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Lecture XIII Assignment • Abstract a research article that utilizes a force platform to collect data. The article should be related to your academic area of interest. Be prepared to give a 10 minute presentation in the next class. Concentrate your presentation and abstract on the methods, procedures, and technical aspects of the use of the force platform. • Solve problems 1 and 2 in 5.6 Problems Based on Kinetic and Kinematic Data in our class text. • Work on completing the force platform laboratory experiment. It is due at the time of the final examination.
Lecture Topics • Examination on models and calculation of joint reaction forces and net muscle moments • Review of results from internal model lab • Force platform lecture • Force platform experiment
Review of Results from Internal Model Lab Internal Model ExperimentInternal Model SubjectInformation and Data Sheet
Limitations Associated with Experiments • Single subject design • Precision in the positioning of the subject • Motivation level of the subject throughout the experiment • Internal model representing reality • Others -------------------------------------------------------- LIMITED GENERALIZABILITY OF RESULTS
Experiment 1 Comparison of Cadaver-Based Measurements of the Moments ofFoot and Shank Segments (Multi-Segment System) and Experimentally Measured Moments
Calculated torque from anthropometry plus bar approximates measured static and free fall torque of shank, foot, and bar.
Experiments 2 and 3 Raw Data Files
Experiments 2 and 3 Biomechanics of Maximum Isometric Knee Flexion Torque for Various Knee Joint Angles Biomechanics of Maximum Isokinetic Knee Flexion Torque for Various Knee Joint Angles and Angular Velocities
Experiments 2 and 3 – Measured and Calculated Isometric Parameters
A relatively small proportion of muscle contraction goes into turning the joint. • Most of the force of muscle contraction goes into compressing the joint, especially when its mechanical advantage is poor.
When the muscle is at its greatest length (largest knee joint angle), it exerted substantially greater contractile force. • The combination of muscle length and mechanical advantage resulted in a relatively constant turning component (Fx) over the range of knee joint positions.
Experiments 2 and 3 – Measured and Calculated Isokinetic Parameters
Experiments 2 and 3 – Measured and Calculated Isokinetic Parameters (continued)
Experiments 2 and 3 – Measured and Calculated Isometric Parameters (continued)
Experiments 2 and 3 – Measured and Calculated Isokinetic Parameters (continued)
Experiments 2 and 3 – Measured and Calculated Isokinetic Parameters (continued)
Experiments 2 and 3 – Measured and Calculated Isokinetic Parameters (continued)
The same pattern existed in the isokinetic contractions as was evident in these isometric contractions (see isometric graph). • A similar pattern is evident among these angular velocities.
An inverse relationship between force of isokinetic contraction and angular velocity was expected. This was not evident at the 165 degree knee angle for these angular velocities, but was evident in the middle knee angles.
The same pattern existed in the isokinetic contractions as was evident in these isometric contractions (see isometric graph). • A similar pattern is evident among these angular velocities.
An inverse relationship between force of isokinetic contraction and angular velocity was expected. This was evident for these angular velocities.
A dynamic relationship existed between muscle length and its ability to exert maximum contractile force for all angular velocities tested.
As muscle length increased, there was an increase in its ability to exert force for all angular velocities. This relationship was relatively constant between 0.37 and 0.4 meters, but appeared curvilinear and increased substantially after achieving a muscle length of 0.4 meters.
The dynamic relationship between muscle length and its ability to exert maximum force of contraction is likely to be related to the a) overlap of actin and myosin myofilaments in the sarcomeres and b) series elastic component of skeletal muscle when length is greater than “resting” length.
For all angular velocities (including the isometric condition), there was an inverse relationship between muscle moment arm and the muscle’s ability to exert maximum force of contraction.
The mechanical advantage of an increased muscle moment arm was not able to compensate for a corresponding decrease in maximum force of muscle contraction.
A curvilinear relationship existed between muscle length and muscle moment arm. • As the muscle moment arm increased, the muscle length decreased.
There appears to a compensatory mechanism in place. The mechanical advantage associated with a longer muscle moment arms is detracted by the loss in ability of the muscle to exert force due to decreases in its length. The opposite is also evident.
For all angular velocities, the torque experienced by the arm of the isokinetic dynamometer was equal and opposite to the torque experienced by the subject’s leg. This is to be expected since the angular velocity of the isokinetic dynamometer is constant for all settings.
Power is the product of torque and angular velocity. • It was previously interpreted that there was a general inverse relationship between angular velocity and the ability of muscle to generate maximum torque.
A direct relationship between the muscle’s ability to generate power and angular velocity is evident. • Of the two factors in determining power (torque and angular velocity), angular velocity appears to dominate.
What effects could internal anatomical differences in the locations of muscle origins and insertions and bone (lever) lengths have on internally measured forces and torques? In other words, what effects would changes in AI, AB, and OB have on internally measured forces and torques? How would these effects manifest themselves in external measures of forces and torques?(See next slide for model figure.)
Influence of Changes in AI, AB, and OB?Other Changes? Definition of Variables F1 – maximum force of hamstring contraction Fc – maximum force applied at pad on mechanical arm Fx –vector component of F1 perpendicular to rigid shaft of shank; turning component of F1 at collective insertion (I) of hamstrings Fy – vector component of F1 parallel to rigid shaft of shank; joint compressive component of F1 at collective insertion (I) of hamstrings 1 – angle between shaft of shank and F1 at I 2 – angle at knee joint center (A) formed by shafts of the thigh and shank AI – distance between collective insertion of hamstrings (I) and knee joint center (A); AI = _______ meters AC – distance from center of cuff to knee joint center (A); AC = _______ meters AB – horizontal distance from knee joint center (A) to a point B located directly above the collective origin (O) of the hamstrings; AB = _______ meters OI – hamstring muscle length OB – distance from O to B; OB = _______ meters OP – distance from O to point P on shaft of shank, OP is parallel to AB AS – perpendicular line from A to O AM – a line from point A that intersects OI, forming a right angle; moment arm of F1 (not drawn on figure)
Several assumptions have been provided about this Hypothetical Model. List at least five additional assumptions which cause this model to be hypothetical as opposed to an actual model. For each of these assumptions, conjecture as to its potential influence on the results of the experiment (i.e., major or minor) and why you think this way.
Additional Assumptions • Two-dimensional versus three-dimensional model • Use of cadaver data • Other?
