## 4 Discussion
By looking further into the research results, we can see in the loaded (stance) gait stage, muscle activities decrease with same load level for the with-Exo walking condition. However, in the unloaded (swing) gait stage, the muscle activities increase. This result suggests that the assistive exoskeleton could effectively share the load with human beings. However, exoskeleton weight and adaptability are not yet good enough. The structure design and control strategy should be reconsidered from an ergonomic point of view to improve the agility, ease of push-off, and adaptability of human-robot systems, or the assistive devices could form a new risk factor for MSDs from loaded carriage. Loading of the lower limb is increased when carrying load. Therefore, a longer ground contact time would increase the amount of time the limb is subjected to increased loading. This may result in increased loading of lower limb structures, such as increased metatarsal compression [24]. Alternatively, a longer ground contact time may be a protective mechanism to lower the rate of loading on the internal structures in order to reduce the potential negative effects of increased load magnitude. It is therefore unclear whether a longer ground contact time is associated with the development of injury, or if it is a mechanism to minimize the risk of injury. Therefore, interventions for increasing muscular strength and endurance of the plantar flexor and knee extensor muscles may be beneficial [2]. EMG data indicated increased ankle plantar, flexor, and knee extensor muscle activity whilst carrying load, in support of the hypotheses. The use of the exoskeleton most significantly increased max and mean muscle activities in the ankle plantar flexor PL in loaded carriage. A significant increase in muscle activation also happened for the knee extensor and hip flexor RF, and the knee flexor and hip extensor ST. However, these significant differences could not be found for the other four pair-wise comparisons (VL, VM, BF, and GL). The very high rates of PL (peroneus longus) muscle activity which were observed may have been the result of a greater contribution of the plantar flexor muscles while knee extensor moments were reduced. Plantar flexor muscle activation has previously been associated with both tibia and metatarsal stress fractures [26]. The suggested interventions to reduce fatigability of the plantar flexor muscles during load carriage activity should be considered, since there is no activation in the exoskeleton ankle joint. Furthermore, the observed significant increases in PL muscle activity which appeared in the exoskeleton assisted load carriage is more likely a result of the loose fastening, rigidity, and inertia of the exoskeleton shoes. Therefore, the structure of exoskeleton shoes and the human-robot shoe interface should be redesigned. In the early stance, the quadriceps muscles act to control knee flexion, while the plantar flexors act at the end of stance to contribute to push-off [27]. In theory, carrying heavier loads is more demanding on the knee joint musculature than that of the ankle. Therefore, the exoskeleton prototype is designed to be activated only in the knee joints. However, the increased GL and PL muscle activity suggests a greater plantar flexor force production is required for push-off when carrying a load of 45 kg, as shown in Figure 4. The activation of the ankle joints is significant. It is clear from the results that load level has a strong influence on the subject's discomfort. The exoskeleton itself also seriously affects the subject's discomfort. However, during the trials with assistance of the exoskeleton, the subject's discomfort increased more slowly with increasing load level. When asked directly, the participants preferred to use the exoskeleton at the heavier load levels of 30 kg and 45 kg. Although no more than a 45 kg load level was tested, ordinal interactions existed in the interaction plot, as shown in Figure 10. That means if the load level was added, the disordinal interaction may appear.