Nine Straps Andrew Charniga

Far more is unknown about injury mechanism in sport than is known. Were it the other way around; for instance, rates of injury, the proliferation of injuries in professional, collegiate and high school sports in America would reflect this (Charniga, 2016 – 2020).

A reverse engineering concept to determine cause of injury beginning with a known outcome, has been presented (Charniga, 2016 – 2019). For the most part, the basic idea has been applied in a general context to explain why serious, even catastrophic injuries such as Achilles rupture should not occur; in movements in sports like basketball and football, tennis and so forth; whereas, weightlifting activities of far greater in intensity than occur in the aforementioned sports; where such negative outcomes are extremely rare.

With that in mind one particularly common circumstance in basketball, football, tennis, baseball and others, would seem to be relatively benign in nature; yet nonetheless, all too often, has a dreadful outcome. For want of a better name let’s call it ‘reaching at 180° planting – heel – to – toe’. A surprising number of athletes suffer an anterior cruciate ligament (ACL) tear simply running about a court or field; upon stepping in this heel – to – toe manner. See depiction in figures 1-3 of NFL quarterback planting foot heel first with knee straight, i.e., @ knee angle of 180°.  

Reaching at 180° planting heel – to – toe

It is common knowledge quadriceps muscles, hence the knee is the shock absorber in heel strike running. In sprinting, where virtually all world class sprinters perform foot strike with the fore – foot, the ankle absorbs most of the force of contact (Novachek, 1997). For instance, Usain Bolt was estimated to generate a foot strike force of up to 1,080 lbs./491 kg. The foot strike impact is dampened by the compliance of the foot’s fascia, tendons and ligaments. Some of the mechanical energy is shifted to the hip; where, this mechanical energy subsequently is re-distributed via rectus femoris, gastronemius and other bi-articular muscles to the foot at take off. This circumstance is both a unique shock absorption and energy redistribution mechanism serving to enhance mechanical efficiency and protect the body from injury; at one and the same time.

A running pace is defined as periods of non – support with heel strike at contact. Conversely, sprinting is defined as even less support time with fore foot strike. In sprinting more energy is generated with less time on the ground; most shock absorption occurs at the ankle. Furthermore, the massive forces generated by a world class sprinter at foot strike are dampened by bi-articular muscles re – distributing mechanical energy to hip at foot strike and shifting it back to the foot at take off.   

“In sprinting … the ankle plantar flexors absorb much of the shock of contact with the ground. Therefore, little power is absorbed at the knee.” Novachek, 1997.

How this re- distribution of mechanical energy mechanism works with heel plant running where forces are significantly smaller than sprinting, yet can result in serious knee injury; is unclear. Especially, with the lowering disposition of center of mass due to the trunk leaning backwards when the athlete is ‘reaching heel – to – toe @180° in track and field jumping, throwing events, and others.  Consider the illustrations of extended leg reaching for the ground; heel first; as depicted in figures 2 – 4. Knee ligaments such as the ACL can tear under such conditions even though forces are decidedly less in running with this heel first strike; than, in sprinting where an athlete strikes the ground with the forefoot.


Figures 1 – 3. In figure 1 elite NFL quarterback suffers ACL tear running with football as he reaches ‘heel – to – toe – at {knee angle} – 180°. In figures 2- 3 world record holder (93 m) javelin thrower plants take off foot with such force to hyper-extend knee without consequence.

One obvious reason mechanical re – distribution of energy mechanism with heel strike may not be as effective that of fore – foot foot strike in sprinting, is the complex interaction of the shock absorbing and enhanced energy propulsion properties of the foot and ankle are lacking with heel strike: 

“the foot is capable of storing strain energy and returning it in an elastic recoil”.“the plantar aponeurosis, the long and short plantar ligaments and the spring ligament are all important in the energy storing mechanism, either as strain energy or by their roles in maintaining the integrity of the arch.” B.F. Ker, et al, 1987

That being said, why does a super elite football player sustain a serious ACL tear (figure 2) ‘reaching @ 180° heel – to – toe’? Whereas, a super elite thrower strikes the ground in essentially an identical heel first with knee angle at 180°; arguably absorbing more energy. Far more power is generated to throw a javelin 90+ meters; such that the knee of the thrower depicted hyper-extends in the process; without suffering adverse consequences (figures 2- 3).

“The magnitude of peak knee extensor moment tends to be greater in running than in sprinting. In running as the knee flexes following initial contact, the quadriceps contract eccentrically. This is seen as power absorption.” Novachek, 1997

Heel – to – toe strike can vary in javelin throw up to hyper-extenison as depicted in figures 2 – 3; to a slight knee bend preceding the follow – through – release of the implement. Some throwers do not even bend the plant knee; but absorb the mechanical energy of the run – up and release with knee hyper – extending, i.e., through the hip. The answer as to why football, basketball and others suffer a serious injury under the same circumstances; my be differences in effectiveness of bi- articular muscles, or tension straps; situated in the lower extremities to re – distribute mechanical energy. Differences which can be the result of insufficient schemes in an athlete’s training; to strengthen these tension straps to perform complex functions in dynamic sport; especially elongation to create and function as a spring like mechanism.

Muscles acting on two joints

There are at least nine (9) cords; muscles with tendon and or fascia attachments connecting two joints of the lower extremities; hence, the name bi-articular. They can perform multiple functions in complex movements such as running, jumping, lifting and so forth. This is why they are not easily assigned simple functions such as a flexion or extension:

  “Bi-articular leg muscles cannot be classified as either extensor or flexor.” Bobbert, M., Ingen Schenau., G. 1986

That being said, this concept raises the first; and probably the most salient issue in sport training as regards injury susceptibility. Kinesiology textbooks anatomy courses and such do not sufficiently elucidate the intricate complexity of muscle interactions in human movement. Consequently, students, coaches, physical therapists, trainers and other products of academia learn of and memorize muscle actions in simplistic models: mostly flexion and/or extension of joints.

For instance, common exercises for hamstring muscles involve simple knee bending seated and or lying; or, a type of deadlift with knees slightly bent. Most academic tests of the strength ratio of quadriceps and hamstrings muscles involve the subject seated on a bench. That being said, most of the special training athletes employ to strengthen these muscles for sport focus on the knee flexing, i.e., under the assumption they are training hamstring muscles. 

Figure 4. Example of an exercise for hamstring muscles commonly applied at all levels of sport. Note: obvious action of gastrocnemius muscles. This exercise does not correspond to the way knee flexor muscles are typically deployed in sports: face down posture and shortening of calf muscles against a fixed knee.

An example of one such exercise is depicted in figure 4. An overly simplistic application of anatomy lessons fosters the delusion this exercise is effective for developing hamstrings to facilitate movement in dynamic sports; and at the same time lower susceptibility to injury. The extent to which hamstrings muscles are active in this exercise; how much of the movement involves gastrocnemius muscles; apparently is not given much thought. Or, for that matter, any notion, training muscles in this posture could have a negative carry over in dynamic movements where the gastrocnemius muscles must relax reflexively to lengthen/stretch; as in the heel – to – toe reaching at 180° (figures 2 – 3).  

Another leg curl exercise is performed face down on a specialized bench; according to Tesch,1993; whom recorded muscle actions with an donut shaped magnetic resonance imaging device (MRI):

“probably the most widely used exercise for hamstring muscles in the gym. It involves the biceps femoris, semitendinosis, sartoris and gracilis. They show only moderate use!”, Tesch, P., 1993

Another version of the same exercise performed seated on a similar exercise machine found in gyms; of the same design as testing machines for hamstrings and quadriceps muscles is likewise a questionable selection for athletes:

gracilis, sartoris, semitendinosis show marked involvement in this exercise. Surprisingly, the biceps femoris is not”. Tesch, P., 1993

With that in mind, training muscles to flex the knees with motion/amplitude – controlled, simple – muscle – contraction exercises as has been enumerated; ignores the fact the hamstring group muscles have more complex functions; simple muscle shortening exercises can inhibit the complex roles the bi-articular muscles of the lower extremities play in effectively re – distributing mechanical energy.

Evidence? A proliferation of hamstring injuries in the NFL, for example. {Charniga, A., “Muscles of the shank, movement of the shin & susceptibility to lower extremity injury”, 2020}

There are four ‘heads’ in the hamstring group. Three of these four parts of the hamstring group are bi – articular; they have complex functions; not relegated to simple flexion at the knee. Hamstring muscles (75% of the group) perform complex functions as transfer straps with other bi-articular muscles of the lower extremities (Novachek, 1997). In addition to a proliferation of lower extremity maladies; hamstring injuries are very common in the NFL; many occurring in the pre – season (see Charniga, 2020).

Few if any of the coaches,  therapists, personal trainers, doctors and so forth who treat, train or otherwise assign remedial and/or strengthening exercises to address this issue consider the role and functions of the gastrocnemius; a thigh muscle i.e., spanning the knee from the shank.

 Gastrocnemius is another set of bi- articular muscles which are instrumental in: “transportation of power from knee to ankle”, (Van Ingen Schenau, 1989).

Most textbooks, coaches and so forth envision heel raises, i.e., simple muscle contraction to produce movement at ankle or foot, as the main function of this muscle. Virtually none, to very few, realize these muscles, besides transport of power, also flex and assist straightening the knee.

Muscles and fascia which cross two joints; such as those listed below; in one manner or another can be involved in re – distribution of mechanical energy in such movements as ‘reaching at 180° planting heel – to – toe’. The re – distribution function of these bi- articular muscles effect mechanically more efficient movements and injury prophylactic.

The bi-articular muscles involved in bending the knee, also shift the mechanical energy of a  straight leg foot strike; transitioning the leg from reaching heel – to – toe as depicted in figures 2 – 3. This potential for shifting mechanical energy in no way shape or form, bears any resemblance to the simple knee flexion exercises as depicted in figure 4.

If anything, simple knee flexion exercises done in sufficient quantity would prove a hindrance to the quick stretch, lengthening of these muscles, i.e., the efficient functioning, under the circumstances of ‘reaching at 180° heel to toe’. Why? The athlete doing those exercises risks a uni – dimensional development of the muscle complex originating above and below the knee. Treating these muscles as just knee flexors instead of multi functional transfer straps, an injury may originate from internal resistance of muscles which do not effectively lengthen; energy of the movement is not effectively dissipated or otherwise re – distributed.

Listed below are nine muscles which in one way or another can function to flex or extend the knee in rapid complex exercises such as running, jumping and so forth, i.e., all from an erect posture.

Biarticluar/two -joint muscles which cross the knee joint:

Hip to knee:

/ Rectus femoris

/ Sartoris

/ Gracilis

/ Semitendinosis

/ Semimembranosis

/ Biceps femoris (long head)

/ Tensor fascia latae

Knee to ankle:

/ Gastrocnemius: medial and lateral heads

Figure 5. World record holder javelin thrower plants left foot with straight leg and bows ankle as the body shifts forward producing more movement to re – distribute mechanical energy.  

A litany of injuries can be found in American sports such as football, basketball where athletes step – heel – to – toe – at -180°  and suffer serious injury such as an ACL tear as depicted in figure 2; or, with knee bending (see Charniga, 2020  “Muscles of the Shank, Movement of the Shin and Susceptibility to Lower Extremity Injury”) and/or landing from a jump. The question arises as to why these athletes are injured relatively easily under what are normal conditions in their respective activities, i.e., power output under these circumstances are sub – maximal.

However, apparently there is little if any connection of catastrophic knee injury with events where maximum power generated such as the javelin, high jump, long jump and triple jump. These athletes take off under analogous circumstances and with more mechanical energy inflicted on joints and soft tissues. One would expect high injury rates due to the greater stress tendons and ligaments. But the literature does not support a high knee injury rate for these events of maximum intensity; compared for instance; to sub – maximum circumstances in American sports such as basketball and football.

Furthermore, some research relative to an ACL injury in javelin for instance, is a good example. One study examined a single ACL injury in a single female thrower out of more than 100 javelin throwers. The cause of injury was deduced to be the woman planted her take off leg at different angle than her other throws. The recommendation was for her to throw with a “soft landing”. This is of course is oxymoronic. The whole purpose of rapid run up, plant and release is to transfer the inertia of the body to the implement. Controlled movements (soft planting) under conditions of maximum strain are unrealistic:

“The ACL injury in this study occurred during the first 30% of the delivery phase, most likely during the first 25% of the delivery phase. A stiff landing of the left leg with a small knee flexion angle was the primary contributor to this injury. Javelin throwers may have a soft left leg landing with a flexed knee, which may help them prevent ACL injuries without compromising performance.” Dai, B., et al, 2015

Against that backdrop, research of this type cannot be any value unless all the variables involved which as has been presented in a number of essays on injury that the exercises, the biomechanics of exercise performance must be taken into account. Obvious cause and effect of injury susceptibility can easily be overlooked where there is an aberrant injury rate among athletes in different disciplines – with all sharing many of the same strength and conditioning regimes, rehab therapeutics and so forth.


/ strength training of bi-articular muscles which perform complex functions in dynamic sports with simple flexion/extension exercises may be a root cause of many hamstring injuries;

/ the ubiquitous hamstring injury may simply emanate from a lack of understanding the interconnected redundancy  of the two – joint muscles of the lower extremities.


/ 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

/ 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