Carl, the hurdles are certainly a fascinating event on the global level at the moment, and I will try to share some general thoughts for coaches on performance which take into account anatomy, with reference to the best athletes of recent times in Oliver, Robles, Xiang, and Trammell below.
As you say anatomical dimensions are especially important in hurdles, and it’s crucial that inherent characteristics are controlled. I would suggest researchers need to go beyond height and weight to at least leg length as a standard and probably tibia/femur as well. For instance in this paper https://w4.ub.uni-konstanz.de/cpa/article/viewFile/4902/4545
Yada and colleagues suggests contact lengths were different for elite 100m sprinters vs 11.02 mean performers, height 1.80+/-0.05 vs 1.74+/-0.04with data of:
1.00 +/- 0.05vs 0.92+/-0.05 with P<0.001
When corrected for height
0.55+/-0.03vs0.53+/-0.03 and significance is now only P<0.05 and means are with 1 standard deviation.
If the authors had controlled for leg length (even leg length and segment lengths) rather than simply height, especially given the subjects were Japanese sprinters being compared to elites who are likely to be predominately of West African descent, I would suggest the correlation would go not just from P<0.001 to means within 1SD, but disappear altogether.
The Nature of the Hurdles vs Sprinting
Perhaps the most important differences between flat sprinting and the hurdles is the
1. a) The presence of an acyclical action in the hurdle clearance step.
b) The requirement of transition to a cyclical action of striding between hurdles.
2. a) the extreme postures encountered during hurdle clearance and subsequent landing, with strong implications for centre of mass position.
b) The requirement to stabilize the resultant changes in COM position during landing of the hurdle clearance step.
In my opinion efficient hurdle performance is defined not only by the action of clearing the hurdle but equally by the ability to maintain rhythm and efficiency during the transition from hurdle clearance to cyclical stepping. The nature of hurdle clearance will strongly influence the transition at ground contact but of itself is subject to high inter-individual variation (inherent anthropometry and mobility) and not a completely reliable indicator in its own right. Any deficits in transition to the cyclical action may manifest initially in loss of technique at the subsequent hurdle and thereafter in all subsequent hurdles if recovery of kinematics does not occur.
Through training and in the context of their individual anthropometry, athletes should probably attempt to modify hurdle clearance posture to allow horizontal velocity and reduced flight times only to the extent that landing can be executed in skillful manner allowing maintenance of rhythm.
Potential Key Factors influencing the Hurdle Clearance and Transition to Striding
i) Leg Length/Center of Mass Height- Because the height of hurdles is a non-modifiable external constraint, and not scaled allometrically for the individual, an athlete with a lower COM will need to produce greater ground reaction forces to clear the hurdle with a given posture and with a given step length.
Modifiability: Leg length is not modifiable, very small changes in COM height and "functional leg length" (essentially through alignment of lower extremity) prior to hurdle clearance may be possible.
ii) Weight- There is no external load in the hurdles with a clearance (hurdle collisions aside, though external forces were of note in Daegu 😉 ), and therefore the relationship of GRFs to performance are mass-specific. Athletes with greater weight will need to produce greater GRFs to clear the hurdle with a given posture and step length.
Modifiability: Weight is clearly modifiable, however the effect in relation to performance of weight and body comp changes plateau quickly and are often nearing optimal in elite athletes.
iii) Muscle Mass Distribution: Muscle mass distribution in muscles less essential to locomotion and the stabilization of posture (i.e. upper extremity) will be disadvantageous to clearing the hurdle with a given posture and GRFs.
iv) Weight Distribution – Athletes with greater weight distribution towards the upper extremity will exhibit a greater forward translation of the centre of mass during clearance postures and more significantly at ground contact after the hurdle. Therefore these athletes need to produce greater GRFs to both clear the hurdle and transition to striding with a given posture and step length.
Modifiability: Excessive weight/muscle on the upper body may be modified to some extent through changes to resistance training emphasis though likely have a large genetic component.
i) Leg Length/Center of Mass Height- Athletes with lower COM and leg length will require greater postural deviation of all major joints including the head and neck, arms, trunk, lumbopelvic region, knee, ankle and foot to clear the hurdle with given GRFs and step length.
ii) Weight Distribution- Athletes with greater distribution of mass toward the upper body will show greater translation of the centre of mass anteriorly during clearance and landing, resulting in greater postural deviation with given GRFs and step length.
Modifiability: Dedicated training to address conscious technique may impact posture during hurdle clearance. Dedicated training to address mobility of important joints may allow improved posture and lesser deviation at particular joints during hurdle clearance, therefore allowing improved position of the COM.
Implications for Flight Time in Theoretical Examples
1. An Athlete with Less Anatomical Leg Length or COM Height
This athlete must produce greater vertical take-off GRF's in order to clear the hurdle, which will increase flight time. The only way this athlete can reduce flight time independent of GRFs and step length is to modify their posture, BUT this athlete must have the mobility and skill to modify their posture in a manner that does not induce excessive anterior translation of the COM and a loss of rhythm at contact during landing of the hurdle clearance step.
2. An Athlete with Greater Weight or Anatomically Superior (toward the upper extremity) Weight Distribution
This athlete again must produce greater vertical take-off GRF's in order to clear the hurdle, which will increase flight time. Again the only way this athlete can reduce flight time independent of GRFs and step length is to modify their posture, BUT this athlete must have the mobility and skill to modify their posture in a manner that does not induce excessive anterior translation of the COM and a loss of rhythm at contact during landing of the hurdle clearance step AND this will be exceedingly difficult to achieve with notable weight distribution toward the upper extremity.
Both these thereotical athletes can also maintain rhythm through increased braking GRF's at landing from the hurdle clearance rather than through changes in posture and skill/efficiency, but this may present siginificant time losses through increase ground time as well as increased metabolic cost. As such, efficient hurdle performance may be defined not only by the action of clearing the hurdle but equally by the ability to maintain rhythm and efficiency during the transition from hurdle clearance to cyclical stepping afterwards.