Abstract – Background: This study aimed to clarify the changes in stiffness variables when maximal speed sprinting performance was developed through long-term training.
Methods: Nine well-trained male athletes performed maximal effort 60-m sprints before and after the completion of six months of winter training. In both experiments, sprinting motion at maximal speed was recorded with a high-speed camera and simultaneously ground reaction force (GRF) was measured. Spatiotemporal and stiffness variables were then calculated.
Results: Sprinting speed was significantly developed (P=0.001) through longer step length (P=0.049). While the leg stiffness did not change (from -539±126 to -558±180 N/kg/m) (P=0.686), the vertical stiffness significantly increased (P=0.001) from -1507±346 to -2357±704 N/kg/m due to increase and decrease in vertical GRF and descent of whole body center of gravity, respectively. Moreover, whereas knee joint stiffness remained constant (from -0.228±0.080 to -0.213±0.084 Nm/kg/°) (P=0.448), ankle joint stiffness was significantly developed (P=0.002) from -0.165±0.031 to -0.210±0.032 Nm/kg/° due to a respective increase and decrease in ankle plantarflexion moment and ankle dorsiflexion angle.
Conclusions: The results demonstrate that the development of maximal speed sprinting performance through longer step length is accompanied by increases in vertical and ankle joint stiffness, and this shows the importance of vertical and ankle stiffness for improving maximal speed sprinting performance.
Coaching Implications: Leg Stiffness at Ground Contact: The Spring Mass Model (SMM)
The Spring Mass Model refers to movement, such as sprinting, that is a result of a body mass bouncing along two springs; one spring compressing to propel the body upward and forward while the other spring swings forward to prepare for ground contact. Leg stiffness, which allows the leg to act as a spring, depends on the stretch-shortening capacities of muscles and varies across different joints (hip, knee, and ankle) and with different angles and speeds.
The spring compresses (loads) as muscles lengthen during the first half of ground contact (eccentric contraction) and rebounds (yields) during the second half as the muscle shortens (concentric contraction). The energy released during the yielding phase depends on the amount of force applied to the ground during the loading phase. The mechanical characteristic of the spring is expressed as leg stiffness which affects the duration of the stance phase (ground contact) and the vertical displacement of the center of mass. This allows individuals to sprint with a variety of different stride rates and stride lengths. The model is under control of the central nervous system (CNS) and the CNS can modify the spring.
Without leg stiffness at ground contact, athletes would be incapable of applying enough force to the ground to successfully complete the start, acceleration, and maximum speed phases of a short sprint. At impact, the force would be absorbed and reactive forces decreased, resulting in loss of vertical and horizontal velocity. With less vertically-directed force, ground contact times increase, and hip height tends to drop.
Leg stiffness, which allows the leg to act as a spring, depends on the stretch-shortening capacities of muscles and varies across different joints (hip, knee, and ankle) and with different angles and speeds. At ground contact, there is little additional bending of the knee. During the stance phase, horizontal braking forces cause some deceleration to help the swing leg move forward. During the mid-phase of ground contact, the yielding energy phase occurs in the opposite direction to propel the sprinter upward and forward. Energy is stored during the downward force (eccentric loading) in the elastic tissues of the limb and released in the opposite direction through elastic recoil. Both the stretch and reflex occur more rapidly in sprinters who spend less time on the ground.
With each step during the acceleration and maximum speed phase of a sprint, leg stiffness increases to counter the braking force at ground contact and overcome the force of gravity . In addition, the foot is in contact with the ground for a shorter time and there is less time to apply force. The inability to apply proper leg stiffness is a formula for the “slows” and greatly reduces speed of movement. Without leg stiffness at ground contact, ground forces and horizontal velocity would be greatly reduced.
Fatigue and Leg Stiffness.During competition in team sports, executing repeated short sprints with minimum recovery between each repetition produces changes in the spring-mass model. Vertical stiffness decreases after repeated sprints and negatively affects performance due to fatigue from failure to recover fully between sprints. In a fatigued state, ground force production and leg stiffness declines, which reduces stride frequency and maximum speed.
Source: Ryu Nagahara and Koji Zushi. 2017. Development of maximal speed sprinting performance with changes in vertical, leg and joint stiffness. J Sports Med Phys Fitness. Dec; 57(12): 1572-1578.
