Introduction
Throughout the twentieth century, thousands of eyewitness reports of giant bipedal apes, commonly referred to as Bigfoot or Sasquatch, have emanated from the montane forests of the western United States and Canada. Hundreds of large humanoid footprints have been discovered and many have been photographed or preserved as plaster casts. As incredulous as these reports may seem, the simple fact of the matter remains -- the footprints exist and warrant evaluation. A sample of over 100 footprint casts and over 50 photographs of footprints and casts was assembled and examined, as well as several examples of fresh footprints.

Tracks in the Blue Mountains
The author examined fresh footprints first-hand in 1996, near the Umatilla National Forest, outside Walla Walla, Washington. The isolated trackway comprised in excess of 40 discernible footprints on a muddy farm road, across a plowed field, and along an irrigation ditch. The footprints measured approximately 35 cm (13.75 in) long and 13 cm (5.25 in) wide. Step length ranged from 1.0 - 1.3 m. Limited examples of faint dermatoglyphics were apparent, but deteriorated rapidly under the wet weather conditions. Individual footprints exhibited variations in toe position that were consistent with inferred walking speed and accommodation of irregularities in the substrate. A flat foot was indicated with an elongated heel segment. Seven individual footprints were preserved as casts.

Evidence of a Midtarsal Break
Perhaps the most significant observation relating to this trackway was the evidence of a pronounced flexibility in the midtarsal joint. Several examples of midfoot pressure ridges indicate a greater range of flexion at the transverse tarsal joint than permitted in the normal human tarsus. This is especially manifest in the footprint figured below, in which a heel impression is absent. Evidently, the hindfoot was elevated at the time of contact by the midfoot. Due to the muddy conditions, the foot slipped backward, as indicated by the toe slide-ins, and a ridge of mud was pushed up behind the midtarsal region.

Patterson-Gimlin Film Subject
In October 1967, Roger Patterson and Bob Gimlin claimed to have captured on film a female Bigfoot retreating across a loamy sandbar on Bluff Creek, in northern California. The film provides a view of the plantar surface of the subject's foot, as well as several unobstructed views of step cycles. In addition to a prominent elongated heel, a midtarsal break is apparent during midstance and considerable flexion of the midtarsus can be seen during the swing phase. The subject left a long series of deeply impressed footprints. Patterson cast single examples of a right and a left footprint. The next day the site was visited by Robert Laverty, a timber management assistant and his sales crew. He took several photographs including one of a footprint exhibiting a pronounced pressure ridge in the midtarsal region. This same footprint, along with nine others in a series, was cast two weeks later by Bob Titmus, a Canadian taxidermist. A model of inferred skeletal anatomy is proposed here to account for the distinctive midtarsal pressure ridge and "half-tracks" in which the heel impression is absent. In this model the Sasquatch foot lacks a fixed longitudinal arch, but instead exhibits a high degree of midfoot flexibility at the transverse tarsal joint. Following the midtarsal break, a plastic substrate may be pushed up in a pressure ridge as propulsive force is exerted through the midfoot. An increased power arm in the foot lever system is achieved by heel elongation as opposed to arch fixation.

Photo credit: Roger Patterson Photo credit: Lyle Laverty

Noteworthy is the documentation of the tracks of this same individual on a number of earlier occasions. One of the first of these was photographed by Peter Byrne near Bluff Creek in 1960. Two others were cast by Al Hodgson, of Willow Creek, one on a logging road near Notice Creek in 1962(?) and another on Bluff Creek in 1963. Another instance was photographed extensively by John Green and Rene' Dahinden on the Blue Creek Mountain Road in 1967, just over one month before the Patterson-Gimlin film was shot.

Photo credit: Peter Byrne

Photo credit: John Green

Additional Examples of "Half-Tracks"
A number of additional examples of footprints have been identified that exhibit a midtarsal break, either as a pronounced midtarsal pressure ridge or as a "half-track" produced by a foot flexed at the transverse tarsal joint . Each of these examples conforms to the predicted relative position of the transverse tarsal joint and elongated heel. The first example is documented by a set of photographs taken by Don Abbott, an anthropologist from the British Columbia Museum, in August 1967. These footprints were part of an extended trackway, comprising over a thousand footprints, along Blue Creek Mountain Road, in northern California.

Photo credit: Dan Abbott

Deputy Sheriff Denny Hereford was one of several officers investigating footprints found by loggers on the Satsop River, in Grays Harbor County, Washington, in April 1982. The subject strode from the forest across a logging landing, then doubling its stride, left a series of half-tracks on its return to the treeline. Note the indications of the fifth metatarsal and calcaneocuboid joint on the lateral margin of the cast. The proximal margin of the half-track approximates the position of the calcaneocuboid joint.

	Photo credit: Henner Fahrenbach

Example of Foot Pathology
The track of an individual with a presumed cripple foot was discovered in Bossburg, Washington in 1969. The malformed right foot has been previously misidentified as a case of talipes equinoverus (clubfoot). However, it is consistent with the general condition of pes cavus, specifically metatarsus adductus or possibly skewfoot. Its unilateral manifestation makes it more likely that the individual was suffering from a lesion on the spinal cord rather than a congenital deformity. Regardless of the epidemiology, the pathology highlights the evident distinctions of skeletal anatomy. The prominent bunnionettes on the lateral margin of the foot mark the positions of the calcaneocuboid and cuboideometatarsal joints, which are positioned more distal than in a human foot. This accords with the inferred position of the transverse tarsal joint and confirms the elongation of the heel segment. Furthermore, deformities and malalignments of the digits permit inferences about the positions of interphalangeal joints and relative toe lengths, as depicted in the reconstructed skeletal anatomy depicted below.

Photo credit: Rene' Dahinden

Relative Toe Length and Mobility
Variations in toe position are evident between footprints within a single trackway, as well as between individual subjects. In some instances the toes are sharply curled, leaving an undisturbed ridge of soil behind toe tips resembling "peas-in-a-pod." In other instances the toes are fully extended. In either case, the toes appear relatively longer than in humans. Among the casts made by the author in 1996 is one in which the toes were splayed, pressing the first and fifth digits into the sidewalls of the deep imprint, leaving an impression of the profile of these marginal toes. This is the first such case that I am aware of. Expressed as a percent of the combined hindfoot/midfoot, the Sasquatch toes are intermediate in length between those of humans and the reconstructed length of australopithecine toes. Furthermore, the digits frequently display a considerable range of abduction.

Photo credit: Wes Sumerlin

Compliant Gait
The dynamic signature of the footprints concurs with numerous eyewitness accounts noting the smoothness of the gait exhibited by the Sasquatch. For example, one witness stated, "...it seemed to glide or float as it moved." Absent is the vertical oscillation of the typical stiff-legged human gait. The compliant gait not only reduces peak ground reaction forces, but also avoids concentration of weight over the heel and ball, as well as increases the period of double support.

Photo credit: Roger Patterson

Conclusions
Human walking is characterized by an extended stiff-legged striding gait with distinct heel-strike and toe-off phases. Bending stresses in the digits are held low by selection for relatively short toes that participate in propulsion at the sacrifice of prehension. Efficiency and economy of muscle action during distance walking and running are maximized by reduced mobility in the tarsal joints, a fixed longitudinal arch, elastic storage in the well developed calcaneal tendon, plantar aponeurosis and deep plantar ligaments of the foot.
In contrast, the Sasquatch appear to have adapted to bipedal locomotion by employing a compliant gait on a flat flexible foot. A degree of prehensile capability has been retained in the digits by maintaining the uncoupling of the propulsive function of the hindoot from the forefoot via the midtarsal break. Digits are spared the peak forces of toe-off due to the compliant gait with its extended period of double support. This would be a efficient strategy for negotiating the steep, broken terrain of the dense montane forests of the Pacific and Intermountain West, especially for a bipedal hominoid of considerable body mass, The dynamic signatures of this adaptive pattern of gait are generally evident in the footprints examined in this study.