Abstract
Purpose: The deep muscles in the flexor compartment of the leg, comprised of tibialis posterior (TP), flexor digitorum longus (FDL) and flexor hallucis longus (FHL) contribute to ankle plantarflexion, ankle inversion and toe flexion. Investigation of these muscles may have future clinical applications given their prominent role in dynamically supporting the medial longitudinal arch of the foot. Without the action of the deep flexors, most notably TP, the arch may collapse, resulting in adult acquired flat foot. The aim of this study was to characterise the anatomy of the deep leg flexors and produce accurate data detailing the proximal muscle attachments and muscle architecture.
Methods: Four legs (4 cadavers, 2 males, mean age: 82 ± 11.8 years) were dissected. Muscle architectural parameters (e.g., muscle, tendon and musculotendinous junction length [MTJ], and physiological cross-sectional area [PCSA]; fascicular length and volume) were measured. Proximal insertion sites were quantified to obtain cross sectional area [CSA] using a 3D laser scanner and the 3D area of insertion for each muscle was digitally measured, using Blender. All data were entered into Microsoft Excel. Anatomical dimensions were normalised to leg length, and descriptive statistics were calculated.
Results: Of the three deep flexors, TP was the largest for all architectural parameters (e.g. muscle PCSA TP (16.3 ± 1.0cm2) FDL(1.9 ± 0.2) cm2, FHL (8.2 ± 3.4cm2) or volume TP (34.6 ± 6.2cm3), FDL (11.0 ± 2.1cm3) and FHL (31.2 ± 12.8cm3)) except distal MTJ and free tendon length. Proximal insertion sites were more complex in TP and there was greater muscle fusion from overlying muscles compared to FDL and FHL. Tibialis posterior had a mean proximal insertional cross-sectional area of 111.4 cm2 compared to the smaller FHL (14.7 cm2) and FDL (20.9 cm2). The proximal MTJ comprised a large percentage of muscle length, ranging from 69% (TP) to 82.5% (FDL) of the total muscle length.
Conclusion:
These findings demonstrate that of the deep flexors, TP is the largest in terms of both its insertional CSA and PCSA. Physiological cross-sectional area directly reflects the force production of a muscle, meaning that TP is likely the strongest invertor and plantarflexor of these three muscles, highlighting its important role in maintaining the medial longitudinal arch. The long proximal MTJs span at least two-thirds the length of each muscle, and this knowledge is clinically relevant for diagnosing injuries that occur in this region of the muscle-tendinous unit. Further study is needed to investigate the muscle fusion, innervation, and compartmentalisation; anatomically separate regions that are activated differentially during specific movements. The presence of these compartments may change how we consider clinical applications such as neuromuscular stimulation to strengthen muscles.