Extrapolation of time series of EMG power spectrum parameters in isometric endurance tests of trunk extensor muscles

The aim of the present study was to test the viability of using short isometric contractions of trunk extensor muscles to perform an assessment of their endurance capacity. To this aim two types of analysis were performed. First, electromyographic (EMG) mean power frequency (MPF) slopes with respect to time as estimated over shorter fixed periods were compared to slopes estimated over the full contraction period of a contraction sustained until the endurance time. Second, the relationship between MPF slope estimates as estimated over various periods and the endurance time of the muscle group was evaluated. Five subjects performed three isometric trunk endurance tests at 25%, 50% and 75% of their maximum voluntary contraction (MVC), respectively. EMG signals of the left and right multifidus, iliocostalis and longissimus muscles were continuously recorded and spectral parameters were calculated. The MPF appeared to decrease consistently during all endurance tests. The extrapolation from a MPF time series of half the estimated contraction period to the time series of the complete contraction period gave reasonable results at all force levels, when data from several electrode locations were incorporated in a single slope estimate (mean or steepst slope). The accuracy of the prediction of trunk extensor endurance on the basis of these parameters describing the MPF time series over half the estimated contraction period was satisfactory. Endurance time predictions from yet shorter periods were unreliable

Control of support limb muscles in recovery after tripping in young and older subjects

Older people fall more often after tripping than young people due to a slower development of mechanical responses. This might be due to age-related changes in muscle properties, but also to changes in motor control. The purpose of the present study was to determine whether (a) timing and sequencing of muscle activation and (b) the magnitude and rate of development of muscle activation in recovery after a trip differs between young and older subjects. We focused on the support limb, as it contributes to recovery after tripping by counteracting the forward angular momentum. Ten young (25 years) and seven older (68 years) men and women walked over a platform and were tripped several times at different points in the gait cycle. Kinematics and EMG of the support limb muscles were measured. After tripping, rapid EMG responses (60-80 ms) were observed in hamstring and triceps surae muscles in both young and older subjects. A slightly increased delay (11 ms) was found only in the soleus muscle of the older subjects. The muscle activity patterns (timing and sequencing) were similar in young and older subjects, but the magnitude and rate of development of muscle activity were significantly lower in older subjects. Especially the lower rate of development of muscle activation in the support limb of older subjects is likely to reduce the rate of force generation, which can contribute to inadequate recovery responses and falls. © Springer-Verlag 2004

Concurrent validity of questions on arm, shoulder and neck symptoms of the RSI QuickScan

Purpose: The objective was to determine the concurrent validity of questions on arm, shoulder and neck symptoms of an Internet-based questionnaire. In addition, the inter-observer reliability of physical examinations by occupational physicians was investigated. Methods: A total of 160 employees of a Dutch occupational health service were approached, of which 106 participated. Right after the assessment of arm, shoulder and neck symptoms using a self-administered questionnaire, each participant was examined by two occupational physicians. The presence of symptoms in the past 7 days was compared to the physical examinations. The participation of two occupational physicians allowed us to study also the inter-observer reliability. Results: Overall, the concurrent validity of the symptom questions of the questionnaire can be defined as poor to moderate with κ values between 0.16 and 0.53. Detecting the presence of symptoms (

Handle height and expectation of cart movement affect the control of trunk motion at movement onset in cart pushing

As unexpected sudden unloading of the trunk may cause low-back injury, the objective of the present study was to investigate whether handle height and the expectation of cart movement in pushing affect trunk control at movement onset. Eleven healthy male participants pushed a 200-kg cart with handles at shoulder and hip heights. The cart would suddenly move when externally released (externally triggered condition) or when static friction was overcome (self-initiated condition). Before self-initiated cart movement, trunk stiffness and muscle activity were significantly higher than before an externally triggered onset at comparable pushing force. Lower muscle activity and trunk stiffness at shoulder height compared with the hip height before the onset resulted in higher trunk inclination after the onset. In conclusion, higher preparatory activation of trunk muscles serves to increase trunk stiffness in anticipation of cart movement and may reduce the impact of the perturbation associated with the onset of cart movement. Statement of Relevance: Sudden cart movement in pushing causes an unexpected unloading perturbation to the trunk. This perturbation was shown to cause uncontrolled trunk movement, which may explain how pushing tasks can be associated with low-back injury. Effects of handle height and awareness of the subjects of the possible cart movement suggest directions for prevention. © 2011 Taylor & Francis

Trunk muscle recruitment patterns in patients with low back pain enhance the stability

Study Design. A comparative study of trunk muscle recruitment patterns in healthy control subjects and patients with chronic low back pain was conducted. Objective. To assess trunk muscle recruitment in patients with low back pain. Summary of Background Data. Conflicting evidence has been reported on the level and pattern of trunk muscle recruitment in patients with low back pain. The disparities can be explained partly by methodologic differences. It was hypothesized that trunk muscle recruitment patterns may be altered in patients with low back pain to compensate for reduced spinal stability. Methods. For this study, 16 patients with low back pain and 16 matched control subjects performed slow trunk motions about the neutral posture and isometric ramp contractions while seated upright. Ratios of electromyographic amplitudes and estimated moment contributions of antagonist over agonist muscles and of segmentally inserting muscles over muscles inserting on the thorax and pelvis only were calculated. In addition, model simulations were performed to assess the effect of changes in muscle recruitment on spinal stability. Results. The ratios of antagonist over agonist, and of lumbar over thoracic erector spinae electromyographic amplitude and estimated moment contributions were greater in the patients than in the control subjects. The simulation model predicted that these changes would effectively increase spinal stability. Conclusions. Trunk muscle recruitment patterns in patients with low back pain are different from those in healthy control subjects. The differences are likely to be functional with respect to enhancement of spinal stability in the patients

Hamstrings co-activation in ACL-deficient subjects during isometric whole-leg extensions

It has been reported that anterior cruciate ligament (ACL)-deficient subjects increase the level of hamstrings activation and this has been interpreted as a means to cope with increased anterior tibial laxity in the knee. This study aimed to establish to what extent co-activation strategies in ACL-deficient subjects are load level and knee angle dependent. Eleven chronic ACL-deficient and 15 control subjects were positioned in a range of postures and asked to exert a feedback controlled vertical ground reaction force (GRF; 30, 60% and maximum), while horizontal forces were not constrained. Surface electromyography of the leg muscles and GRF were measured. In postures with the knee over and in front of the ankle, ACL-deficient subjects generated, respectively, 2.4 and 5.1% MVC more hamstrings activation than control subjects. Enhanced hamstrings co-activation in ACL-deficient subjects was more apparent in extended than in flexed knee angles. For both ACL-deficient and control subjects, hamstrings co-activation was larger in males than in females. It is concluded that ACL-deficient subjects show a task dependent increase in hamstrings co-activation, but its clinical significance remains to be shown. © Springer-Verlag 2009

Longitudinal and transversal displacements between triceps surae muscles during locomotion of the rat

The functional consequences of differential muscle activation and contractile behavior between mechanically coupled synergists are still poorly understood. Even though synergistic muscles exert similar mechanical effects at the joint they span, differences in the anatomy, morphology and neural drive may lead to non-uniform contractile conditions. This study aimed to investigate the patterns of activation and contractile behavior of triceps surae muscles, to understand how these contribute to the relative displacement between the one-joint soleus (SO) and two-joint lateral gastrocnemius (LG) muscle bellies and their distal tendons during locomotion in the rat. In seven rats, muscle belly lengths and muscle activation during level and upslope trotting were measured by sonomicrometry crystals and electromyographic electrodes chronically implanted in the SO and LG. Length changes of muscle-tendon units (MTUs) and tendon fascicles were estimated based on joint kinematics and muscle belly lengths. Distances between implanted crystals were further used to assess longitudinal and transversal deformations of the intermuscular volume between the SO and LG. For both slope conditions, we observed differential timing of muscle activation as well as substantial differences in contraction speeds between muscle bellies (maximal relative speed 55.9 mm s-1). Muscle lengths and velocities did not differ significantly between level and upslope locomotion, only EMG amplitude of the LG was affected by slope. Relative displacements between SO and LG MTUs were found in both longitudinal and transversal directions, yielding an estimated maximal length change difference of 2.0 mm between their distal tendons. Such relative displacements may have implications for the force exchanged via intermuscular and intertendinous pathways