We reject our first hypothesis because athletes with UTTA had shorter running ProsL than BioL lengths at mid-stance and take-off, even though standing ProsL lengths were 2.4 cm (2.6%) longer than BioL lengths. Based on our results (Appendix 1), to achieve equivalent running leg lengths across speeds at touchdown and take-off, athletes with UTTA should set their RSP height so that standing ProsL length is 2.8–4.5% longer than BioL length. Our findings support our second hypothesis because athletes with BTTA had ProsL Lratio 3.0-3.4% less than BioL Lratio in NA athletes at mid-stance and take-off. These results indicate that MASH regulations for athletes with BTTA do not result in equivalent running ProsL lengths compared to BioL lengths. Thus, to achieve equivalent running leg lengths at touchdown and take-off across speeds, athletes with BTTA should set their RSP height so that their standing ProsL lengths are 2.1–3.9% longer than their presumed standing BioL lengths.
Compared to running BioL lengths, ProsL lengths are shorter at mid-stance and take-off in athletes with UTTA. At mid-stance, a shorter ProsL than BioL could be due to differences between BioL and ProsL stiffness. A ProsL is 3.6–12% less stiff than a BioL in athletes with UTTA running at 5–6 m/s,[1, 22, 23] which yields greater ProsL compression per unit force than a BioL. A shorter ProsL length at take-off likely reflects the inability to actively plantarflex during running. ProsL Lratio was less than BioL Lratio in athletes with UTTA, and ProsL Lratio was less in athletes with BTTA than BioL Lratio in NA athletes, and the difference in Lratio between legs increased for touchdown and takeoff at faster speed. The interaction between leg type and speed could be due to changes in foot-strike patterns with faster speeds. NA runners typically choose a rearfoot-strike pattern at slower speeds and transition to forefoot/midfoot-landing at faster speeds.[24] A forefoot- or midfoot- versus rearfoot-strike pattern has been associated with ~ 10° greater ankle joint plantarflexion at touchdown,[24] which would increase BioL length at touchdown.
Comparable running leg lengths between a ProsL and BioL can be achieved by increasing RSP stiffness or height. ProsL stiffness would affect leg length at midstance, but not at touchdown or take-off, but running with stiffer RSPs may incur injury and/or impair performance by increasing metabolic energy expenditure of athletes with BTTA.[25] For every 1 cm increase in RSP height in athletes with BTTA there was a 0.63–0.88 cm increase in running ProsL length during touchdown, mid-stance, and take-off. However, using a taller RSP may decrease step frequency, increase ground contact time,[26] and affect knee angle at touchdown, mid-stance, and take-off during running.[27]
There are no athletics regulations that limit the BioL lengths of NA athletes or the ProsL length of athletes with UTTA. However, athletes with BTTA must adjust their RSP height so that they do not exceed their presumed MASH to compete in sanctioned athletics events.[9, 10] MASH estimates an athlete’s barefoot standing height based on body segment dimensions and population studies of NA.[11] On top of the uncertainty and potential bias in estimating an athlete’s standing height, running leg length is not universally proportional to standing leg length. The MASH rule forces athletes with BTTA to run with shorter leg lengths than presumed leg length based on their biological dimensions. Moreover, in sanctioned athletics events, NA athletes race wearing shoes with midsole heights of ≤ 2 cm.[28] Thus, to allow athletes with BTTA to compete with equivalent running leg lengths as non-amputee athletes, they should be allowed to increase their RSP height by 2.1–3.9% of their presumed standing BioL lengths based on running speed, and then add another 2.0 cm to account for footwear midsole height. For example, the current MASH rule estimates[9, 29] that the fastest ever 400 m athlete with BTTA would have BioL lengths of 0.944 m if they had not been amputated. Using average running speed from this athlete’s fastest 400 m performance (9.01 m/s) and adding 2.0 cm to account for footwear midsole height, our results suggest this athlete’s standing ProsL lengths should be 0.998 m to achieve equivalent running leg lengths as a NA athlete with standing BioL lengths of 0.944 m. In other words, this athlete would need to increase their standing ProsL lengths 5.4 cm beyond their MASH regulated ProsL lengths to attain similar running leg lengths as proportional NA athletes.
This study has potential limitations. First, we calculated leg length as the distance between the hip joint center and a marker on the distal end of the shoe or RSP. We used these markers rather than center of pressure data, which are noisy at touchdown and take-off. The use of metatarsal markers to measure leg length does not incorporate toe length. Toe plantarflexion may increase BioL length by up to ~ 7.0 cm at take-off[30] and thus the difference between running BioL and ProsL lengths may be even greater. We used one RSP model and future studies that assess other RSP configurations would inform RSP height prescription to achieve equivalent running ProsL and BioL lengths across speeds.