S14 ESMAC 2012 abstract / Gait & Posture 38 (2013) S1–S116

flexors 41% and knee extensors 28%. On the non-involved side 78%,44% and 38% respectively. The corresponding numbers in the con-trol group were 66%, 48% and 33%. There was a moderate correlation(r = −0.479) between knee extensor muscle strength and doublesupport time on the involved side.

Discussion & conclusions: We found that muscle strength doesnot seem to correlate with muscle work performed during walk-ing, although plantar flexors on the involved side are used just over100% of the maximum voluntary strength. The markedly increaseduse of plantar flexor muscles could in long term have implicationsand to some extent explain reported deterioration in walking andsigns of fatigue and overuse in adulthood [3]. From a gait perfor-mance perspective muscle strength has no major influence in thismildly involved and high functioning group as opposed to moreseverely involved children with a different CP pattern [1,2]. Weconclude that muscle strengthening of the knee and ankle muscleswould most likely not be beneficial in this patient group. How-ever, a previous study found decreased concentric muscle workduring walking from plantar flexors on both the involved and non-involved side in unilateral CP, with increased muscle work from hipextensors bilaterally [5]. We speculate that strengthening of the hipextensor muscles could result in improved gait performance.

References

[1] McNee AE. Increases in muscle volume after plantarflexor strength training2009.

[2] Nyström E. Walking ability is related to mm strength in children with cerebralpalsy 2008.

[3] Opheim A. Walking function, pain, and fatigue in adults with cerebral palsy:2009.

[4] Örtqvist M. Reliability of a new instrument for measuring plantarflex muscle-strength.2007.

[5] Riad J. Power generation in children with spastic hemiplegic cerebral palsy. 2008.

http://dx.doi.org/10.1016/j.gaitpost.2013.07.035

O23

Simulation of muscle contracture in cerebralpalsy (SiMusCP). validation of a decisionsupport system for hamstrings lengthening

Eric Desailly 1, Abdennour Sebsadji 1, DanielYepremian 1, Farid Hareb 1, Néjib Khouri 2

1 Fondation Ellen Poidatz, Motion Analysis Unit, StFargeau Ponthierry, France2 Armand Trousseau Hospital, Department ofPediatric Orthopaedic Surgery, Paris, France

Introduction: Musculoskeletal modeling associated with gaitanalysis help to exclude indication of a hamstrings lengthening (HL)by the objectification of a non-functional impact of a supposed mus-cular contracture [1]. It does not allow a positive diagnosis of theindication of HL. That is why we developed a customizable mus-culoskeletal model able to analyze the muscles kinematics duringwalking and to simulate the maximum muscle length from clinicalgoniometric measurements (SiMusCP). This connection introducesa new diagnostic approach, theoretically exhaustive, of the possiblecausality of hamstrings contracture on CP children gait impair-ments. The purpose of this study is to evaluate the real contributionof this procedure to the therapeutic decision.

Patients/materials and methods: 42 cerebral palsy children(12 ± 3 years) were divided into two groups: 31 (G1 = 60 lowerlimbs) and 11 (G2 = 20 lower limbs), respectively having followedand not-followed HL among all the associated surgeries. All patientshad clinical gait analysis before and 1.9 ± 0.8 years after surgery.The lower limbs were classified, improved or not improved by

HL, with a validated supervised classification system (linear SVM).SiMusCP procedure is performed retrospectively on the basis ofclinical data and preoperative gait analysis. Maximum functionalhamstrings lengths were computed during gait. A specific mus-culoskeletal model was used at this purpose [2] and to realizedirect kinematics simulations of the hamstrings maximum clini-cal lengths corresponding to the angles measured by experimentedphysicians testing the modified popliteal angles of the patients. HLshould not have been realized if hamstrings maximum functionallength or hamstrings maximum clinical length were short but onlywhen both were short. The concordance between the predictionsfrom the simulation and the actual outcome of the surgery wasevaluated.

Results: The optimal clinical and functional thresholds wererespectively identified by ROC curves as healthy subjects’ meanhamstrings maximum walking length +0.85 SD and −0.6 SD.SiMusCP procedure had a sensitivity of 87.5% and a specificityof 65%. The positive predictive value was 83.3%. The intensity ofthe connection between the result of surgery and the indicationproduced by SiMusCP were significantly (p < 0.001) very high (Coef-ficient of Yule Q = 0.86)

Discussion & conclusions: SiMusCP requires significant rigorin the collection of data (morphological, clinical, or from the CGA)used as input. This decision support system can improve HL resultsby making outcomes more predictable.

References

[1] Arnold A. S. et al., Gait & posture, 23(3), 273-281.[2] Desailly, E. et al., Journal of Biomechanics, 43(13), 2601-2607.

http://dx.doi.org/10.1016/j.gaitpost.2013.07.036

O24

Force enhancement and force depression in amodified muscle model used for muscleactivation prediction

Natalia Kosterina, Ruoli Wang, Anders Eriksson,Elena M. Gutierrez-Farewik

KTH Mechanics, Stockholm, Sweden

Introduction: Force depression and force enhancementinduced by active muscle shortening and lengthening, respectively,represent muscle history effects. The objective of the study wasto apply the history effect in a musculoskeletal model used fordynamic movements simulation. Due to the limitations in measur-ing muscular force in vivo and relying on motor control featureto prevent unexpected force changes, the muscle history wasintroduced in evaluated muscle activation.

Patients/materials and methods: A muscle model depend-ing on the preceding contractile events together with the currentparameters was developed for OpenSim software using as a basisthe activation patterns obtained from the software, with its com-mon implicit activation-force expressions. An improved musclemodel was applied in dynamic simulations of heel-raise in uprightposition and squat movements. Muscle activations were computedusing joint kinematics and ground reaction forces recorded fromthe motion capture of seven individuals. In the muscle-actuatedsimulations, a modification was applied to the computed activation,and was compared to the measured electromyography data.

Results: The modified activation was closer to experimentallyobserved activation in 3 out of 4 studied muscles. If the modificationapplied to muscular force, the history gives a small but visible effectfor the squat and heel-raise. The modification is more significant forsimulations of fast movements with a load.

ESMAC 2012 abstract / Gait & Posture 38 (2013) S1–S116 S15

Discussion & conclusions: A skeletal muscle model wasenhanced by a history phenomenon using a simple formula [1,2].The history modification improves the existing muscle modeland gives more accurate description of underlying activations inmusculoskeletal system movement simulation. Though currenttechniques do not enable us to validate the result fully, the supple-ment improved description of skeletal muscle force and showedthe importance of the modification in demanding tasks.

References

[1] Kosterina N, Westerblad H, Eriksson A. Mechanical work as predic-tor of force enhancement and force depression. Journal of Biomechanics2009;42(11):1628–34.

[2] Kosterina N, Westerblad H, Eriksson A. History effect and timing of force pro-duction introduced in a skeletal muscle model. Biomechanics and Modeling inMechanobiology 2011, http://dx.doi.org/10.1007/s10237-011-0364-5.

http://dx.doi.org/10.1016/j.gaitpost.2013.07.037

O25

Compensatory strategies for excessive muscleco-contraction at the ankle

Ruoli Wang 1, Elena M. Gutierrez-Farewik 1,2

1 Royal Institute of Technology, Department ofMechanics, Stockholm, Sweden2 Karolinska Institutet, Department of Women’s andChildren’s Health, Stockholm, Sweden

Introduction: Co-contraction is the concurrent activation ofagonist and antagonist muscles (antagonistic pairs) across the samejoint. In some gait disorders, e.g. spastic gait, the temporal sepa-ration and magnitude differences of activities between agonist &antagonist muscles are frequently attenuated and motor controlbecomes poor. The aim of this study was to identify the necessarycompensatory mechanisms to overcome excessive co-contractionof the soleus-tibialis anterior pair using induced acceleration anal-ysis (IAA) to retain a normal walking pattern.

Patients/materials and methods: Nine healthy adults (age:30 ± 3 years) were examined using a motion capture system (ViconMX40). Ground reaction forces were obtained from two force-plates (Kistler). Surface EMG signals (Motion Laboratory System)were recorded from the biceps femoris long head (BFLH), rectusfemoris (RF), medial gastrocnemius (GAS), soleus (SOL), and tib-ialis anterior (TA) bilaterally. The simulations were performed inOpenSim, which consisted of scaling, inverse kinematics, residualreduction algorithm (RRA) and computed muscle control (CMC)

[1]. IAA was used to compute contributions of primary ankle dor-sif/plantarflexors and knee flexor/extensors to the accelerations ofankle and knee joints. The agonist and antagonistic muscles can beidentified by defining the one with lesser activation as antagonist[2]. Three co- contraction levels (normal, medium and high) weresimulated by increasing the activation of the antagonist muscle.The response of other muscles to the excessive co-contraction ofSOL-TA was computed by repeating CMC and IAA after constrainingexcitations of SOL-TA at each co-contraction level.

Results:

Discussion & conclusions: Results of the simulation indicatedthat with a high level of the SOL-TA co-contraction, one could stillperform normal walking through other means. When increased co-contraction was simulated in the SOL-TA by increasing excitationof TA in the 2nd sub-phase, GAS was the largest compensator atthe ankle and knee. In addition, the net joint accelerations fromthe ankle and knee muscles were generally unchanged, which indi-cated that the ankle and knee muscles alone are able to compensatefor increased co-contraction at the ankle joint.

References

[1] Delp SL, et al. IEEE Transactions on Biomedical Engineering 2007.[2] Falconer K, et al. Electromyography and Clinical Neurophysiology 1985.

http://dx.doi.org/10.1016/j.gaitpost.2013.07.038