5″-diameter force sensing resistors placed below the metatarsal heads of the 1st, 4th, and 5th toes and the heel pad (Fig. 1; Myomonitor IV; Delsys Inc., Natick, MA, USA; Interlink Electronics, Camarillo, CA, USA).24 The pressure sensor system allowed for immediate collection and processing of 20–30 sequential steps.11 and 20 The pressure sensors were protected and held in place by five-toed socks (Injinji). Plantar pressure recordings were used to determine the average stride frequency, which in combination with running speed, further provided average stride length. The fraction of the stride period, where plantar pressure was measurable, underestimated the stance

phase, because the plantar pressures did not include the last portion of stance between when the pressure is generated from the balls of the feet to the toes. Cytoskeletal Signaling inhibitor The duty cycle, or the fraction of the stride during which one foot was on the ground, was determined from the light video (208 fps; AVT Venetoclax clinical trial Pike 032C Camera, Allied Vision Technologies, Newburyport, MA, USA). To determine the running style (FFS vs. RFS) used by each runner, FSA was measured at each speed under both barefoot and shod conditions ( Fig. 2). FSA, the angle between the horizontal and the line from the ankle to the 5th toe at the time of contact between the runner’s foot and the horizontal, was corrected for the rest angle ( Fig. 2). 6

Runners were recorded with a high-speed light camera for 6 s of the 30 s recording period (208 fps). Each video trial included at least seven complete strides of running at each speed for each condition. To categorize each trial, FSA < 8° represented an FFS trial and FSA > 8° represented an RFS trial (NIH ImageJ; Fig. 2). 6 The average FSA across the four speeds for each subject Casein kinase 1 determined if the individual ran with an FFS or RFS style when barefoot and when shod. Consistently FFS (CFFS) runners always used an FFS style and consistently RFS (CRFS) runners used an RFS style during both footwear conditions. Runners in the “shifter” group used an FFS when barefoot

and an RFS when shod. Surface electromyography (sEMG) determined the activity patterns of the medial and lateral gastrocnemii muscles with a wireless data logger and a laptop computer (Myomonitor IV; EMGworks, Delsys Inc.) The subjects wore the myomonitor around their waist with the provided adjustable belt. Over the surface of each muscle, the hair was shaved, if necessary, and the skin surface was lightly exfoliated to increase electrode conductance. Bipolar electrodes (Delsys) were adhered lengthwise along the medial and lateral gastrocnemii muscles one-third of the way down the tibia and at the midpoint of the muscle based on measurements made by a B-mode, real-time ultrasound machine (210DX; 7.5 MHz linear transducer, Medasonics, Mountain View, CA, USA).25 The wires between the right foot and hip were secured to the leg with self-adherent athletic wrap to minimize movement artifact in the sEMG signal.