  Inverse dynamic analysis using musculoskeletal modeling is a powerful tool,
which is utilized in a range of applications to estimate forces in ligaments,
muscles, and joints, non-invasively. To date, the conventional input used in
this analysis is derived from optical motion capture (OMC) and force plate (FP)
systems, which restrict the application of musculoskeletal models to gait
laboratories. To address this problem, we propose a musculoskeletal model,
capable of estimating the internal forces based solely on inertial motion
capture (IMC) input and a ground reaction force and moment (GRF&M) prediction
method. We validated the joint angle and kinetic estimates of the lower limbs
against an equally constructed musculoskeletal model driven by OMC and FP
system. The sagittal plane joint angles of ankle, knee, and hip presented
excellent Pearson correlations (\rho = 0.95, 0.99, and 0.99, respectively) and
root-mean-squared differences (RMSD) of 4.1 $\pm$ 1.3$\circ$, 4.4 $\pm$
2.0$\circ$, and 5.7 $\pm$ 2.1$\circ$, respectively. The GRF&M predicted using
IMC input were found to have excellent correlations for three components
(vertical:\rho = 0.97, RMSD=9.3 $\pm$ 3.0 %BW, anteroposterior: \rho = 0.91,
RMSD=5.5 $\pm$ 1.2 %BW, sagittal: \rho = 0.91, RMSD=1.6 $\pm$ 0.6 %BW*BH), and
strong correlations for mediolateral (\rho = 0.80, RMSD=2.1 $\pm$ 0.6%BW ) and
transverse (\rho = 0.82, RMSD=0.2 $\pm$ 0.1 %BW*BH). The proposed IMC-based
method removes the complexity and space-restrictions of OMC and FP systems and
could enable applications of musculoskeletal models in either monitoring
patients during their daily lives or in wider clinical practice.
