Relation of muscular contractions to mechanical deformation in the human tibia during different locomotive activities

Bone deformation is widely accepted as one of the most important factors in bone adaptation. It was hypothesized that the largest load on bone is primarily caused by muscle forces. Nevertheless, in vivo bone deformation amplitude and regimes measurements are still technically challenging, especially in humans. Furthermore, the relationship between the local muscle activities and the bone loading pattern remained largely unclear. Therefore, taking human shank as an object, one of the purposes of the present thesis was to investigate the tibia loading regimes, in terms of tibia segment deformation pattern, during different locomotor activities in humans. Moreover, the role of calf muscle activities in tibia deformation regimes was further examined.
Utilizing a novel optical segment tracking (OST) approach, the in vivo tibia segment deformation regimes in five male volunteers were investigated during walking, running, stair ascent and isometric muscle contraction. The reliability study of the OST approach suggested that the OST approach is capable of assessing in vivo tibia deformation regimes with high resolution, accuracy and repeatability. Most importantly, the surgical procedure of the entire experiments was well tolerated by the test subjects. During the stance phase of walking, results suggested that the proximal tibia primarily bends to the posterior aspect, medial aspect and twists to the external aspect with respect to distal tibia. The peak to peak (p2p) antero-posterior (AP) bending angles increased linearly with the vertical ground reaction force and speed, respectively. Similarly, p2p torsion angles increased with the vertical free moment. However, the exact relationship between the tibia deformation and reaction force or moment was different between individuals. P2p AP bending angle during treadmill running was speed-dependant and larger than walking. In contrast, p2p tibia torsion angle was smaller during treadmill running than walking. Furthermore, peak posterior bending and peak torsion occurred during the first half (21%-22%) and second half (72%-76%) of the stance phase of walking, respectively. Two noticeable peaks of torsion with forefoot contact (38% and 82% of stance phase), but only one peak of torsion with full foot contact (78% of stance phase) were found during stair ascent. The peak-to-peak torsion angle was larger with forefoot contact than full foot contact during both the stance phase of stair ascent and running. The tibia deformation regimes were characterized mostly by torsion than bending during isometric plantar flexion.
To conclude, bending and torsion predominated the tibia deformation regimes during the investigated activities. The relationship of tibia deformation with locomotor speed and ground reaction forces bears highly individual-related information. Unexpectedly large torsional deformation was induced by forefoot exercises and isometric contraction. Together with the specific fixed phase relationship between torsional deformation angles and local muscle activities, it seems that tibia torsion deformation is closely related to the local muscle contractions. It thus can be speculated that the torsional deformation might be another candidate, besides the compression and tension, to drive the long bone adaptation.