Static loading of the knee joint results in modified single leg landing biomechanics.

BACKGROUND: External loading of the ligamentous tissues induces mechanical creep, which modifies neuromuscular response to perturbations. It is not well understood how ligamentous creep affects athletic performance and contributes to modifications of knee biomechanics during functional tasks. HYPOTHESIS/PURPOSE: The purpose of this study was to examine the mechanical and neuromuscular responses to single leg drop landing perturbations before and after passive loading of the knee joint. METHODS: Descriptive laboratory study. Male (n = 7) and female (n = 14) participants' (21.3 ± 2.1 yrs., 1.69 ± 0.09 m, 69.3 ± 13.0 kg) right hip, knee, and ankle kinematics were assessed during drop landings performed from a 30 cm height onto a force platform before and after a 10 min creep protocol. Electromyography (EMG) signals were recorded from rectus femoris (RF), vastus lateralis (VL), vastus medialis (VM), semimembranosus (SM), and biceps femoris (BF) muscles. The creep protocol involved fixing the knee joint at 35° during static loading with perpendicular loads of either 200 N (males) or 150 N (females). Maximum, minimum, range of motion (ROM), and angular velocities were assessed for the hip, knee, and ankle joints, while normalized EMG (NEMG), vertical ground reaction forces (VGRF), and rate of force development (RFD) were assessed at landing using ANOVAs. Alpha was set at 0.05. RESULTS: Maximum hip flexion velocity decreased (p < 0.01). Minimum knee flexion velocity increased (p < 0.02). Minimum knee ad/abduction velocity decreased (p < 0.001). Ankle ROM decreased (p < 0.001). aVGRF decreased (p < 0.02). RFD had a non-significant trend (p = 0.076). NAEMG was significant between muscle groups (p < 0.02). CONCLUSION: Distinct changes in velocity parameters are attributed to the altered mechanical behavior of the knee joint tissues and may contribute to changes in the loading of the leg during landing.

Repetitive trunk loading leads to faster trunk movement in response to external perturbation.

The purpose of the present study was to examine trunk movement pattern responses to mechanical perturbation before and after two different repetitive trunk flexion-extension loading schemes. Spatial and temporal parameters were studied to understand the trunk recovery from an anteriorly directed perturbation. Eighteen male and female subjects (18-27 yrs) participated in active and passive trunk flexion-extension, performed seven days apart. Subjects performed 60 trunk flexion-extension repetitions in each condition. Subjects either volitionally moved their trunks (active condition) or relaxed while a dynamometer controlled the movements (passive condition). Trunk perturbations occurred before and immediately after two 30 repetition sessions. Initial responses included latency measures of trunk displacement and peak trunk velocity (VP). Temporal measures included perturbation onset to initial trunk movement (TD), movement initiation to VP (TMIPV), and perturbation onset to VP (TPPV). Recovery measures included peak recovery velocity (VPR), recovery time (TR), velocity slope (VS), and recovery slope (RS). Differences between loading sessions were present for TPPV (p < .05), TR (p < .05), and RS (p < .05). Overall, the results indicate repetitive loading leads to lower resistance to perturbation, but faster recovery from perturbation although no differences to active or passive repetitive loading was observed.

A potential mechanism by which torque output is preserved in cerebral palsy during fatiguing contractions of the knee extensors.

INTRODUCTION: The purpose of this study was to compare agonist and antagonist electromyography (EMG) during an isokinetic fatigue protocol in subjects with cerebral palsy (CP) and typical development (TD). METHODS: Nine individuals with CP and 11 with TD completed 50 repetitions of maximum concentric knee extensions (KE) and flexions (KF) at 60°/second. RESULTS: Rate of decline in peak torque for KE was significantly less in CP compared with TD. Rate of decline in agonist EMG was not significantly different between groups, but the rate of decline in antagonist EMG was significantly greater in CP. There were no differences between groups for KF. CONCLUSIONS: Declining agonist EMG occurred in parallel with declining antagonist hamstring activity in CP, decreasing the relative opposing force and resulting in a lesser decline in net torque. This finding illustrates a potential mechanism by which net torque is preserved in those with CP who are inherently weaker.

Comparison of trunk muscle reflex activation patterns between active and passive trunk flexion-extension loading conditions.

The aim of the present study was to determine the effects of trunk flexion-extension loading on the neuromuscular reflexive latencies and amplitude responses of the trunk musculature. Eighteen male and female subjects (18-27yrs) participated in active and passive trunk flexion extension, performed ∼7days apart. Subjects performed 60 trunk flexion-extension repetitions. Surface electromyography (EMG) was collected bilaterally from paraspinal and abdominal muscles. In the active condition, subjects volitionally moved their trunks, while in the passive condition the dynamometer controlled the movements. The trunk was perturbed before and immediately after 30 repetitions. Latency of muscle onset, latency of first peak, latency of maximum peak, and peak EMG amplitude were evaluated. No differences between conditions, sides, or perturbation session were apparent. Overall latencies were shorter in females (p<.05) and abdominal muscles compared to paraspinals (p<.05). Thoracic paraspinal muscle amplitudes were greater than all other muscles (p<.05). Based upon the present results, the neuromuscular system engages trunk flexor muscles prior to the paraspinals in order to provide possible stabilization of the trunk when flexor moments are generated. Overall, the results indicate no difference in response of the neuromuscular system to active or passive repetitive loading.

Passive trunk loading influences muscle activation during dynamic activity.

INTRODUCTION: Repetitive loading of the low back tissues induces tension relaxation with a corresponding variation in the myoelectric response of the neuromuscular system, which may influence low back health. The purpose of this study was to observe trunk muscle activities before and after a passive cyclic trunk flexion-extension exercise. METHODS: Nineteen participants were subjected to sagittal plane loading of the low back tissues. These individuals performed toe-touching before and over 1 hour after load termination. RESULTS: Lumbar paraspinal activity cessation occurred significantly sooner over time during lumbar and pelvic flexion post-loading, as compared with pre-loading. Lumbar paraspinal activity then re-initiated later in lumbar extension. There was no change in peak or average muscle activity during the procedures. CONCLUSIONS: It is apparent that passive loading of the viscoelastic lumbar tissues results in a prolonged neuromuscular adaptation, followed by possible neuromuscular compensation sustained within 1 hour of load termination.

Passive repetitive loading of the lumbar tissues influences force output and EMG during maximal efforts.

Loading of the low back tissues induces tension-relaxation in the viscoelastic connective tissues. The extent to which repetitive loading influences the muscle activation and subsequent muscle force production has not been fully explored. The purpose of this study was to examine the myoelectric activity of the trunk muscles during maximal flexion and extension exertions before and after a passive trunk flexion-extension protocol. Nineteen subjects performed three trials of maximal efforts in trunk flexion and extension while seated in an upright position. Surface electromyography (EMG) recordings were collected bilaterally from paraspinal (thoracic, TP, lumbar LP), rectus abdominis (RA), and external oblique muscles. A 10-minute passive trunk flexion-extension protocol was used to repetitively load the lumbar tissues at a rate of 0.17 rad/s, through the subjects' range of motion. The main findings included a significant reduction in moment output during extension efforts (p < 0.05) with significant reductions in the average EMG from the TP and LP muscles during extension (p < 0.05). In flexion, peak and average EMGs were also significantly reduced (p < 0.05). The results indicate a significant reduction in the ability of the trunk extensors to output force, but this may be due to the increased compliance of the connective tissues rather than modified neuromuscular signals to the paraspinal muscles. However, neuromuscular changes were apparent in the TP and RA muscles suggesting a modified control mechanism was present.

Trunk muscle activation during sub-maximal extension efforts.

Neuromuscular fatigue of the trunk musculature, particularly lumbar paraspinal and abdominal muscles, is important in when evaluating motor control of the trunk. Activation of agonists and antagonists trunk muscles was hypothesized to change during sub-maximal isometric trunk extension efforts. Thirteen women were positioned in 30 degrees of trunk flexion and performed maximal voluntary isometric contraction in trunk extension against an isokinetic dynamometer. One of two sub-maximal efforts (50% and 70%) was performed to induce neuromuscular fatigue on two different days. Surface electromyography of the lumbar paraspinal (LP), rectus abdominis, and external oblique muscles was recorded during each session. Torque output, median frequency of the power density spectrum, and normalized integrated electromyography were analyzed using repeated measures analysis of variance to evaluate trends in the data over time. Paraspinal muscles showed signs of fatigue in both conditions (p<0.05) Abdominal activity did not increase during the 70% condition, but showed a non-significant trend (p=0.07), coinciding with the reduced median frequency of LP muscles. The neuromuscular system modulates its motor control strategy to identify the muscle activation levels necessary to maintain force output. This information is necessary in the evaluation of contributing mechanisms to trunk stability in furthering preventative and rehabilitative treatments.

Trunk extensor fatigue influences trunk muscle activities during walking gait.

Prolonged physical activities may introduce risks for low back injury due to the adapted neuromuscular response of the system once neuromuscular fatigue is present. Trunk extensor muscles were fatigued in fourteen healthy women to observe myoelectric changes in the trunk musculature during walking trials performed before and after fatigue conditions. Sub-maximal efforts at 50% and 70% maximal trunk extension effort were performed until the pre-determined levels could not be sustained. Surface electromyography (EMG) from lumbar paraspinal (LP), rectus abdominis (RA), external oblique (EO) muscles were recorded during fatigue conditions and pre and post fatigue walking trials. Infrared sensors were used to time participants as they walked. Footswitches attached to the right heel were used to record heel contacts, and were time synchronized with the EMG signals. LP and RA activity burst peaks shifted in time at contralateral heel contacts (p<0.05) in the 70% condition, while RA amplitude increased (p<0.05) and EO burst peak temporal shifts (p<0.05) were present in the 50% condition. Reduced ability of the paraspinal muscles to support the trunk after fatigue onset may be a contributing factor, lending to diminished spine stiffness in attenuating ground reaction forces.

Interaction of viscoelastic tissue compliance with lumbar muscles during passive cyclic flexion-extension.

Human and animal models using electromyography (EMG) based methods have hypothesized that viscoelastic tissue properties becomes compromised by prolonged repetitive cyclic trunk flexion-extension which in turn influences muscular activation including the flexion-relaxation phenomenon. Empirical evidence to support this hypothesis, especially the development of viscoelastic tension-relaxation and its associated muscular response in passive cyclic activity in humans, is incomplete. The objective of this study was to examine the response of lumbar muscles to tension-relaxation development of the viscoelastic tissue during prolonged passive cyclic trunk flexion-extension. Activity of the lumbar muscles remained low and steady during the passive exercise session. Tension supplied by the posterior viscoelastic tissues decreased over time without corresponding changes in muscular activity. Active flexion, following the passive flexion session, elicited significant increase in paraspinal muscles EMG together with increase in the median frequency. It was concluded that reduction of tension in the lumbar viscoelastic tissues of humans occurs during cyclic flexion-extension and is compensated by increased activity of the musculature in order to maintain stability. It was also concluded that the ligamento-muscular reflex is inhibited during passive activities but becomes hyperactive following active cyclic flexion, indicating that moment requirements are the controlling variable. It is conceived that prolonged routine exposure to cyclic flexion minimizes the function of the viscoelastic tissues and places increasing demands on the neuromuscular system which over time may lead to a disorder and possible exposure to injury.

A proposed method for determining peak power in the jump squat exercise.

In recent years a great deal of research has been published using peak power (PP) in the jump squat (JS) exercise as a measure of athletic performance. However, no standardized method for the determination of PP exists at this time to accurately evaluate this variable. Our proposed method (PM) for determining PP (PPPM) in the JS uses the product of vertical ground reaction forces and velocity of the center of mass of both the subject and the external resistance of a loaded Olympic bar. Fifteen male subjects with a mean age of 27 +/- 3 years, weight of 78 +/- 17 kg, and height of 175 +/- 10 cm participated in this study. PP was measured in the JS at five different testing loads (30%, 35%, 40%, 45%, and 50% body weight) based on methods commonly discussed in the literature to compare PP results of previous methods to those obtained using the PM. Paired t-tests at different load levels were used for statistical analysis with an overall alpha = 0.05. The average PP among five testing loads, measured by the PM, was 3782 +/- 906 W. PP derived from the product of force and velocity of the bar alone was 72% lower than PPPM at 1057 +/- 243 W (P < 0.0001). The PP estimated by the product of bar velocity and vertical ground reaction forces of the bar plus the subject was 8% higher than PPPM at 4100 +/- 844 W (P = 0.0001). Our results indicate that using the methods traditionally reported in the literature may cause an overestimation of PP during athletic performance. Using the PM in future research will facilitate test validity and enable the generalization of results outside the scope of specific research projects.

Flexion-relaxation response to cyclic lumbar flexion.

BACKGROUND: The epidemiology classify cyclic lumbar flexion as a risk factor for the development of cumulative low back disorder. Experimental biomechanical data confirming the epidemiology in humans are lacking. The purpose of this study, therefore, is to investigate the flexion relaxation response to sustained cyclic lumbar flexion in humans. METHODS: Twelve normal college aged males performed deep cyclic lumbar flexion at 0.1 Hz for 9 min while recording lumbar paraspinal electromyogram and kinematic data. FINDINGS: The most important observation of the study was the significant increase in the angular excursion of myoelectric silence during the deep part of the flexion phase. The observed increase in myoelectric silence consisted of earlier cessation of EMG during flexion and delayed activation of trunk extensors during extension. EMG magnitude, during flexion, increased approximately 30% over trials (P < 0.0001), and a smaller (10%), but, significant (P < 0.02) EMG magnitude increase was also observed during the extension phase of the cycle. Spasms, an indication of micro damage to viscoelastic tissues, were sporadic and appeared more frequently later in the session and mostly during the silent period. INTERPRETATIONS: It was concluded that increased myoelectric silence during prolonged cyclic flexion-extension demonstrates an enhanced flexion-relaxation phenomenon which reduces lumbar stability and may be detrimental to low back health. The presence of spasms confirm that sustained cyclic lumbar flexion results in micro damage in the viscoelastic tissues. Overall, a neuromuscular disorder was evoked due to a relatively short period of unloaded cyclic lumbar flexion.