodes of LBP.     

Fear of Movement Is Related to Trunk Stiffness in Low Back Pain

Background

Psychological features have been related to trunk muscle activation patterns in low back pain (LBP). We hypothesised higher pain-related fear would relate to changes in trunk mechanical properties, such as higher trunk stiffness.

Objectives

To evaluate the relationship between trunk mechanical properties and psychological features in people with recurrent LBP.

Methods

The relationship between pain-related fear (Tampa Scale for Kinesiophobia, TSK; Photograph Series of Daily Activities, PHODA-SeV; Fear Avoidance Beliefs Questionnaire, FABQ; Pain Catastrophizing Scale, PCS) and trunk mechanical properties (estimated from the response of the trunk to a sudden sagittal plane forwards or backwards perturbation by unpredictable release of a load) was explored in a case-controlled study of 14 LBP participants. Regression analysis (r ²) tested the linear relationships between pain-related fear and trunk mechanical properties (trunk stiffness and damping). Mechanical properties were also compared with t-tests between groups based on stratification according to high/low scores based on median values for each psychological measure.

Results

Fear of movement (TSK) was positively associated with trunk stiffness (but not damping) in response to a forward perturbation (r² = 0.33, P = 0.03), but not backward perturbation (r² = 0.22, P = 0.09). Other pain-related fear constructs (PHODA-SeV, FABQ, PCS) were not associated with trunk stiffness or damping. Trunk stiffness was greater for individuals with high kinesiophobia (TSK) for forward (P = 0.03) perturbations, and greater with forward perturbation for those with high fear avoidance scores (FABQ-W, P = 0.01).

Conclusions

Fear of movement is positively (but weakly) associated with trunk stiffness. This provides preliminary support an interaction between biological and psychological features of LBP, suggesting this condition may be best understood if these domains are not considered in isolation.     

Trunk stiffness increases with steady-state effort

Trunk stiffness was measured in healthy human subjects as a function of steady-state preload efforts in different horizontal loading directions. Since muscle stiffness increases with increased muscle activation associated with increasing effort, it is believed that coactivation of muscles helps to stiffen and stabilize the trunk. This paper tested whether increased steady-state preload effort increases trunk stiffness. Fourteen young healthy subjects each stood in an apparatus with the pelvis immobilized. They were loaded horizontally at directions of 0, 45, 90, 135 and 180 degrees to the forward direction via a thoracic harness. Subjects first equilibrated with a steady-state load of 20 or 40% of their maximum extension effort. Then a sine-wave force perturbation of nominal amplitude of 7.5 or 15% of maximum effort and nominal period of 250ms was applied. Both the applied force and subsequent motion were recorded. Effective trunk mass and trunk-driving point stiffness were estimated by fitting the experimental data to a second-order differential equation of the trunk dynamic behavior. The mean effective trunk mass was 14.1kg (s.d.=4.7). The trunk-driving point stiffness increased on average 36.8% (from 14.5 to 19.8N/mm) with an increase in the nominal steady-state preload effort from 20 to 40% (F(1,13)=204.96, p<0.001). There was a smaller, but significant variation in trunk stiffness with loading direction. The measured increase in trunk stiffness probably results from increased muscle stiffness with increased muscle activation at higher steady-state efforts.