Freelap Friday Five with Pierre Samozino
Pierre Samozino is an assistant professor in Biomechanics at the University Savoie Mont Blanc in France. His research activities focus notably on muscle mechanical properties in relation to sport performance. The central part of his current research is to propose new concepts and simple methods to better understand the muscular determinants of explosive performance (jumps, sprints, change of direction) and make their evaluation possible for the greatest number of sports practitioners.
Freelap USA: Regarding comparisons of jumping to sprinting in terms of being force- or velocity-dominant, can you be one and not the other? How much can we tell about sprinting from how an athlete jumps?
Pierre Samozino: Even if the concepts and the terms are very similar between PFV profiles in jumping and sprinting, there are some important differences between both: notably how to interpret the obtained variables, the correlation between their magnitudes, and the underlying mechanisms8. Besides being related to two different movement patterns, the main difference is that force production in sprinting is performed in two dimensions (while only in one main direction in jumping), and only the horizontal force production (the one effective in accelerating forward) is considered in the PFV profile.
So, a high horizontal force production (whether low or high velocities) can be achieved by a high lower limb force production and/or an effective horizontal orientation of the force onto the ground. The effectiveness of force application is less (not) involved in PFV profiles in jumping. Moreover, while a PFV profile in jumping can be drawn from the same movement performed at different loads (and so the same muscle group involved over the whole FV relationship), a PFV profile in sprinting integrates lower limb force production in a slight different body configuration (from the first steps of the sprint to the peak velocity). Therefore, there are different contributions of the different muscles over a given individual PFV profile, and these muscle groups are slightly different from a jumping PFV profile.We recommend assessing PFV profile in both jumping & sprinting, especially at high and elite levels. Click To Tweet
Our group recently conducted a study2on 557 sport men and women to test the correlations between PFV profiles in jumping and in sprinting. The overall results showed some correlations for heterogeneous groups, but the correlation magnitudes decreased for higher-level athletes. The low correlations generally observed between jumping and sprinting mechanical outputs suggest that both tasks provide distinctive information regarding the PFV profile of lower limb muscles8, for the reasons discussed above. Therefore, we recommend the assessment of the PFV profile both in jumping and sprinting to gain a deeper insight into the maximal mechanical capacities of lower-body muscles, especially at high and elite levels.
Freelap USA: Are there any potential links between sprinting FV profile and hamstring injuries?
Pierre Samozino: Sprinting PFV profile well describes the horizontal force production capacities over the different velocities characterizing an all-out acceleration. This horizontal force production during sprinting involves hip extensors, and notably hamstring muscles7. In the opposite view, hamstring injury is the most frequent injury in sports with sprint and acceleration. So, there are undoubtedly some links between sprinting PFV profile and hamstring injuries, and our group is currently trying to better understand those links.There are undoubtedly some links between sprinting PFV profile and hamstring injuries. Click To Tweet
A first study showed that soccer players with a hamstring injury presented, at the moment they returned to training, a lower value of maximal horizontal force production capability at low velocities (variable F0 in the FV profile) compared to uninjured players. This was then confirmed by a case study two years later4. This showed that the sprinting PFV profile is sensible enough to detect a remaining deficit in horizontal force production ability at return to play after a rehabilitation process. Longitudinal studies are currently in progress to study the potential use of sprinting PFV profiling as a hamstring injury screening tool for a prevention-performance win-win strategy.
Freelap USA: How does an individual body structure fit into force and velocity profiling (such as limb lengths, tendon lengths, etc.)? Is this something that we can quantify or at least make an opinion on, and if so, would we want to treat these individuals any differently in training?
Pierre Samozino: The power-force-velocity profile is related to individual structural factors, which are not the same for maximal force capacities (capacity to produce high level of force at low velocity) and for velocity capacities (capacity to produce force at high velocity). The structural factors involved in the maximal force production have been well documented with the effect of high muscle size (notably cross-sectional area) and pennation angle6.
Concerning the capacity to produce force at high velocities, greater fascicle lengths and lower pennation angles are thought to be important, in addition to neuromuscular factors. When multi-joint movements are considered, limb length can affect force production capability by changing the lever arms involved at the different joints: longer limbs present higher kinematic advantages (increasing velocity qualities) but lower mechanical advantages (decreasing external force production). Otherwise, rate of force development is expected to affect force production at high velocities more than at low velocities. Besides being related to neuromuscular factors, rate of force development depends on tendon stiffness: the higher the stiffness, the more efficient the force transmission to the limbs, and the faster the force production.
All these different structural factors, along with the neuromuscular ones, affect the individual power-force-velocity profile. When you test the PFV profile of an athlete, it is important to keep in mind that different neuromuscular and structural factors are encompassed in the macroscopic mechanical PFV variables. So, when an athlete presents a deficit in one specific PFV variable, it is important to know the underlying neuromuscular and structural mechanisms in order to help focus specific training on some of them when it is possible (complex to train the limb length for instance!).
Freelap USA: How can we explain that the force/velocity relationship (from which the power-force-velocity profile is obtained) describes an inverse relationship between force and velocity (force decreases when velocity increases)?
Pierre Samozino: The inverse F-V relationship is often misunderstood since we have in mind that if we increase the force applied to the ground (or to an object), we increase the velocity of the latter. And the F-V relationship says the opposite: force and velocity change in opposite ways. In fact, there is no opposition between these two observations. They just do not refer to the same thing.
The first one (velocity increases when force applied increases or when resistance decreases) is the expression of the fundamental principles of dynamics: Newton’s laws of motion. They are the mechanical constraints imposed by Earth’s physical laws on human movements (and all other objects).During sports, physical laws are the same for everybody, muscle mechanical properties are not. Click To Tweet
The second one (force decreases when velocity increases) corresponds to the mechanical properties of the neuromuscular system, and so to the mechanical constraints imposed by the biology on human all-out movements. When physics says that velocity depends on force (2nd Newton’s law of motion), physiology says that force depends on velocity (F-V relationship). During sport activities, physical laws are the same for everybody, muscle mechanical properties (PFV profile) are not. And ballistic push-off performance is the best solution regarding both mechanical constraints. Note that the widely used load-velocity relationship integrates both principles of dynamics and neuromuscular capabilities, while the force-velocity relationship only characterizes the latter, which makes it of greater interest.
Freelap USA: What are some different contributing factors to how high an athlete performs a countermovement jump? With this in mind, what is the reliability of a CMJ versus other types of jumps?
Pierre Samozino: For any kind of vertical jumps, and from a macroscopic mechanical point of view, performance, and so jump height, depends on the net mechanical impulse developed in the vertical direction. The latter is the product of net vertical force (force developed by the athlete minus his body weight) and the push-off duration. So, jumping performance requires an athlete to develop the highest force possible over the highest time possible.
But this is impossible, since the higher the force, the shorter the push-off duration. Additionally, in some sports, athletes cannot increase the push-off time since, due to their opponents, they have to jump very quickly over a short push-off distance. Thus, the ability to develop a high impulse cannot be considered a muscle capability.
The recent work of our team has shown that, for a given push-off distance (depending on the sport activity and athlete preference), the net vertical impulse, and so the jumping performance, depends on both the lower-limb muscle maximal power output and force-velocity profile9,10. The force-velocity profile represents, for each athlete, the balance between the capacity to produce a high level of force at low velocity (F0) and the capacity to produce force at a high velocity (V0). An optimal balance exists between these two independent qualities: the higher the imbalance, the lower the performance.
This is true for squat jumps (SJ) or countermovement jumps (CMJ). The only difference between both is that the countermovement enhances the athlete’s lower limb force-production capabilities, mainly by increasing the maximal power, that shifts the force-velocity relationship toward the top and the right2. The force-velocity profile can also slightly change according to the athlete. This higher force production ability is related to the preceding eccentric phase leading to a higher level of force at the beginning of the concentric phase of the push-off, and so higher net impulse1. These changes, with different magnitudes across individuals, lead to higher performances.
That being said, there is no big difference in reliability between a CMJ and SJ; both of them are highly reliable. The only thing different is the slightly more complex standardization of the squat depth in CMJ compared to SJ, but it can easily be done using a rubber band extended under the bottom. Also, we observed that squat depth is very reliable after familiarization.
1. Bobbert MF, Gerritsen KG, Litjens MC, and Van Soest AJ. “Why is countermovement jump height greater than squat jump height?” Medicine & Science in Sports & Exercise.1996;28(11):1402-1412.
2. Jimenez-Reyes P, Samozino P, Cuadrado-Penafiel V, Conceicao F, Gonzalez-Badillo JJ, and Morin JB. “Effect of countermovement on power-force-velocity profile.” European Journal of Applied Physiology. 2014;114(11):2281-2288.
3. Mendiguchia J, Samozino P, Brughelli M, Schmikli S, and Morin J-B. “Progression of Mechanical Properties during On-field Sprint Running after Returning to Sports from a Hamstring Muscle Injury in Soccer Players.” International Journal of Sports Medicine. 2014;35:690-695.
4. Mendiguchia J, Edouard P, Samozino P, Brughelli M, Cross M, Ross A, Gill N, and Morin J-B. “Field monitoring of sprinting power-force-velocity profile before, during and after hamstring injury: two case reports.” Journal of Sports Science. 2016;34(6):535-541.
5. Mendiguchia J, Martinez-Ruiz E, Edouard P, Morin J-B, Martinez-Martinez F, Idoate F, and Mendez-Villanueva A. “A Multifactorial, Criteria-based Progressive Algorithm for Hamstring Injury Treatment.” Medicine & Science in Sports & Exercise. 2017;49(7):1482-1492.
6. Morales-Artacho AJ, Ramos AG, Pérez-Castilla A, Padial P, Argüelles-Cienfuegos J, de la Fuente B, and Feriche B. “Associations of the Force-Velocity Profile with Isometric Strength and Neuromuscular Factors.” International Journal of Sports Medicine. 2018 Oct 5. doi: 10.1055/a-0644-3742
7. Morin J-B, Gimenez P, Edouard P, Arnal P, Jimenez-Reyes P, Samozino P, Brughelli M, and Mendiguchia J. “Sprint acceleration mechanics: the major role of hamstrings in horizontal force production.” Frontiers in Physiology. 2015;6:404.
8. Morin J-B and Samozino P. “Interpreting power-force-velocity profiles for individualized and specific training.” International Journal of Sports Physiology and Performance. 2016;11(2):267-272.
9. Samozino P, Rejc E, Di Prampero PE, Belli A, and Morin JB. “Optimal Force-Velocity Profile in Ballistic Movements. Altius: citius or fortius?” Medicine & Science in Sports & Exercise. 2012;44(2):313-322.
10. Samozino P, Edouard P, Sangnier S, Brughelli M, Gimenez P, and Morin JB. “Force-velocity profile: imbalance determination and effect on lower limb ballistic performance.” International Journal of Sports Medicine. 2012;35 (6):505-510.