Hamstring injury in Sport

Andrew Charniga

www.sportivnypress.com

 

“All skilled movement involves controlling which muscles are tensed and which muscles are relaxed. This combining of muscles occurs throughout the body so that all body positions are achieved through the delicate balance of tension and relaxation in each pair of muscles.”  Nadori, L. “PHYSICAL PREPARATION OF WEIGHTLIFTERS” Proceedings of the Weightlifting symposium 1993 Olympia Greece      

Athletes and non – athletes alike suffer injury in sporting endeavors such as hamstring pulls and tears, Achilles ruptures, knee ligament tears and so forth. However, one does not hear of these problems in the animal world; where such super predators as Cheetahs generate far more power, jump further and run much faster than the fastest human. The obvious reason why: there are no coaches, doctors, trainers and the like training the animals. 

Numerous papers have addressed the problem of a ubiquitous hamstring injury; especially in sports with repetitive sprinting such as football, rugby, soccer, track and field and others. Most observations/conclusions of these data gathered from biomechanical analysis of hamstring injuries from running and so forth are very limited in scope with a number of essential shortcomings:

/ a narrow focus on the specific injury circumstance such as the late swing phase of sprinting;

/ strengthening hamstrings is the answer as weak muscles are believed to predispose one to this type of injury;

/ neglect to consider a connection between injury susceptibility and training/ rehabilitation exercises;

/ neglect to compare rates of injury in other events where the stress on hamstrings is large; but, injury rate is not particularly high;

/ ignore the redundancy factor: many muscles in the body can contribute to movement of a single joint in an infinite of combinations.  

 

Figures 1 & 2. Two illustrations of the eccentric loading on the hamstring group in the performance of the classic weightlifting exercises.  In both the top (clean) and lower (snatch) photos the male athletes straightening knee joints are approaching a knee angle of 180º; with barbell in hands and shoulder joints in front of the vertical line of the barbell. This is a classic example of a large shift in the loading to muscles of the hip and rear thigh preceding a re-bending of the knees under the barbell. Conversely, the photos of the two females avoid such a loading of hamstrings by re-bending knees earlier than the males and shifting the loading back onto the legs especially the ankle muscles.  Charniga photos.

Some factors connected with hamstring injuries from the literature:

/ biceps femoris long head (BFLH) is the most commonly injured muscle of the hamstring group ;(1,2,3)

/ hamstring injuries in sprinting occur at late swing phase of the gait {just before foot strike}; (1,2,3)

/ the eccentric phase (swing phase of running) for the hamstring group is the time of most risk for injury; when muscle tendon length is greatest just before foot strike; (1,2,3)

/ fatigue impairs contraction power and is therefore a factor increasing risk of hamstring injury (1,2,3);

/ hamstring injury can be perceived as a failure of the muscle – tendon spring mechanism, i.e., to lengthen and recoil like a spring (1);

/ the long head of biceps femoris (BFLH) is injured the most because it is subject to more force to complete a longer stretch than the other parts of the hamstring group; (1,2,3)

/ semimembranosis (SM) is most common site of hamstring injury in dance; (1,2,3)

/ hamstrings undergo large loads in lengthening during swing phase exceeding those in stance phase; (1,2,3)

/ the negative work in the swing phase is associated with injury vulnerability because eccentric work has been connected with muscle damage; (1,2,3)

/ an ankle injury increases the chance of a subsequent first time hamstring injury; (3)

/ the force does not cause the hamstring injury as much as does the magnitude of active strain during lengthening;

/ the force absorbed in the swing phase in eccentric muscle contraction of hamstrings is generally considered the injury precipitating factor; (1,2,3)

/ strengthening the hamstrings with eccentric loading is believed an effective strategy to prevent injury; (3)

/ the biarticular hamstrings undergo a stretch–shortening cycle during sprinting, with the lengthening phase occurring during terminal swing and shortening phase commencing just before foot-strike”. (1,2,3)

“There is no direct correlation between strength and sport results.” A.N.Vorobeyev, 1988

Figure 3. World’s strongest athlete OM Yun Chol dropping under world record weight of more than three times bodyweight; his body significantly exceeds the acceleration of free falling body: made possible by forcefully pulling against the bar combined with extremely fast relaxation of muscles. Charniga photo

Factors completely absent in all of this literature is the intrinsic, critical role of muscle relaxation in running and other endeavors in sport which feature explosive lower extremity effort.

For instance, relaxation of the hamstrings and the other two – joint muscles of the lower extremities; especially the gastrocnemius muscles; preceding lengthening of the leg spring in the late swing phase are not considered in the literature. Most research neglects mention of  gastrocnemius, a thigh muscle; in the etiology of a hamstring injury. These muscles cross the knee from below; any extension of the knee joint up to 180° can only occur if these muscles relax and lengthen, i.e., knee extension is affected from an ‘overlap’ or a ‘choke’ point of muscle attachments from above and below. 

The western literature for the most part, describe the actions of the hamstrings in running and other ballistic movements; as either concentric (shortening) or eccentric (lengthening) isometric (tension without movement) muscle contraction. No one considers muscle relaxation in the lengthening period of the late swing phase when injury frequently occurs. There is little in the western literature which delves into the role of muscle relaxation in coordination and production of muscular power in sport. For instance, a sprinters legs must flex and straighten with such speed as to preclude only simplistic models of periods of muscle contraction; be it eccentric or concentric.

Soviet era research of muscle relaxation dates back to at least the 1930s. The critical role of muscle relaxation; especially the speed of muscle relaxation; has been consistent theme in efficacy of sport technique in East European literature ever since those days. For example, from Matveyev:

…”coordination enslavement is the incomplete relaxation of muscles after contraction; or, a slow switch to the state of relaxation.” L.P. Matveyev, 1977

In actuality, movement amplitude is limited, first of all, by tension of the muscles’ antagonists. Due to this flexibility indices depend on the ability to combine loosening of the extended (lengthening) muscles with the tensing of moving muscles.” L.P. Matveyev, 1977

Muscles have to relax very fast in dynamic exercises especially high power exercises like sprinting. Otherwise, movements are hindered by what Matveyev refers to as “coordination enslavement”. This ‘enslavement’ occurs when  an athlete’s muscles (agonists) encounter internal resistance from a failure to relax; or, a slow switch to relax tension in antagonist muscles. A  connection between excessive tension in muscle antagonists to the etiology of hamstring injuries is not considered; especially from muscles such as the bi-articular gastrocnemius which cross the knee from below. 

However, this factor of excessive antagonist tension is implied by the observation hamstring pulls in running and sprinting are a failure of the hamstring group to function as a spring mechanism, i.e., there is resistance to the lengthening of the hamstring group; very likely from two – joint (bi – articular) muscles such as gastrocnemius as the leg straightens; specially in late swing phase of running.

Indeed, if the late swing phase of running and sprinting is the most common instant of hamstring injury onset; and, this injury is in effect a “failure of the involved muscles – tendons to function successfully as a spring mechanism”; how is it the failure (injury) occurs before the spring mechanism reaches its geometric limit: a knee angle of at least or even greater than 180°?

That being said, the literature looks to the perceived strength deficit between this muscle group and the quadriceps as the linchpin to injury susceptibility; hence the testing with machines (Zvijac, J. et al, 2014) and promotion of such exercises as the ‘Nordic curl’  (see figure 6).

The question as to how the hamstring muscles ‘weaken’ with exercises like sprinting; which has absolute specificity to the circumstances of hamstring pull onset, i.e., they are exercised concomitantly with all the running muscles; and, thus predisposes them to injury; is left unanswered because the question is not; or simply rarely raised?

That question is all  the more pertinent; since athletes continue suffer injury despite; but, more likely because of doing these prophylaxsis exercises (figure 6).

Of the exercises which appear in the literature to prevent hamstring injury by strengthening/stretching hamstring muscles; little is to be found to cultivate the lengthening of gastro – soleus group concomitantly with hamstring muscles. This is strange; the bi-articular gastrocnemius muscle is a thigh muscle which must lengthen along with hamstring group as the thigh is straightened just before foot strike in sprinting; when most hamstring injuries are purported to occur. So, at very least, a few exercises where gastronemius (triceps surae) undergo lengthening tension should be incorporated into the training of the susceptible athlete.  

Figures 4-5. Elite female weightlifter inadvertently dropped 107 kg on her back forcing her knees from a flexed to fully straight disposition, i.e., an instantaneous violent stretching – lengthening of hip – hamstring – gastro-soleus muscles linked as a chain. Uninjured she went on to lift this weight and even tried to lift a world record in the second exercise (the clean and jerk). The young woman’s  hamstring group, gastrocnemius and other biarticular muscles crossing from above and below the knee performed as an effective spring mechanism: stretching and recoiling. She simply got up and walked away.  Charniga photos

Figure 6. Professional football player who suffered a relatively severe  hamstring injury (on right) running on a football field. On left he is practicing simple muscle shortening hamstring exercise (on left) which activates concomitantly, muscles crossing knee from above (hamstring group especially biceps femoris short head) and gastrocnemius muscle from below knee.  

Furthermore, the literature is lacking in any cause and effect link between susceptibility to hamstring injuries and strength training exercises such as the one on the left in figure 6. Simple exercises like the one depicted do not replicate the specific conditions of injury onset in sprinting – the late swing phase; exercises like this even inadvertently involve other muscles such as gastrocnemius. The research focuses on the hamstring group; muscles which cross knee from above; excluding the gastrocnemius, a thigh muscle which crosses knee from below. Muscles crossing knee from above and below are most certainly involved simultaneously in lengthening; especially in the late swing phase of sprinting.

It is not a good idea to do muscle shortening exercises like the one depicted above. The hamstring group and the other bi-articular muscles of the leg spring have to function like a bow string – stretching; accumulating strain energy and subsequently recoiling elastically; releasing strain energy. This bow string action would seem to be a critical component for sprinting, running and so forth.

Athletes in dynamic sports shouldn’t be training to ‘ball up’ muscles in simple bodybuilding type, muscle shortening exercises like the one depicted in the figure 5. Yet, it is this type of exercise and others like it; right out of some textbook; performed on the floor, or bench which are most commonly employed.

Relaxed Stretching

The term ‘eccentric contraction’ of the hamstring group of muscles in the late swing phase, is somewhat misleading. A more accurate descriptive would be ‘relaxed stretching’. For example:

“Usually, the earlier the antagonists are included in the work the slower the movement, the more difficult it is to achieve relaxation. Therefore it is necessary to achieve a short period of preliminary tension in the antagonists. One can achieve this by improving the appropriate coordination of the nerve – muscle apparatus, as well as by developing the strength and elastic qualities of the muscles in order that the “braking” of the movement occurs later towards the end by means of the stretching of more powerful and elastic antagonists.” V. Kuznyetsov ,1975

The eccentric contraction of hamstrings in the late swing phase in running referred to in the literature is a braking action to prevent injury from hyper-extension of the involved joints and enhance mechanical efficiency through transfer of energy. As indicated in the quote from Kuznyetsov above; muscle relaxation is critical; allowing the “braking movement” to take place later as the leg straightens prior to ground contact; otherwise this would slow the movement; an antithetical to speed.

Consequently, a ‘relaxed stretching’ phase is crucial for speed of movement. Teaching an athlete to develop eccentric strength of hamstring group to prevent injury with grinding, slow eccentric strength exercises is probably counterproductive; because sprinting actually requires a rapid onset of a late ‘braking phase’ as the leg swings forward and extends for ground contact, i.e., low tension followed by tension from stretching. 

‘Choke Points’

A relaxed -stretching phase followed by the braking – phases of the hamstring and gastrocnemius muscles is not replicated in any meaningful way in the simple strengthening exercises; especially with: leg curls seated or lying, the ‘nordic curls’; or ham glut exercise; as two of the three ‘choke points’: the knee and ankle; are fixed. 

For instance, consider what appears to be a certain disaster; ultimately a non – event, in figures 3-4, where a barbell fell on the back of an elite female weightlifter. Muscles crossing knee from above and below such as bi-articular hamstring, gracillis, sartorius, gastrocnemius and others reflexively relax under the shock of the falling weight; not stiffening in eccentric contraction. The energy of the fall is dissipated by the young woman’s reflexive “relaxed stretching”; much like pulling on and then releasing a bow string. 

The circumstances surrounding the certain – injury – sans – injury depicted in figures 3-4 may hold a key to the ubiquitous pulled hamstring conundrum. The hamstring group as well as other bi-articular muscles which cross the knee from above and below performed like a bow string as she fell and the weight struck her back.

The most obvious thing a spring will do is to reverse a movement.” Alexander, R. McN Elastic Mechanisms of Animal Movement,1988

A bow string initially is stretched relatively taut. When the string is drawn back it accumulates elastic energy as it stretches further; elastic energy is released when the string is released. That is why arrows can be propelled with such speed and power. The bow string doesn’t (should not) break under normal preliminary taut stretch to stretch – recoil conditions. 

Training with bow – string movements with light bouncing or ‘supra stretching’ at the full extension hip, knee and ankle ‘choke points’ to help prevent hamstring pulls seems a logical solution; considering the most cited instant of hamstring injury occurs in the late swing phase of running and/or sprinting before the knee joint has even fully straightened, i.e., reached 180º.

The falling young woman’s bi-articular ‘bow strings’: hamstring group, gastrocnemius and others, were rapidly and briefly drawn taut with significant force such that the knee and hip joints were forcefully straightened against their geometric limits; before springing back as would be anticipated from a functioning spring mechanism. Hamstring injury literature is lacking this ideation: the athlete’s spring mechanism extends as one piece from heel to hip; and not just hip to knee. Confining one’s analysis of the elastic  functioning of the hamstring group, i.e., hip to knee; neglects the concept of the entirety of the leg spring spanning heel to hip.

Virtually all hamstring rehab or preventative supplementary exercises which can be found in the literature; in one way or another; do not replicate the action of a ‘bow string’; especially the simultaneous relax – stretch – recoil action of the bi-articular muscles which cross the knee from above and below.  For instance: 

“as the biceps femoris is more susceptible to injury because it exerts more force to complete a longer stretch in the same amount of time than either the semimembranosus or semitendinosus muscles, it seems feasible to suggest that, if these other muscles of the hamstring group could be trained to stretch further during hamstring action then the biceps would require less force to sustain eccentric muscle action.” (1) Dolman,B., Verrall, G. Reid, I.

If the situation described by Dolman, is indeed the most likely circumstance precipitating hamstring injury; the single most salient premise from the literature as to hamstring injury onset is the muscle tendon complex fails to function as a spring mechanism. This premise should at the very least be the starting point for one to devise and/or to select appropriate exercises to help prevent hamstring pulls. Consequently, the idea to train the hamstrings; especially the other two bi-articular muscles in that group (simimebranosis, semitendonosis) as  well the other bi-articular muscles to “stretch further during hamstring action” is the starting point to select special exercises.

For instance, the exercise depicted in figure 7, aka, Belayev bends, can be an appropriate movement for special exercise selection; or, at least one variation which replicates the very similar conditions of stretching with tension. To perform this exercise the knees are locked with heels together; toes can be raised slightly. The athlete should bend by leaning backwards almost to the point of falling over. Leaning backwards while bending reduces the moment on the back and shifts the strain to the lengthening gluteus – hamstring – gastro- soleus complex. The entire chain from hip – to – heel; lengthens under the strain of the lean back motion.

The entire muscle tendon complex from hip – to – heel lengthens as one piece in this exercise. Most importantly the muscles which cross the knee from above and below are lengthened simultaneously like a bow string as all three ‘choke points’: hip – knee- ankles are stretched.

Bounce lightly; performing a ‘supra – stretch’ a few times at the low point of the bend before standing erect.  A lengthening tension is created and released as the athlete bends and straightens up.

Figure 7. Good morning technique popularized by world weightlifting champion Vladimir Belayev (USSR) for stretching erector spinae muscles as well as the muscles tendons and ligaments of the rear leg spring: gastrocnemius, hamstring and gluteus M. The muscles originating from above and below the knee perform as a single muscle – tendon spring mechanism with knees straight and toes raised slightly to further lengthen the gastrocnemius M. 

Redundancy

“Do a slow progressive stretch, but never bounce during stretching.”

A feature of the human body’s redundancy which seems to not have received attention in the copious literature of hamstring injury pertains to injury onset most often occurs before the knee reaches full extension: about 180°.  If the injury is considered a failure of the muscle – tendon complex to perform as a spring mechanism; is it odd the injury occurs before the ‘spring’ is fully stretched within its geometric constraints?  Geometric constraints refer to the natural limitations of the body’s joints. For instance, the geometric constraints of the knee joint is about an angle of 180°. Movement past this angle is usually from excessive force and can result in injury.

‘Supra – Stretching’ Choke Points of the Leg Spring Against Geometric Constraints

“….We should begin to think of tendons as springs and muscles as tensioners of the springs”

… the hamstrings absorb energy at the knee and generate energy at  the hip. … since the overall change in length of the hamstrings is minimal, the hamstrings as a whole can be considered to neither absorb nor generate energy.   … the hamstrings can be thought to function as an ‘energy strap’ , transferring energy from the moving tibia to the  pelvis to aid in hip extension.”  Novachek, T. “The biomechanics of running”, 1997

Considering the quote from Novachek: hamstrings can be conceptualized as ‘energy straps’; which is also true of the body’s other bi- articular muscles; transferring energy about the body. Consequently, training these biological ‘straps’ to shorten in strength training exercises such as ‘nordic curlse’ leg curls on a bench and others is a non sequitor (it does not follow); these muscles are not being prepared to function as elastic mechanisms to effect transferance of energy. The energy transfer metaphor applied to bi- articular muscles means the hamstring group serves at least multiple crucial functions to enhance mechanical efficiency and act as an injury prophylaxis; by lengthening and recoiling as a spring – like mechanism; improbable if trained chronically to ball up into a fist.

The suggested exercises are essentially ‘supra – stretches’ against the geometric constraints of the entire leg spring. These stretches over the (hip, knee and ankle joints) ‘choke points’ are not intended to be a silver bullet; they are a logical alternative to the simplistic concentric/eccentric contraction exercises which are sorely lacking in specificity to what most commonly occurs at onset of most hamstring injuries.  

Some ‘supra – stretches’ exercises/techniques to help prevent hamstring strain

/ Belayev bends with a bar or light weight;

/ Seated good mornings with knees straight toes raised;

/ Leg spring stretches seated on the floor with legs straight; raise the one leg at a time with rubber band fastened over forefoot; with the strong dorsi-flexing of the foot to stretch the leg spring from heel to hip; with springy action at the end to stretch dynamically the muscle tendon complex as an elongated spring;  

/ Gymnastic kicks;   

/ High jumper bar kicks with ‘supra – stretching (see figure 7);

/ Seated good morning with weight behind head;

/ Seated heel to hip stretch with band;

/ Seated heel to hip stretch with band and bouncing supra – stretch;

/ Jumper kicks to a bar;

Links to some supra – stretching exercises which can be useful in hamstring injury prophylaxis:

1/  https://youtu.be/-fYGShMu9UE

Belayev bends Legs straight

2/ https://youtu.be/rt0a4Vs4gew

Belayev bends legs straight side view

3/ https://youtu.be/tuL2Wz-zR-0

Good morning legs straight toes pointed seated

4 / https://youtu.be/gs9fAC1BivY

Gymnastic kicks

5/ https://youtu.be/YT-_wMdpfZs

Jumper kicks to a bar

/ 6 Seated heel to hip stretch with band

https://youtu.be/oL_a7WQZNEc

Figure 7. Weightlifter performing dynamic stretching of the leg spring by kicking foot above the head with straight leg; and holding with bouncing movement. 

8/ Seated heel to hip stretch with band with bouncing

Afterword

“Weightlifters have a distinctive strength topography; first and foremost of which is the high development of the extensor muscles. The strength ratio of the qualified lifter’s extensor to flexor muscles is as follows: the arm  (elbow joint) 1.6:1, trunk (ilio – femoral and lumbar joints) 4.3:1, ankle (ankle joint) 5.4:1, thigh (knee joint) 4.3:1. This is indicative of the distinct and harmonious development of the weightlifter’s various muscle groups.” A.N. Vorobeyev, 1988

As has been noted the overwhelming majority of research in the area of hamstring injury begins and ends with the sports or activities where hamstring injury is prevalent: running, sprinting, soccer, football, basketball and others. Little if any information is to be found from research with athletes/sports for whom a hamstring injury is not so common; weightlifters for instance.

For example, the why as to the how the young woman in figures 4- 5 was not injured; even though, logic would dictate, she should have succumbed to a hamstring pull (figures 1&2); is not a mystery; a functional spring will spring back when stretched.

The western literature has many references to testing for a strength ratio of quadriceps to hamstring of 100:60, i.e, a hamstring strength of 60% to quadriceps strength considered optimum. This optimum strength ratio is purported to be part of an hamstring injury prophylaxis. Yet, Vorobeyev’s 1988 textbook for the USSR’s institutes of sport shows the weightlifter’s strength topography has considerable asymmetry; with a ratio of knee extensors to flexors to be 4.3 – 1. Yet, hamstring injury are not particularly common; despite the huge loading of the weightlifter’s lower extremity muscles are subject.


Figure 8. Weightlifter in the  phase of the pull (shins are vertical) where  a large loading of hamstring muscles occurs. Charniga photo

Conclusions

“The body is filled with springs that are just as good (as the artificial feet of Oscar Pisitoris). They are called ligaments. They are called tendons. In addition the body has muscles which can do far greater things than just springs.” Hugh Herr MIT, NBC evening news 8/6/12

The single factor from the literature which most succinctly and logically states the problem of hamstring pulls/tears is the assertion those muscles fail to function as a spring mechanism (1); especially when most injuries occur in late swing phase of running just before contact with the support; and, typically, prior to knee joint reaching full extension of approximately 180°. The ‘supra – stretching’ movements suggested, address those conditions; whereas the simple muscle shortening exercises most often employed in weight rooms and therapy sessions should be contraindicated.

The obvious deficit in the hamstring injury literature is the narrow focus on the strength of hamstring group of muscles as the centerpiece of the injury problem; while neglecting the elastic spring like qualities of the three ‘choke points’, hip, knee and ankle joints as a single unit. Thigh and hip muscles – the gastrocnemius and the gluteus maximus overlap the attachments of hamstring muscles at the knee and pelvis, respectively; creating potential ‘choke points’ of resistance to the lengthening of hamstring group in the swing phases of running and sprinting.   

It would seem a no – brainer, all the muscles of the leg spring have to cooperate with hamstring group so as not to elicit resistance to stretching as the leg spring is extended in running, sprinting and the like. That is consistent, yet rarely if ever spelled out in the literature. Simple muscle shortening exercises which have been discussed and depicted in figure 5; are in this regard, counterproductive. Whereas ‘supra – stretching’ exercises are a logical alternative. 

References

/ (1) Dolman, B., Verrall, G., Reid, I., “Physical principles demonstrate that the biceps femoris muscle relative to the other hamstring muscles exerts the most force: implications for hamstring muscle strain injuriesMuscles Tendons Ligaments J., 2014 Jul – Sep; 4(3): 371 – 377 Published online November 17, 2014

/ Novachek, T. “The Biomechanics of Running”, Gait and Posture, 7:77-95:1998

/ Van Ingen Schenau, G.J., “From rotation to translation: Constraints on multi – joint movements and the unique action of bi-articular muscles”, Human Movement Science, 8:301 – 337:1989

/ Bobbert, M.F., Huijing, P.A., Jan Van Ingen Schenau, G., “An Estimation of Power Output and Work Done by the Human Triceps Surae Muscle – Tendon Complex in Jumping,” Journal Of Biomechanics, 19:11:899-906,1986

/ Bobbert, M.F., Jan Van Ingen Schenau,G., “Coordination in Vertical Jumping,” Journal Of Biomechanics, 21:3:249 – 262, 1988.

/ Bobbert, M.F., Huijing, P.A., Jan Van Ingen Schenau, G., “A Model of the Human Triceps Surae Muscle – Tendon Complex Applied to Jumping,” Journal Of Biomechanics, 19:11:887 – 898, 1986

/ Prilutsky, B. I., Zatsiorsky, V.M., “Tendon Action of Two Muscles: Transfer of Mechanical Energy Between Joints During Jumping, Landing and Running. J. Biomechanics 1994:27:1:25 – 34.

/ Tesch, P., Muscle meets magnet 1993

/ (2) Schache, A., Dorn, T., Blanmch2, P., Brown, N., Pandy, M., “Mechanics of the Human Hamstring”, J Foot and Ankle Research 2018: 11:7 PMID: 29492109; doi: 10.11186/s13047- 018 – 0247-4; PMCID: PMC5828071

/ (3) Malliaropoulos, N., Bikos, G., Meke, M., Vasileios, K., Valle, X., Lohrer, H., Maffuli, N., Padhiar, P., “Higher frequency of hamstring injuries in elite track and field athletes who had a previous injury to the ankle – a 17 year observational cohort study”, PMID: 29492109; J. Ankle Res. 2018: 11:7 published online2018 feb 26. doi: 10.11186/s13047-018-0247-4

/ Dai, B., et al, “Biomechanical characteristics of an anterior cruciate ligament injury in javelin throwing, Journal of sport and Health science xx (2015):1-8

/ Charniga, A., “Muscles of the shank, movement of the shin & susceptibility to lower extremity injury”. 2020 www.sportivnypess

/ Charniga, A., “Nine Straps” 2020 www.sportiCharniga, A., “Muscles of the shank, movement of the shin & susceptibility to lower extremity injury”. 2020 www.sportivnypessvnypess

/ Nadori, L. “Physical preparation of weightlifters” Proceedings of the IWF Weightlifting symposium. 1993 Olympia Greece      

/ Werkhausen, A., Albracht, K., Cronin, N., Mewier, R., et al, “Modulation of muscle – tendon interaction in the human triceps surae during an energy dissipation task” Journal of Experimental Biology (2017) 220, 4141-4149 doi:10. 1242/jeb. 164111

/ Roberts, T., Azizi, E., “Flexible mechanisms: the diverse roles of biological springs in vertebrate movement”, J Exp Biol. 2011; 214 (3): 353- 361

/ Kuznyetsov, V.V., Special Strength Training For Athletes, Soviet Russia, 1975; Translated by Andrew Charniga

/ Zvijac, J., Toriscelli, T., Merrick, W., Papp, D., Kiebzak, G., Isokinetic Concentric Quadriceps and Hamstring Normative Data for Elite Collegiate American Football Players Participating in the NFL Combine” National Strength and conditioning Assosciation journal, :28:04: April 2014:875 – 883

/ Latish, M., Zatsiorsky, V., Biomechanics and Motor Control, Elsevier, New York, 2016