Plantar flexor prestretch in the block start
Hanging the heel over the block must be considered, along with block spacing and angles, in the context of power output and alterations to individual posture. I am not sure about hanging the heel over re joint stiffness, and particularly on the front leg it reducing the available surface upon which to apply force may be unhelpful re muscle activation. I would suggest that a greater length of the triceps surae in the blocks may allow higher velocity due to improved timing and co-ordination of the relatively sequential extension of hip-knee-ankle in the block start. Research of interest
Longer plantarflexors or increased pre tension on knee extensors?
https://www.tandfonline.com/doi/abs/10.1080/02640410500131753
Nine male sprinters performed maximal sprint starts from the blocks that were adjusted either to 40° or 65° to the horizontal… Muscle- tendon lengths of the medial gastrocnemius, soleus, vastus medialis, rectus femoris and biceps femoris were calculated from the set position to the end of the first single leg contact…block velocity was greater (P < 0.01) in the 40° than in the 65° block angle… the initial lengths of the gastrocnemius and soleus of the front leg in the block at the beginning of force production until half way through the block phase were longer (P < 0.001) in the 40° than in the 65°…initial length and the length in the middle of the block phase were also longer in the 40° than in the 65° block angle…for both the gastrocnemius (P < 0.01) and soleus (P < 0.01- 0.05) of the rear leg. In contrast, the initial lengths of the rectus femoris and vastus medialis of the front leg were longer (P < 0.05) in the 65° than in the 40° block angle condition. All differences gradually disappeared during the later block phase. The peak ankle joint moment (P < 0.01) and power (P < 0.05) during the block phase were greater in the 40° than in the 65° block angle condition for the rear leg. The peak ankle joint moment during the block phase was greater (P < 0.05) in the 40° block angle for the front leg, whereas the peak knee joint moment of the rear leg was greater (P < 0.01) in the 65° block angle condition. The results suggest that the longer initial muscle- tendon lengths of the gastrocnemius and soleus in the block phase at the beginning of force production contribute to the greater peak ankle joint moment and power and consequently the greater block velocity during the sprint start.
Increased ankle stiffness allows more efficient transmission of forces from hip/knee?
https://www.ncbi.nlm.nih.gov/pubmed/22098532
One elite sprinter performed 10 maximal sprint starts…The lower-limb joints predominately extended and revealed a proximal-to-distal sequential pattern of maximal extensor angular velocity and positive power production. Pearson correlations revealed relationships (P < 0.05) between ankle stiffness (5.93 ± 0.75 N x m x deg(-1)) and selected performance variables. Relationships between negative power phase ankle stiffness and horizontal (r = -0.79) and vertical (r = 0.74) centre of mass velocities were opposite in direction to the positive power phase ankle stiffness (horizontal: r = 0.85; vertical: r = -0.54). Thus ankle stiffness may affect the goals of the sprint push-off in different ways, depending on the phase of stance considered.
Elongation = plantarflexion + better proximal to distal sequencing?
https://www.ncbi.nlm.nih.gov/pubmed/1585060
The sprint starts of 12 skilled collegiate sprinter/hurdlers were filmed for four different sprint start conditions. ..The subjects employed their preferred right-left leg placement in the blocks, while the anterior-posterior spacing of the front block with respect to the starting line and the amount of forward lean in the set position were varied. Four positions were constructed that accounted for anthropometric differences. The four positions consisted of combinations of two arm orientations (forward lean and perpendicular to ground) and two front block to starting line distances (bunched and elongated). … The results indicated that the elongated starting positions resulted in greater horizontal displacement, greater propelling impulse, increased first step toe-off velocity, and greater average velocity through a 2-m speed trap…Forward lean tended to result in greater vertical velocity at block clearance and horizontal velocity at first step toe-off, whereas perpendicular arm positioning resulted in greater 2-m speed trap velocity.
Stiffness but not tension?
https://www.ncbi.nlm.nih.gov/pubmed/17215551
Pre-tensed and conventional starts that exert, respectively, large and small forces against the starting blocks in the "set" position (0.186 vs. 0.113 N per newton of body weight) were analyzed. The starts were videotaped, and the horizontal forces exerted on feet and hands were obtained from separate force plates. In the pre-tensed start, the legs received larger forward impulses early in the acceleration (0.18 vs. 0.15 N x s per kilogram of mass in the first 0.05 s), but the hands received larger backward impulses (-0.08 vs. -0.04 N x s x kg(-1)). At the end of the acceleration phase, there was no significant difference in horizontal velocity between the two types of start and only trivial differences in the center of mass positions. The results did not show a clear performance change when the feet were pressed hard against the blocks while waiting for the gun.
Earlier EMG is possibly helpful
https://www.springerlink.com/content/h7pkxp283803k3w3/
Eight male sprinters were filmed running three maximal starts over 3 m on a long force platform. ..EMG activities of the gastrocnemius caput laterale muscle (GA), vastus lateralis muscle (VL), biceps femoris caput longum muscle (BF), rectus femoris muscle (RF) and gluteus maximus muscle (GM…Total reaction time (TRT) was defined as the time from the gun signal until a horizontal force was produced with a value 10% above the base line. Pre-motor time was defined as the time from the gun signal until the onset of EMG activity and motor time (MT) as the time between the onset of EMG activity and that of force production. ..Significant positive correlations were observed between MT and maximal horizontal force and the velocity of the centre of gravity during the last observable contact on the blocks (P<0.05-0.01). The EMG activities of the muscles analysed demonstrated large individual variations until the end of the first contact after the blocks. This resulted in non-observable MT in some individual cases. In general, however, despite the complex multijoint character of TRT, its fractions could be analysed during the early phases on the blocks. To optimize starting action it is desirable that all the important muscles should be activated before any force can be detected against the blocks.