Laboratory Experiments: Measurement and Interpretation of Muscle Force Vectors, Moments, and Power for Knee Flexion

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

2a. Neatly plot on one sheet of graph paper Fx versus 2, Fy versus 2, and F1 versus 2 for maximum isometric contractions for the knee joint angles of 165, 150, 135, 120, 105, and 90. Distinguish between the three lines.

What interpretation did you make of this data?

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)

2b. Neatly plot on one sheet of graph paper Fx versus 2, Fy versus 2, and F1 versus 2 for maximum isokinetic contractions for the knee joint angles of 165, 150, 135, 120, 105, and 90 for the three angular velocities. Use the same scale for this plot as was used in 2a. For the nine lines, distinguish between the three parameters and three angular velocities.

For 2a. and 2b., explain the relationships that exist between knee joint angle (2) and the force of muscle contraction (F1), joint turning component (Fx) of muscular contraction, and joint compressive component (Fy) of muscular contraction. Are these relationships similar between the isometric and isokinetic contractions? Explain. Is there a pattern, going from the isometric contractions to faster and faster isokinetic contractions? In other words, is there a relationship between angular velocity and the three force vectors? Explain.

What interpretation did you make of this data?

The same pattern existed in the isokinetic contractions as was evident in the 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.

What interpretation did you make of this data?

The same pattern existed in the isokinetic contractions as was evident in the 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.

Neatly plot on one sheet of graph paper the force of muscle contraction (F1) versus muscle length (OI) for the isometric and three isokinetic contractions. Distinguish between the four lines. Can muscle force-velocity and length-tension relationships justify these results? Explain.

What interpretation did you make of this data?

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.

Even though the isokinetic dynamometer maintained a constant angular velocity (as opposed to what typically is the case in an isotonic contraction) the following relationships were evident: 1. Force-velocity: With increased velocity there was a general trend for the knee joint flexors to be able to exert a decreased maximum force of contraction. This is also typical of what is seen in maximum isotonic contractions.

Even though the isokinetic dynamometer maintained a constant angular velocity (as opposed to what typically is the case in an isotonic contraction) the following relationships were evident: 2. Length-tension: A curvilinear relationship between maximum force of contraction and muscle length was evident. This is somewhat similar to what is typical at the muscle fiber level in which maximum force of contraction is dependent on the interaction between the overlap of the actin and myosin myofilaments and the tension associated with theseries and parallel elastic components

4. Neatly plot on one sheet of graph paper the mechanical advantage (moment arm) of the hamstrings to the knee joint center versus F1 for the isometric and isokinetic contractions for the knee joint angles of 165, 150, 135, 120, 105, and 90. Distinguish between the four lines. Is there an inverse relationship between mechanical advantage and F1? Explain.

What interpretation did you make of this data?

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.

5. Neatly plot on one sheet of graph paper the mechanical advantage (moment arm) of the hamstrings to the knee joint center versus and the muscle length (OI) of the hamstring muscles. What is the relationship between mechanical advantage and hamstring length? Explain.

What interpretation did you make of this data?

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 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.

6. Neatly plot on one sheet of graph paper [(Fx)(AI)] versus 2 and [(Fc)(AC)] versus 2 for the isokinetic contractions of 30/second [(/6) (radians/second)] for the knee joint angles of 165, 150, 135, 120, 105, and 90. Note that clockwise moments about the knee joint center (A) are negative and counterclockwise moments are positive. Distinguish between the two lines. An isokinetic dynamometer is said to provide “accommodating resistance.” Explain this relationship in regard to constant angular velocity.

What interpretation did you make of this data?

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 shank. This is to be expected since the angular velocity of the isokinetic dynamometer is constant for all settings.

Neatly plot on one sheet of graph paper power versus angular velocity for the three isokinetic contraction conditions for the knee joint angles of 165, 150, 135, 120, 105, and 90. What relationship exists between power and angular velocity? Explain. What relationship exists between maximum power in each of the three isokinetic contraction conditions and the joint angle at which it occurred? What are plausible explanations for this relationship?

What interpretation did you make of this data?

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)

9. 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?

Force Platform Lecture