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This study investigates the pathological toe and heel gaits seen in human locomotion using neuromusculoskeletal modelling and simulation. In particular, it aims to investigate potential cause-effect relationships between biomechanical or neural impairments and pathological gaits. Toe and heel gaits are commonly present in spinal cord injury, stroke and cerebral palsy. Toe walking is mainly attributed to spasticity and contracture at plantar flexor muscles, whereas heel walking can be attributed to muscle weakness of biomechanical or neural origin. To investigate the effect of these impairments on gait, this study focuses on the soleus and gastrocnemius muscles as they contribute to ankle plantarflexion. We built a reflex circuit model based on previous work by Geyer and Herr with additional pathways affecting the plantar flexor muscles. The SCONE software, which provides optimisation tools for 2D neuromechanical simulation of human locomotion, is used to optimise the corresponding reflex parameters and simulate healthy gait. We then modelled various bilateral plantar flexor biomechanical and neural impairments, and individually introduced them in the healthy model. We characterised the resulting simulated gaits as pathological or not by comparing ankle kinematics and ankle moment with the healthy optimised gait based on metrics used in clinical studies. Our simulations suggest that toe walking can be generated by hyperreflexia, whereas muscle and neural weaknesses partially induce heel gait. Thus, this 'what if' approach is deemed of great interest as it allows investigation of the effect of various impairments on gait and suggests an important contribution of active reflex mechanisms to pathological toe gait. KEY POINTS: Pathological toe and heel gaits are commonly present in various conditions such as spinal cord injury, stroke and cerebral palsy. These conditions present various neural and biomechanical impairments, but the cause-effect relationships between these impairments and pathological gaits are difficult to establish clinically. Based on neuromechanical simulation, this study focuses on the plantar flexor muscles and builds a new reflex circuit controller to model and evaluate the potential effect of both neural and biomechanical impairments on gait. Our results suggest an important contribution of active reflex mechanisms to pathological toe gait. This 'what if' based on neuromechanical modelling is thus deemed of great interest to target potential causes of pathological gait.

The knee and ankle extensors as human primary antigravity muscle groups are of utmost importance in a wide range of locomotor activities. Yet, we know surprisingly little about how these muscle groups work, and specifically, how close to their maximal capacities they function across different modes and intensity of locomotion. Therefore, to advance our understanding of locomotor constraints, we determined and compared relative operating efforts of the knee and ankle extensors during walking, running, and sprinting.Using an inverse dynamics biomechanical analysis, the muscle forces of the knee and ankle extensors during walking (1.6 m·s), running (4.1 m·s), and sprinting (9.3 m·s) were quantified and then related to maximum forces of the same muscle groups obtained from a reference hopping test that permitted natural elastic limb behavior.During walking, the relative effort of the ankle extensors was almost two times greater compared with the knee extensors (35% ± 6% vs 19% ± 5%, P0.001). Changing walking to running decreased the difference in the relative effort between the extensor muscle groups, but still, the ankle extensors operated at a 25% greater level than the knee extensors (84% ± 12% vs 63% ± 17%, P0.05). At top speed sprinting, the ankle extensors reached their maximum operating level, whereas the knee extensors still worked well below their limits, showing a 25% lower relative effort compared with the ankle extensors (96% ± 11% vs 72% ± 19%, P0.01).Regardless of the mode of locomotion, humans operate at a much greater relative effort at the ankle than knee extensor muscles. As a consequence, the great demand on ankle extensors may be a key biomechanical factor limiting our locomotor ability and influencing the way we locomote and adapt to accommodate compromised neuromuscular system function.

The EMG of the soleus muscle was recorded with bipolar electrodes chronically implanted in rats aged 10-30 days. Changes in the activity pattern were studied in relation to motor development. The firing pattern and shape of motor units were studied at higher resolution. EMG activity was closely related to motor behaviour and posture. At P11, soleus was only phasically active during movements. Tonic EMG activity appeared with age, and by P16 the activity pattern was similar to that of the adult. The activity level also increased markedly with age. This was paralleled by a change in hindlimb posture. The development of tonic activity precedes the development of dendrite bundles in the motoneuron pool of the soleus muscle, which does not support the hypothesis that bundle formation is causally related to tonic activity in postural muscles. At young ages, the tonic EMG was characterized by a clear oscillation pattern with a frequency ranging from 7 to 20 Hz, the higher frequencies occurring at older ages. This oscillation disappeared around P16, which suggests that it is related to functional development of the stretch reflex. Periods of high frequency firing were observed in the EMG; the highest firing frequencies occurred during locomotion. The maximum firing frequency increased steeply to about 80 Hz at P18. It is suggested that bursts of high frequency firing are related to the development of monoaminergic innervation of the spinal cord. During the first three weeks, motor unit action potentials often showed complex shapes of long duration and considerable spike-to-spike variability. Computer simulation of the summation process of fibre potentials at the electrode showed that the small fibre diameters and high variability in diameter at young ages are major factors with respect to the generation of complex potentials.