Abstract
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 (lMTU) 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 lMTU compared to that of typically developing (TD) children, and exerts higher (2) passive and (3) active forces at shorter lengths, within gait-relevant lMTU 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 lMTU's per each hip and knee angle combination and the gait-relevant lMTU range, and to analyze gait relevant spastic muscle force - lMTU data. lMTU values were normalized for the participants’ thigh lengths. Two-way ANOVA was used to compare the patients’ lMTU 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 lMTU within gait cycle) and muscular force characteristics (the percent force at shortest lMTU of peak force, either in passive or in active conditions) within gait-relevant lMTU range.
Results
ANOVA showed that lMTU's of spastic GRA are shorter (on average by 15.4%) compared to those of TD. At the shortest gait-relevant lMTU, 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 - lMTU 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.
