# Biomechanical Analysis of Gait Compensation Strategies as a Result of Muscle Restriction
## 1 Introduction  
Human physical ability deteriorates with aging. An important cause is the reduction in muscle strength, mainly owing to the loss of muscle fibers and muscle cross-sectional area [1, 2, 3, 4]. In fact, a loss of at least 30% of muscle mass is expected by the age of 80 years [5, 6]; this loss is greater in the lower limbs than in the upper limbs [7]. For instance, with regard to the lower limbs, significant reductions in the muscle areas of the gluteus maximus, gluteus medius, psoas major, rectus femoris (RF), vastii (VAS), biceps femoris short head (BFS), and gastrocnemius (GAS) muscles have been reported in the elderly when compared to the young [8]. With the reduction of the lower-limb muscle mass, the gait suffers from several impairments because the loss of mass causes changes in the activity magnitude of the affected muscles and their periods of action along the gait cycle. For instance, a shorter range of activity of the GAS muscle has been reported in elderly with recurrent falls [9], and a lower activity of the same muscle has been found in the elderly during the late stance phase [10]. This last might explain the elderly less-powered ankle push-off [11]. [PERSON] et al. [12] reported a reduction of 25-35% in the muscle strength of the leg extensors in 70-80-year-old men. In addition, a peroneal nerve disfunction, that is a peripheral neuropathy commonly seen in the elderly, causes a lower activity or paralysis ofthe tibialis anterior (TA) muscle that in turn leads to foot drops (a gait abnormality where the forefoot drops when getting closer to the ground) [13]. Thus, the motion pattern of the lower-limb joints deviates from the normal gait pattern, partially because of the effects of muscle weakness, and partially as a result of unconscious compensation measures that the human takes to minimize the effects of muscle weakness and maintain safe ambulation [14]. For instance, an increased hip power has been reported in the elderly during the stance phase and has been suggested as a compensation for their lower-powered push-off in the late stance phase [15]. Owing to a weak TA muscle during the swing phase, the elderly exert a reduced ankle dorsal flexion; however, to maintain a safe ground clearance, the knee and/or hip joints compensate with a higher flexion [16]. According to the simulations of [PERSON] [17] regarding the effects of muscle weakness on a lower limb, a weakened muscle can compensate for itself by exerting higher activity (as measured by electromyography), or can be compensated for by other muscles. For instance, the main leg muscle groups can compensate in the following manner: (a) The weakness of the GAS and soleus (SO) muscle groups increases the activity of the BFS muscle and decreases the activity of the TA muscle. Both the GAS and BFS muscles are responsible for knee flexion; thus, the BFS compensates for the GAS function. Additionally, the GAS and the SO muscles are responsible for plantar flexion, and the disability to exert plantar flexion is compensated by lowering the activity of the TA muscle that is responsible for the ankle dorsal flexion. (b) The weakness of the hamstring (HAM) and VAS muscles (which are responsible for knee flexion/hip extension and knee extension, respectively) increases the activity of these same muscle groups, and decreases the activity of the iliospoas (ILPS) muscle (responsible for hip flexion). (c) The weakness of the RF muscle, which is responsible for knee extension and hip flexion, increases the activity of the ILPS and VAS muscles, and decreases the activity of the BFS muscle. However, joint compensation motions vary widely, and the mechanisms for explaining how and when they occur have not been sufficiently investigated. Furthermore, the observation and analysis of such compensation motions are necessary to understand the mechanisms of human motion with aging.