Length tension relationship | S&C Research
The length tension relationship is the observation that isometric force exerted by a a pilot study on the implications for the prevention of hamstring injuries. If the alteration of the eccentric length-tension relationship could be used as a primary risk factor of hamstring muscle strains, the findings of the present study. Rehabilitation and prevention strategies for hamstring injuries hamstring injury (hamstring-injured group [HG]) . flexion, the length-tension relationship.
The equipment has an input impedance of two mega ohms MW and a common-mode rejection ratio of MW. Two pairs of active surface electrodes with a diameter of Procedures Initially, research procedures were explained to volunteers and they signed a consent form agreeing to take part in the study.
Assessments were performed only in the non-dominant leg. The dominant leg was considered to be the leg chosen by the participant to kick a ball Two assessments were made three weeks apart.
All participants completed assessments as planned.
Prevention and rehabilitation of hamstring injuries
After determination of the lower limb to be assessed, the skin was shaved and cleaned with alcohol. Pairs of active electrodes were placed over the area of greatest muscle bulk of LV and BF of the non-dominant leg, according to direction of muscle fibers. A reference electrode was placed over the acromion. After electrode placement, participants were positioned in the dynamometer seat with 90 degrees of hip flexion.
Knee joint axis was aligned with the axis of rotation of the dynamometer and the adaptation to support the knee was positioned right above the lateral maleoli Figure 1.
Movement amplitude was limited to 0 to 90 degrees of knee flexion for all volunteers in the initial and final assessments. Volunteers were requested not to resist movement of the dynamometer lever. Movement of the dynamometer lever begun at 90 degrees of knee flexion, proceeded until complete extension and then passively returned to flexion. During the passive return, the isokinetic dynamometer software registered the torque generated by leg weight, isokinetic lever weight and the force produced by energy conserved in the elastic elements of muscles around the joint.
Such torque was defined as the passive torque of the flexion return movement.
During evaluation of passive torque, electromyographic data from LV and BF were registered simultaneously at a frequency of Hz to monitor of muscle activity and ensure that the test was performed passively. The two muscles were monitored because they are representative of activity of quadriceps and hamstrings. Only trials in which muscle activity did not exceed two standard deviations from the mean activity registered during rest were considered for analysis.
The program was executed right after each measure in order to guide the decision to accept or discard the test. After measurement of passive torque, a hamstring isokinetic performance test total isokinetic torque was conducted with equal positioning and test amplitude.
Length tension relationship
Only data referring to concentric contraction of hamstrings during knee flexion were used for analysis. Volunteers performed three sub maximal practice trials before data collection. After the test, data provided by the software regarding coefficients of variation of knee flexors torque and angle of peak torque measures in each repetition were analyzed. Repetitions presenting the greater and smaller values for angle of peak torque were discarded, and thus only five values were left for analysis.
This procedure intended to guarantee greater consistency of the data. Participants that needed to repeat the measure had a rest time of 10 minutes before performing the new trial.
Prevention and rehabilitation of hamstring injuries | Functional Resistance Training
In the total isokinetic torque assessment, the dynamometer software provides torque data with or without correction of the effects of gravity on the lever and body segments masses. As the torques generated by the effect of gravity are present in the passive torque and total isokinetic torque measurements, it would be annulated during data reduction by the subtraction of passive torque form total isokinetic torque.
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Therefore, data were collected without corrections for the effects of gravity. Data collected in the passive knee flexion measure and in the five most consistent repetitions of isokinetic hamstring assessment were transferred to a computer for analysis. Torque data were filtered with a fourth-order Butterworth filter and a 1.
Passive torque values for each joint angle were subtracted from the total torque curve obtained in the isokinetic assessment. The remaining torques represented the hamstring active torque. Statistical Analysis Paired t tests were used to analyze differences between mean peak active torque values of hamstrings between the two assessments test and retest.
Test-retest reliability of the measures described in this study was determined through Intraclass Correlation Coefficients ICC and corresponding confidence intervals. ICC values vary between 0. For clinical measures, reliability indices should exceed 0. The level of significance of 0. Table one presents the values of active peak torque of all volunteers for the first and second assessments.
The Intraclass Correlation Coefficient ICCthat was used to analyze test-retest reliablity of the hamstring active torque values obtained with the methods described in this study, demonstrated a correlation of 0. Once again results were not changed when male subjects were excluded from analysis. In this case the ICC was 0. Results demonstrated that the method analyzed in this study is reliable and can provide objective information about changes of the hamstrings torque-angle curve caused by alterations in muscle length.
Objective determination of muscle length is necessary in order to document the efficacy of therapeutic interventions aimed at modifying the structure of muscles5, Changes in the alignment of body segments and available range of movement of a joint are associated with alterations in muscle length8, Therefore, the good reliability indices found for hamstring active peak torque values indicates that this method can be useful in studies evaluating the efficacy of treatment programs designed to change muscle length in order to correct posture or increase range of movement.
Flexibility is measured as the maximum range of movement of a joint7,15, However, some authors suggest that a possible explanatory mechanism for flexibility increases after stretching or strengthening programs is related to increased tolerance to stretch, and not to changes in muscle length18, With increased tolerance to stretch, the individual allows the application of greater forces on the joint during a flexibility assessment, and thus greater values are obtained for range of movement Additionally, results obtained in the flexibility assessment can vary according to the force applied during the examination.
If the forces applied during examination are not controlled, factors as increased tolerance or a desire of the participant to demonstrate progress can interfere in the results of maximum range of movement.
Therefore, alterations of this measure may not reflect a real change in the structure of muscle tissue. As the analysis of the length-tension curve of a muscle does not have the inherent limitations of flexibility measures, it can indirectly inform about muscle length5,6,9. However, it is impossible to obtain direct measures of the length tension relationship in humans.
Therefore, angle of peak torque measures obtained through analysis of the torque-angle curve were used in studies that investigated the effects of interventions such as muscle strengthening5,23 and stretching7,24 on muscle length.
These studies used the measure of angle of peak torque to infer about muscle length, but reliability estimates were not reported, and the passive torque was not subtracted from total torque5,7.
However, as passive and active torques are both influenced by muscle length14 and in these studies an intervention protocol was used with the objective of increasing muscle length7, it is possible that no modifications in total torque would be observed at the end of the study, although passive resistance and the length-tension relationship may have suffered changes.
In case a decrease in passive torque and a shift of the length-tension curve active component in the direction of greater muscle length occur simultaneously, the angle of peak total active torque may not change because the two effects could cancel each other when total active torque is considered. Therefore, if the objective of a study is to evaluate the impact of an intervention on muscle length, the utilization of a method that does not take off passive torque to calculate the angle of peak torque can obscure study results.
One limitation of the present study refers to restrictions in the clinical applicability of the method to measure the angle of active peak torque, since it demands an isokinetic dynamometer and such equipment is not common in clinical settings.
Although the methodology proposed in this study minimizes patient and examiner interferences in the determination of measures, the investigation of muscle length assessment procedures that are suitable not only to scientific research but also to the clinical routine is necessary.
Another possible limitation is related to the characteristics of the sample, which had few male subjects. However, as demonstrated in statistical analysis, this factor did not have any interference in results. Results of the present study indicate that hamstrings produce maximum active torque around 25 degrees of knee flexion.
In the determination of angle of peak torque, the torque-angle curve profile is influenced by mechanical factors, related to muscle lever arms, and physiological factors, related to the length-tension relationship For some muscles, such as elbow flexors, there is a predominance of mechanical factors leverage in the determination of the torque-angle curve profile. Scand J Med Sci Sports ; Clinical predictors of time to return to competition and of recurrence following hamstring strain in elite Australian footballers.
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