Patient specific musculoskeletal modeling to relate intra operative muscle force data to gait in cerebral palsy

Increased pathological resistance against knee extension is a characteristic pathological condition in cerebral palsy (CP). Active and passive forces of spastic knee flexor muscles have been associated with the high forces which constrain the knee movement in flexed positions. However, studies quantifying these forces directly are rare. The aim of this study was to determine (1) if the range for spastic muscle-tendon complex length is comparable to that of healthy muscle and (2) how spastic muscle force changes as a function of muscle-tendon complex length. Musculoskeletal patient specific models were developed using OpenSim software by using the gait analysis data of CP patients. The results suggest that the muscle-tendon complex length trends of the patients are similar to those of healthy individuals. The difference between the minimum and maximum MTL is 13% and 8% for healthy ST-GRA muscles respectively, while this difference oscillates between 7% and 14% for spastic ST muscles and 4%-9% for spastic GRA muscles of the patients. In addition, at longer muscle-tendon complex lengths higher passive and active forces of spastic muscles were measured and the dominant component of the total force is the active muscle force. For the GRA, peak active muscle force is more than 5 times that of passive force. For the ST, peak active muscle force is approximately 4 times that of passive force. The outcome of this assessment suggests that mechanical characteristics of spastic muscles may not necessarily differ from those of healthy muscles. This implies that, an explanation for the restricted joint movement of CP patients via the consideration of spastic muscles as shortened and mechanically abnormal muscles is a simplistic one.