OpenSim

In the 1990s Delp and Loan introduced a musculoskeletal modelling software that allows the user to create, alter and evaluate models of many different musculoskeletal structures (Delp, 1990). This software is used extensively to create computer models of musculoskeletal structures and to simulate movements (Delp et al., 2007). Models of the lower and upper extremities have been developed to examine the following biomechanical parameters in gait laboratory testing:

 Studying how surgical changes in musculoskeletal geometry (e.g. origin-to-insertion path) and muscle-tendon parameters (e.g. optimal muscle-fibre length and tendon slack length) can affect the moment-generating capacity of the different muscles on the human body(Hoy et al., 1990b);

 Examining the biomechanical consequences of surgical procedures including tendon surgeries, osteotomies and joint replacements (Delp and Maloney, 1993, Delp and Zajac, 1992, Delp et al., 1995, Erdemir and Piazza, 2004);

 The lower-extremity models have been used to estimate muscle-tendon lengths, velocities, moments arms during normal and pathological gait (Arnold et al., 2000, Arnold et al., 2006, Jonkers et al., 2006, Kimmel and Schwartz, 2006);

 To investigate the causes of abnormal gait (Piazza and Delp, 1996, Kerrigan et al., 1998, Higginson et al., 2006).

OpenSim is an open-source platform for modelling, simulating and analysing the neuromusculoskeletal system. It includes low-level computational tools that are invoked by an application (figure 5.37).

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Figure 5.37: A screenshot from OpenSim, which includes a musculoskeletal model of the lower extremities.

5.8.7.1The OpenSim model

The Gait2392 model is a three-dimensional, 23 degree of freedom computer model of the human musculoskeletal system was used for muscle property data analysis. It is also compatible with Visual3D *.mot exported files and scaling procedure. The model was created by Darryl Thelen (University of Wisconsin-Madison) and Ajay Seth, Frank C. Anderson, and Scott L. Delp (Stanford University). It features 92 musculo-tendon actuators to represent 76 muscles in the lower extremities and torso (figure 5.38).

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The musculoskeletal model describes not only the geometric relationships of the muscles and bones (the musculoskeletal geometry), but also the muscle-tendon parameters and it consists of coordinates for muscle attachments and a model for each muscle tendon compartment.

The model’s bone geometry of the shank and foot were adopted from (Stredney, 1982). The model of the lower extremity consists of seven rigid-body segments: pelvis, femur, patella, tibia/fibula, talus, foot (which includes the calcaneus, navicular, cuboid, cuneiforms, metatarsals), and toes. Reference frames are fixed in each segment (Delp, 1990). Figure 5.39 shows location of the body-segmental reference frame.

Figure 5.39: The coordinate systems of the bone segments (Delp, 1990).

 Pelvis: The pelvic reference frame is fixed at the midpoint of the line connecting the

two anterior superior iliac spines;

 Femur: The femoral frame is fixed at the centre of the femoral head;

 Tibia: The tibial frame is located at the midpoint of the line between the medial and

lateral femoral epicondyles;

 Patella: The patellar frame is located at the most distal point of the patella;

 Talus: The talar frame is located at the midpoint of the line between the apices of the medical and lateral malleoli;

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 Calcaneus: The calcaneal frame is located at the most interior, lateral point on the posterior surface of the calcaneus;

 Toe: The toe frame is located at the base of the second metatarsal.

5.8.7.2Ankle, subtalar, and metatarsophalangeal joints

The ankle, subtalar and metarsophalangeal joints are represented as a frictionless revolutes as shown in figure 5.40.

Figure 5.40: The ankle (ANK), subtalar (ST) and metatarsophalangeal (MTP) joints with axes and orientation (Delp, 1990).

Each foot is modelled using two segments: a hindfoot and the toes. The hindfoot joins with the shank via a two degrees of freedom (dof) joint and the toes joins with the hidfoot via a one degree of freedom hinge joint. The metatarsophalangeal axis is rotated by – 8 degrees on a right-handed vertical axis to minimize disarticulation of the joint. In this research, the entire foot segment was rigid due to the need to place marker positions through holes drilled in shoe uppers. The foot segment was used as a single rigid body in this research.

5.8.7.3Muscle geometry

The paths of the muscles were based on geometric data (i.e., musculo-tendon origin and insertion sites) as reported by (Delp, 1990). In all case, tendons were assumed to attach at a point to the bone. The muscle-tendon actuators in the lower extremity portion of the model were defined on the anatomical landmarks on the bone surface models. In some cases, for example the soleus, origin and insertion landmarks are sufficient for describing the muscle path. In other cases, where muscle wraps over bone or is constrained by retinacula,

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intermediate points or “via” points are introduced to represent the muscle path more accurately. The number of points activated for the muscle can depend on body position. Straight-line segments were used whenever an actuator can run freely from one point to another (for example SOL muscle), with intermediate or via points introduced to model contact of the muscle with bony prominences. Cylinders were used to model the path of muscle when the muscle wraps completely around the underlying bone and/or other muscles (for example a medial and lateral GAS cylinder was utilised to simulate the tissues swapping around the medial and lateral condyles of the femur). This was designed because by using a via-cylinder rather than a series of via-points, muscle moment arms at some of the joints can be represented more accurately (Anderson and Pandy, 1999). The parameters for the cylinders at the knee were estimated by inspecting MR images of the hamstrings and gastrocnemius muscles as reported by (Reicher, 1993).

5.8.7.4Model Anthropometry

The model mass and inertial properties for all segments, except for foot segment, were based on average anthropometric data of the five male subjects who participated in that study (age 26 ± 3 years, height 177 ± 3 cm and weight 70.1 ± 7.8 kg). All data were recorded according to the methods described by (McConville et al., 1980). The mass, position of the centre of mass and principal moments of inertia for each segment in the model except for foot segment were calculated by averaging the anthropometric data for the subjects (Anderson and Pandy, 1999).

The mass of the rearfoot and toes in the models were similar to the mass of the whole foot reported by (McConville et al., 1980) plus the mass of a size 10 tennis shoe.