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408 Lab 7

# 408 Lab 7 - EXSC 408L Introduction to Biomechanics Lab 7...

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EXSC 408L Fall ‘11 Introduction to Biomechanics Lab 7 - Joint Kinetics Page 1 of 12 Lab #7 - Joint Kinetics and Internal Forces Purpose: The objective of this lab is to understand how to calculate net joint forces (NJFs) and net joint moments (NJMs) from force data. Upon completion of this lab you will: Know how to use a step-by-step method to generate the correct free body diagrams (FBDs) and mass acceleration diagrams (MADs) for the lower extremity during the time of peak vertical reaction force of a movement. Understand the difference between a net joint force (NJF) and a net joint moment (NJM). Know how to use force plate data, along with FBDs, to calculate NJFs and NJMs at the ankle, knee, and hip. Know how to determine which sets of muscles are activated during a movement. Introduction: In human movement, there are typically multiple moments acting on the system, which can be opposed by muscle forces. A moment is generated when a force acts on an object at a distance from its center of mass. We want to determine the overall mechanical demand placed on the muscles that cross a joint due to these moments. In other words, we want to know how much muscular force is needed for a given movement. I. NET JOINT MOMENTS A net joint moment is the minimum moment required at a joint to obtain the observed kinematics. A net joint force is the minimum force required at a joint to obtain the observed kinematics. In a static analysis, there is no motion at the joint, so the sum of the moments at the elbow must be zero. There is no rotation. This does not mean that the NJM is zero! Σ M = 0 M elbow = NJM - M arm-hand - M barbell = 0 In addition to the NJM, we need to know the moment arm of the muscle creating the NJM in order to determine the muscle force required for the motion. The moment arm of a muscle is the perpendicular distance between the joint and the line of action of the muscle. Once you solve for the NJM and the moment arm of the muscle, the mechanical demand placed on the elbow muscle can be calculated. NJM = F muscle x d Fmuscle Note : If more than one muscle contributes to the NJM then the NJM cannot be attributed directly to any one muscle that crosses the joint. In this case, the contribution of each muscle to the NJM can only be estimated. The calculated muscle moment is the sum of all of the muscle actions involved. In this case, since the arm is being held in flexion, we can say that the net muscle moment is due to the action of the elbow flexors, but we cannot say exactly which elbow flexors are most involved.

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EXSC 408L Fall ‘11 Introduction to Biomechanics Lab 7 - Joint Kinetics Page 2 of 12 II. STATIC ANALYSIS Imagine statically holding a barbell with your forearm in a horizontal position, parallel to the ground. If the elbow joint center is considered to be the axis of rotation, there are two negative moments acting on
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408 Lab 7 - EXSC 408L Introduction to Biomechanics Lab 7...

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