The above discussion raises some important implications regarding the complex and adaptive nature of sensorimotor integration, and the demand it places on neuroprocessing, particularly from the perspective of healthy ageing and the infirm. It is clear that whilst visual inputs in particular are a crucial component of sensorimotor control, there is nonetheless a delicate balance and weighting of sensory information necessary for such control. This is especially the case in more demanding tasks such as dual tasking scenarios that are typical of everyday life. The emerging evidence for the potential use of feedback as a postural control mechanism may therefore be of particular interest in assisting those who may be particularly affected by complex environments. Although, the balance between the benefits and increased cognitive demands of sensory stimuli during locomotion and dynamic movements are yet to be fully established.
It is also clear from the fossil record that whilst definitive adaptations for terrestrial bipedalism are clear from as early as 4.5 and 3 Ma, there is considerable debate about the exact nature of bipedalism in early hominins such as Australopithecus afarensis. The consensus that the species did not have a full waist but was certainly substantially capable of effective bipedal walking, at least over short distances, therefore makes it a crucial species for consideration by a human analogue study of segment coupling, as performed in this project. With the full transition to striding bipedalism complete in Homo ergaster, later species of Homo were consequently much more human-like in the type of bipedalism they practiced. As seen in Homo erectus, such species were likely to have been fully competent runners, enabling their rapid expansion and success. Alongside the growing cognitive and balance control abilities discussed, particular interest was placed on these species when considering the sensorimotor implications of this work.
As humans are habitual bipeds, the sole contact that the human body has with the ground is through the plantar surface of the feet. Despite the clear importance this implies for detailing the interactions at this interface with the overall function of foot during gait, recent advances in foot pressure analysis only serve to emphasise the complexity of the issue. The longstanding method for plantar pressure measurement, ten region subsampling (Rosenbaum and Becker, 1997), involves analysing the foot as a series of defined areas, each of which is then allocated a single pressure value. However, with the recent development of techniques to analyse plantar pressures at a pixel by pixel level (Pataky et al., 2008), it has been shown that such subsampling may actually exaggerate or under-represent statistical differences when comparing pressure values. When also considering the very small numbers of pressure plate records typically used in pressure analysis using ten region subsampling, this would suggest a very poor representation of accurate pressure distributions, particularly when considering recent evidence that natural inter- and intra- subject variation in foot pressure distribution has shown to be high, even overlapping that of other apes (Bates et al., 2013b).
The issue of variability is further complicated when considering the morphology and function of the feet of our ancestors. Fossil evidence of foot bones is sparse, and the locomotor conclusions made from even the most complete specimens are the subject of considerable debate due to their complex mosaic of characteristics for both arboreal and terrestrial locomotion (Kidd, 1999, Wood, 1974, Day and Napier, 1964). There are of course fossilised footprints, including the famous trail at Laetoli (Leakey and Hay, 1979), however much controversy still exists surrounding the extent to which actual pressure distribution is reflected in footprints given the insufficiently understood effects of differences in substrate properties. Indeed, recent evidence demonstrates that in modern humans, the overall depth of footprints has a significant effect on pressure distribution (Bates et al., 2013a).
As a consequence, research continues to attempt to find novel solutions to determine the most likely modes of locomotion in our ancestors, particularly those species that had begun to make the transition from dense forest environments to more open grassland and savannahs. Notably this includes
Australopithecus afarensis, made famous by the 3.2 Ma ‘Lucy’ skeleton, which
possessed an apparent mosaic of features for both terrestrial bipedalism (Latimer and Lovejoy, 1989, Johanson et al., 1982) and arboreal locomotion (Stern Jr and Susman, 1983, Susman, 1983). Consequently, a number of theories have been put forward regarding the gait of Australopithecus afarensis
ranging from a chimpanzee-like bent hip bent knee gait (Stern Jr and Susman, 1983, Susman et al., 1984) to fully erect bipedalism much like that that of modern humans (Lovejoy et al., 2002, Crompton et al., 1998, Latimer, 1991). Although computer modelling techniques suggest that the latter is most probable (Crompton et al., 1998), as yet the changes in foot pressure distribution that might have accompanied the transition from arboreal locomotion in dense woodland to habitual bipedalism in open environments are yet to be considered.
Further, despite the known increase in the size of the semicircular canals in Homo (Spoor et al., 1994), and hence enhanced abilities for gaze and head stabilisation, the corresponding implications for adaptations in sensory processing that likely contributed to the efficiency of Homo erectus as a habitual biped, and its success as an endurance runner (Bramble and Lieberman, 2004) and persistence hunter (Carrier et al., 1984), have also as yet remained unassessed.