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3- Load-velocity Relationship in skeletal muscle

The speed at which a muscle changes length (usually regulated by external forces, such as load

or other muscles) also affects the force it can generate. Force declines in a hyperbolic fashionrelative to the isometric force as the shortening velocity increases, eventually reaching zero at

some maximum velocity. The reverse holds true for when the muscle is stretched  – force

increases above isometric maximum, until finally reaching an absolute maximum. This has

strong implications for the rate at which muscles can perform mechanical work (power). Since

power is equal to force times velocity, the muscle generates no power at either isometric force

(due to zero velocity) or maximal velocity (due to zero force). Instead, the optimal shortening

velocity for power generation is approximately one-third of maximum shortening velocity.

Force –velocity relationship: right of the vertical axis concentric contractions (the muscle is shortening),

left of the axis excentric contractions (the muscle is lengthened under load); power developed by themuscle in red.

Motor Unit

A motor unit consists of one alpha motor neuron together with all the muscle fibers it

stimulates. Since the human body contains, on average, 250,000,000 muscle cells and

approximately 420,000 motor neurons, a motor unit will generally consist of a single motor

neuron paired with many muscle fibers. In strength training, the early strength gains seen by

novices are often not gains in size or number of muscle fibers, but activation of motor units that

had been previously dormant. The motor neuron is a specialized type of nervous cell that runs

between the central nervous system and the muscles. Neurons typically consist of a cell body

(the axon) and the dendrites. If a neuron were to be seen as a tree, the axon would be

analogous to the trunk and the dendrites to the branches. Neurons found within the brain

normally have relatively short axons, but neurons that are part of a motor unit — because they

must connect to the muscles of the body — have elongated axons that run through the spinal

cord, and out to the associated muscle fibers. Each muscle fiber is connected to a particular

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dendrite, and it is through the dendrites that messages are relayed between the central

nervous system and the muscle fiber.

Isometric Contraction

An isometric contraction of a muscle generates force without changing length. An example canbe found when the muscles of the hand and forearm grip an object; the joints of the hand do

not move, but muscles generate sufficient force to prevent the object from being dropped.

Question # 6

SIMM is a widely used software system for modeling the musculoskeletal system. OpenSIMM, a

free application with complementary capabilities, has recently been introduced. Together,

these two software systems offer biomechanics researchers unsurpassed capabilities for

modeling and simulation of the musculoskeletal system.

SIMM

SIMM was introduced in the early 1990s and has become adopted by the biomechanicscommunity. This software is now used by hundreds of biomechanics researchers to create

computer models of musculoskeletal structures and to simulate movements such as walking,

cycling, running, and stair climbing. Using SIMM, models of the lower and upper extremities

were developed to examine the biomechanical consequences of surgical procedures including

tendon surgeries, osteotomies and total joint replacements. A lower-extremity model was used

to estimate muscle-tendon lengths, velocities, moment arms, and induced accelerations during

normal and pathologic gait. SIMM has helped bring simulation to biologists who have created

computational models of the frog, tyrannosaur, cockroach, and other animals. Version 5.0 of 

SIMM was released in February 2010, and includes new features designed to aid clinical gait

analysis, such as batch-processing capabilities, calculation of heel strike and toe off events,averaging of multiple trials, and AVI movie output. Although SIMM helps formulate models of 

the musculoskeletal system and create dynamic simulations of movement, it has relatively

limited tools for computing muscle excitations that produce coordinated movement and for

analyzing the results of dynamic simulations. These complementary capabilities are provided by

OpenSIMM.

OpenSIMM

OpenSimm is an open-source software system that lets users create and analyze dynamic

simulations of movement . It is being developed at Simbios, a NIH center at Stanford University

for physics-based simulation of biological structures. It contains modules that scale a generic

musculoskeletal model to fit a specific subject, fit the model to recorded marker data (inverse

kinematics), perform inverse dynamics, and generate muscle-driven forward simulations from

recorded gait data. OpenSimm can import and export most SIMM models. It contains a muscle

editor, model viewer, coordinate viewer, and plotting tool, but no other model editing tools

(e.g., there is no joint editor, body editor, wrap editor, marker editor, deform editor, or

constraint editor). Version 2.0 was released in January 2010, and includes contact modeling,

static optimization to estimate muscle and joint forces, and an application programmer’s

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interface (API) that enables software developers to call OpenSim functions from their own

programs or MATLAB.

The main benefits of OpenSim 2.0 over SIMM 5.0 are that it:

  has a more full-featured model scaling utility (e.g., can require that left and right sides

be of equal size)

  has a more full-featured inverse kinematics utility (e.g., can explicitly specify some joint

angles while using markers to track others)

  contains "residual reduction algorithm" to make recorded motion data more

dynamically consistent with recorded ground reaction forces, resulting in more accurate

inverse dynamics results

  can generate muscle-driven forward dynamic simulations that reproduce recorded gaitdata (using computed muscle control algorithm)

  can perform dynamic simulations without SD/FAST or a C compiler

  has more extensive analysis features for dynamic simulations

LifeMod

LifeMOD is a complete, state-of-the-art virtual human modeling and simulation software

solution. Its advanced capabilities and intuitive graphical interface, developed and refined over

two decades, enable engineers, designers, and others interested in biomechanics to create

human models of any order of fidelity, report true engineering data, and enable rapid and

repetitive testing of designs, all while slashing time, cost, and risk from new product

development.The leading human modeling solution across a wide variety of industries, LifeMOD is used by

more than 600 corporate clients and hundreds of universities and research institutions

worldwide. Many of our orthopaedic customers are realizing productivity increases up to 20%

and decreases in development costs by up to 40% while enhancing innovation and reducing risk.

LifeMOD automatically produces standard plots of force, displacement, velocities, accelerations,

torques, and angles. These powerful post-processing capabilities make creating clear, concise

reports and attention-grabbing presentations complete with animations, plots, and charts, a

simple task. Corporate management or other stakeholders can now truly grasp the ‘what, why,

how and when’ of a given product’s human interaction and subsequent evaluation. 

Important Features:  Easy to use, self-guiding interface and context-sensitive help

  Anthropomorphic databases for automatic model creation

  Inverse and forward dynamics

  Life-like motion with 3D motion-capture import

  Simple to complex muscle modeling

  Automatic joint creation

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  Powerful post-processing and reporting

References 

1.  Delp, S.L., J.P. Loan, M.G. Hoy, F.E. Zajac, E.L. Topp, and J.M. Rosen, An interactive

graphics-based model of the lower extremity to study orthopaedic surgical procedures.

IEEE Transactions on Biomedical Engineering, Vol. 37, pp. 757-767, 1990.

2.  Delp, S.L. and J.P. Loan, A graphics-based software system to develop and analyze

models of musculoskeletal structures. Computers in Biology and Medicine, vol. 25, pp.

21-34, 1995.

3.  Delp, S.L. and J.P. Loan, A computational framework for simulating and analyzing human

and animal movement. IEEE Computing in Science and Engineering, vol. 2, pp. 46-55,

2000.4.  Delp, S.L., Anderson, F.C., Arnold, A. S., Loan, P., Habib, A., John, C., Thelen, D.G.

OpenSim: Open-source software to create and analyze dynamic simulations of 

movement. IEEE Transactions on Biomedical Engineering, vol. 54, pp. 1940-1950,

5.  http://www.musculographics.com/products/simm.html 

6.  http://www.lifemodeler.com/products/lifemod/