Background. Spastic muscles of patients with cerebral palsy (CP) are considered structurally as shortened muscles, that produce high force in short muscle lengths. Yet, previous intraoperative studies in which muscles’ forces are measured directly as a function of joint angle showed consistently that spastic knee flexor muscles produce a low percentage of their maximum force in flexed knee positions. They also showed effects of epimuscular myofascial force transmission (EMFT): simultaneous activation of different muscles elevated target muscle's force. However, quantification of spastic muscle's force - muscle-tendon unit length (l. MTU. ) data during gait is lacking. Aim. Combining intraoperative experiments with participants’ musculoskeletal models developed based on their gait analyses, we aimed to test the following hypotheses: activated spastic semitendinosus (ST) muscle (1) operates at short l. MTU. 's during gait, forces are (2) low at short l. MTU. 's and (3) increase by co-activating other muscles. Methods. Ten limbs of seven children with CP (GMFCS-II) were tested. Pre-surgery, gait analyses were conducted. Intraoperatively, isometric spastic ST distal forces were measured in ten hip-knee joint angle combinations, in two conditions: (i) activation of the ST individually and (ii) simultaneously with the gracilis, biceps femoris, and rectus femoris muscles endorsing EMFT. In OpenSim, gait_2392 model was used for each limb to (a) calculate l. MTU. per each hip and knee angle combination and the gait relevant l. MTU. range, and (b) analyze gait relevant spastic muscle force - l. MTU. data. Two-way ANOVA was used to compare the patients’ l. MTU. to those of the seven age-matched typically developing (TD) children. l. MTU. values were normalized for the participants’ thigh length. (a) was used to test hypothesis (1) and (b) to test hypotheses (2) and (3): in condition (i), the percent of peak force exerted at the shortest l. MTU. calculated per limb was used as a metric for (2). In condition (ii), mean percent change in muscle force calculated within gait-relevant l. MTU. range was used as a metric for (3). Results. Modeling showed that l. MTU. of spastic ST during gait is shorter on average by 14.1% compared to TD. The ST active force at the shortest gait-relevant l. MTU. was 68.6 (20.6)% (39.9–99.2%) of the peak force. Simultaneous activation of other muscles caused substantial increases in force (minimally by 11.1%, up to several folds, with an exception for one limb). Therefore, only the first and third hypotheses were confirmed. Conclusion. The modeling showed in concert with the clinical considerations that spastic ST may be a shortened muscle that produces high force in short muscle lengths. However, this contrasts intraoperative data, which shows only low forces in flexed knee positions. Note that, the model does not distinguish the muscle-belly and tendon lengths. Therefore, it cannot isolate shorter muscle length and how this compares to the data of TD children remains unknown. Yet, the effects of co-activation of other muscles shown intraoperatively to cause an increase of the spastic ST's force are observed also in muscle force - l. MTU. data characterizing gait. Therefore, if indeed spastic ST produces high forces in short muscle-belly lengths alone, elevated forces due to co-activation of other muscles may be considered as a contributor to the patients’ pathological gait. Otherwise, such EMFT effect may be the main determinant of the pathological condition

Introduction and Objective. Clinically, it is considered that spastic muscles of patients with cerebral palsy (CP) are shortened, and produce higher force in shorter muscle lengths. Yet, direct quantification of spastic muscles’ forces is rare. Remarkably, previous intraoperative tests in which muscle forces are measured directly as a function of joint angle showed for spastic gracilis (GRA) that its passive forces are low, and only a small percentage of its maximum active force is measured in flexed knee positions. However, the relationship of force characteristics of spastic GRA with its muscle-tendon unit length (l. MTU. ) is unknown. Combining intraoperative experiments with participants’ musculoskeletal models developed based on their gait analyses, we aimed to test if spastic GRA muscle (1) operates at short l. MTU. compared to that of typically developing (TD) children, and exerts higher (2) passive and (3) active forces at shorter lengths, within gait-relevant l. MTU. range. Materials and Methods. Ten limbs of seven children with CP (GMFCS-II) were tested. Pre-surgery, gait analyses were conducted. Intraoperatively, isometric spastic GRA distal forces were measured in ten hip-knee joint angle combinations, in two conditions: (i) passive state and (ii) maximal activation of the GRA exclusively. In OpenSim, gait_2392 model was used for each limb to calculate l. MTU. 's per each hip and knee angle combination and the gait-relevant l. MTU. range, and to analyze gait relevant spastic muscle force - l. MTU. data. l. MTU. values were normalized for the participants’ thigh lengths. Two-way ANOVA was used to compare the patients’ l. MTU. to those of the seven age-matched TD children to test the first hypothesis. In order to test the second and the third hypotheses, Spearman's rank correlation coefficient (ρ) was calculated to seek a correlation between the muscle's operational length (represented by mean l. MTU. within gait cycle) and muscular force characteristics (the percent force at shortest l. MTU. of peak force, either in passive or in active conditions) within gait-relevant l. MTU. range. Results. ANOVA showed that l. MTU. 's of spastic GRA are shorter (on average by 15.4%) compared to those of TD. At the shortest gait-relevant l. MTU. , the GRA passive force was 84.6 (13.7)% of the peak passive force; and the active force was 55.8 (33.9)% of the peak active force. Passive state forces show an increase at longer lengths, whereas active state force characteristics vary in a patient-specific way. Spearman's rank correlation indicated weak correlations between muscle's operational length and muscular force characteristics (ρ= −0.30 P= 0.40, and ρ= −0.27 P= 0.45, for passive and active states, respectively). Therefore, only the first hypothesis was confirmed. Conclusions. Novel muscle force - l. MTU. data for spastic GRA were obtained using intraoperative data and modelling combined. The modelling showed in concert with the clinical considerations that spastic GRA may be a shortened muscle. However, because the model does not distinguish the muscle-belly and tendon lengths, it cannot isolate shorter muscle belly length and how this compares to the data of TD children remains unknown. Moreover, the absence of a strong correlation between shorter operational muscle length and higher force production either in passive or in active conditions highlights the influence of other factors (e.g., muscle structural proteins, and muscle mechanical characteristics including intermuscular interactions etc.) on the pathology rather than ascribing it solely to the length of a spastic muscle itself