bi - articular muscles are commonly found in the upper and lower extremities of the human body .
the rectus femoris ( rf ) spans the hip and knee , and the gastrocnemius ( ga ) crosses the knee and ankle .
the actions of these muscles at their primary joints have been known for well over 100 years .
the rf is an extensor of the leg , and the ga is a powerful plantarflexor .
the descriptions of these particular actions have been relatively unchanged for many years and appear in most anatomy textbooks [ 2 - 4 ] . however , these muscle action descriptions do not consider the influence the second joint may have on the muscle s action at the primary joint , or vice versa .
for example , considering the ga action at the ankle , how does the plantarflexion ( pf ) torque it generates change as the angles of the knee and ankle change ? at what combination does muscular insufficiency arise ?
advances in technology have made it possible to answer questions of this type , resulting in more detailed descriptions of the bi - articular muscles of the extremities . in our previous papers
, we discussed in some detail issues such as muscle tissue , joint moments and moment arms [ 6 - 8 ] and their effects on bi - articular muscle actions .
the ga is one of 14 muscles that act upon the knee , and nine of these , including the ga are bi - articular .
however , the ga is only one of these nine that acts on both the knee and ankle , and the others cross the knee and hip . at the knee ,
the gas actions oppose those of the quadriceps femoris , acting synergistically with the other primary knee flexors ( biceps femoris , semitendinosus , semimembranosus , gracilis , popliteus , and sartorius ) .
it opposes the action of the dorsiflexors ( e.g. , tibialis anterior , extensor digitorum longus and hallucis longus ) and it is a powerful plantarflexor working with other posterior leg muscles ( e.g. , soleus , tibialis posterior , flexor digitorum and hallucis longus ) .
it is estimated that young males can generate pf torque ranging between 1,000 to 1,780 n .
the ga consists of two heads arising from the posterior aspects of the femoral condyles .
these merge into a common belly that rides on the proximal half of the sural aspect of the leg .
the ga and soleus are collectively referred to as the triceps surae and the innervation is supplied by the tibial nerve entering the proximal segments of the muscles .
these two muscles provide approximately 80% of force of pf , which is a principal component to a large portion of the gait cycle and essential to nearly all forms of human locomotion .
di nardo et al   explored the ga s role in pf using semg to measure activation patterns in the lateral portion of the ga when walking .
these investigators found that the lateral portion of the ga was active in the stance phase when transitioning from flat foot contact to toe - off and then again in the final portion of the swing phase .
riemann et al   investigated the ga s contribution to the stability of the ankle across three ankle angles .
these investigators measured the stiffness values of the ga as the subject s ankles were passively moved from 10 of dorsiflexion through the neutral position and into 10 of pf , while the knee was alternately positioned at 0 and 90. the results showed that the 0 knee angle produced higher stiffness values across the three ankle positions than the 90 knee angle .
the stiffness index increased as the ankle moved from pf to dorsiflexion and actually began early in the neutral position . in our initial project with the ga
, we investigated how the knee and ankle joint angles interacted in the development of knee joint flexion moments .
we used 17 subjects ( 10 females and seven males ) in this project and measured the knee flexion moments produced by the ga at 24 knee and ankle joint positions .
knee angles of 0 ( anatomical position ) , 15 , 30 , 45 , 60 , 75 , 95 , and 105 ( li et al   labeled the knee angles from the anatomical position as follows : 180 , 165 , 150 , 135 , 120 , 105 , 90 , and 75. in landin et al , they used the more common clinical sequence starting with 0 for the anatomical position , and then progressing through 15 , 30 , 45 , 60 , 75 , 90 , and 105 of knee flexion . the li et al sequence was converted to avoid confusion . ) were crossed with three ankle positions ( neutral , 15 dorsiflexed and plantarflexed ) . a biodex system 3 dynamometer controlled
the knee angles and removable casts were used to hold the ankle in each of its three positions .
three dependent measures were of interest : 1 ) passive moment ( pm ) , which was the joint moment without stimulation , and obtained by finding the mean of the joint moment before and after the stimulation period ; 2 ) maximum moment ( mm ) , which was the joint moment during the stimulation ; and 3 ) stimulated moment ( sm ) , which was the joint moment induced by the stimulation and was obtained by taking the difference between the mm and pm . except where unique results were obtained , this review will deal with the mm findings since it represents the moments most applicable to the activities of daily living .
1 ) showed that the mm reduced linearly as knee flexion increased from the anatomical ( 0 ) position .
this reduction became greater as the ankle joint moved from dorsiflexion to pf . moving the ankle from dorsiflexion to
furthermore , mm production was influenced more by ankle joint changes with the knee between 0 and 30 compared to the lesser degrees of knee angle .
the most striking observation was that the knee flexion moment decreased approximately 50% as knee flexion moved from 0 to 30. the reduction in the mm slowed across the 45 - 75 knee angles . when the knee was flexed beyond 75 , the stimulation had little effect on the knee flexion moment .
in other words , the ga s greatest contribution to knee flexion occurred with the knee in full extension .
this was observed across the three ankle joint angles . in a recent follow - up to the li
et al s   paper , landin et al   used similar methods to measure the ga production of a pf joint moment .
significant effects were obtained for the knee . a fully extended knee ( 0 ) and a dorsiflexed ankle ( + 15 ) created the highest ( 24.95  10.1 nm ) torque values , while the 105 knee angle and a plantarflexed ankle ( -15 ) produced the smallest values ( 13.02  4.9 nm ) .
li et al   reported that the ga produced the greatest knee flexion moment in the same joint combination , although the force was not as high ( approximately 9.5 nm ) , as we noted for pf ( approximately 25 nm ) .
we contend that this is most likely due to the disparity in the mechanical advantage of the knee and ankle lever systems . at the knee ,
the ga works in a third - class lever , which produces little mechanical advantage . at the ankle , however , the ga is part of a second - class lever , which has considerable mechanical power . based on our results and those of li et al , it is the 0 knee position and a dorsiflexed ankle that creates the optimal length for the ga to work at either joint .
it is apparently the mechanical properties of the lever system that causes the disparity in the ga s knee and ankle joint moments .
maximum plantarflexion moment across joint combinations .   regarding the lowest torque , the results reported in our more recent paper are somewhat in contrast to li et al . their results showed that flexing the knee to 90 or more , regardless of the ankle position , did not significantly reduce torque any further .
more precisely , the knee flexion moment did not significantly decrease as the knee was flexed to 105 with the ankle plantarflexed , neutral , or dorsiflexed . in our more recent project , we found that the pf joint moment decreased linearly from the 0/+15 joint combination , through the 105/-15 combination , at which point the ga was in its most shortened position .
it is interesting to note that even in the 105/-15 position , the pf mm ( 13.02 nm ) exceeded the knee mm reported by li et al   at the 0/+15 position .
the ga produced a greater pf torque in the position of shortest length than it produced for knee flexion in its most elongated position .
as argued above , this discrepancy is most likely due to the mechanical properties of a second - class compared to a third - class lever .
this lever effect also likely explains the fact that li et al found the smallest knee flexion moment before reaching the shortest position for the ga . the mechanical properties of a third - class lever may have rendered the ga insufficient before it reached its shortest length , whereas the second - class lever for the ga at the ankle allowed it to generate a pf torque force throughout the range motion used in these studies .
further research using knee flexion angles beyond 105 may provide further information on this issue .
li et al   argued that their findings were clinically important due to the ga s perceived role in knee joint stability during various lower extremity movements .
following knee joint traumas such as acl ruptures , the ga s synergistic work with the quadriceps assists in stabilizing the knee during weight bearing motions .
their results suggested that flexed knee positions ( > 30 ) , where the ga has less influence should be avoided , particularly in the early stages of rehabilitation .
rehabilitation exercises that maintain the knee in a position of stability ( < 30 ) appear to be best . maintaining this knee angle while stimulating co - contraction of all the dynamic stabilizers at the knee , which includes the ga , enhances stability through increased tibiofemoral joint compression , joint stiffness , and proprioceptive influences as suggested by wilk et al .
the clinical implications of landin et al   suggest that having a patient seated at the end of an examination table with the foot subject to gravity , while useful for evaluating the triceps surae , is a poor position from which to assess the ga as a plantarflexor .
this position places the knee at greater than 60 of flexion and maintains a somewhat plantarflexed foot , both of which lead to significant declines in ga torque production .
even if the clinician holds the patient s ankle in a dorsiflexed position , the flexed knee will still mask the ga s true strength .
manual testing of the ga in isolation should be performed , whenever possible , with the knee extended and the ankle dorsiflexed to potentially elicit the maximum pf torque from the ga . the recent work with
the ga reveals that the knee angle is the main factor in the development of both a knee flexion and pf moment of the ga .
the ga is a much more powerful plantarflexor than it is a knee flexor , due largely to the mechanical advantage of a second - class lever .
however , it is also possible that the 30 total degrees of the ankle rom used in li et al   and landin et al   was too restrictive .
total ankle rom ( dorsiflexion through pf ) can approach 85 . moving the ankle closer to the extreme ends of the rom
the dorsiflexion range is reduced as the knee moves closer to full extension , and therefore , can not be achieved in all joint combinations .
the direct attachment arises from the anterior inferior iliac spine ( aiis ) of the pelvis , while the reflected attachment emerges from the rim of the acetabulum and the fibrous capsule of the hip joint .
the two proximal attachments quickly blend into a common belly that coalesces into the quadriceps tendon and , in conjunction with the vasti muscles , then inserts on the tibial tuberosity [ 1 - 4 ] . the knee extension work of the quadriceps group is widely known and well documented , with a 60 - 90 knee angle appearing optimal . although the function of the rf as a hip flexor is largely unknown ,
anatomists have traditionally included it with the hip flexor group [ 2 , 3 ] . at the hip
, the rf works in a third - class lever , meaning that the application of the force lies between a proximal fulcrum and a distal resistance . as such , the hip is specialized for range and speed of motion , but high demands are placed on the muscles moving the lever .
consequently , the rf is heavily involved in all swinging actions of the lower extremity . in landin
hsu et al   described this action as placing unique demands on the rf for it is the only sport - related movement that requires substantial hyperextension of the hip at the outset .
this passively stretches the rf , then while assisting ( to some unknown extent ) with the initial hip flexion motion , the rf also quickly becomes involved with extending the leg . as a result , the rf is working simultaneously across two joints and becomes subject to distal strains or tears . from a clinical perspective , the role of rf in hip flexion pertains to muscle imbalance issues .
muscles can react to stress different ways , and an injury , volitional inactivity , or both can produce stress .
postural muscles tend to tighten , while their antagonistic muscles ( the phasic group ) tend to weaken . when postural muscles ( e.g. , iliopsoas , rf , and quadratus lumborum ) around the pelvis tighten , the phasic muscles ( e.g. , rectus abdominis , gluteals , vastus lateralis , medialis , and intermedius ) weaken and imbalances arise . janda   coined the phrase 
the gluteals and abdominals lengthen and weaken , while the iliopsoas and rf shorten and tighten .
this often leads to an anterior pelvic tilt , which in turn causes a variety of musculoskeletal maladies and poor posture .
what makes the lcs particularly troublesome is that sitting , even with good posture , creates conditions favorable to its development .
furthermore , rehabilitation protocols following a variety of lower extremity injuries strengthen the rf and the three vasti muscles are a primary focus of the exercises .
quadriceps strength is a key element in making the knee joint more stable and in slowing the progression of osteoarthritis , particularly in partially menisectomized knees .
however , since the rf is one of the postural muscles that tend to tighten , the potential exists for an exacerbation of a muscle imbalance and lcs .
if the rf plays a significant role in hip flexion , then increasing its strength in a quadriceps regimen presents a quandary .
should the rf not substantially contribute to hip flexion , then the use of quadriceps protocols in rehabilitation can be employed without concern for creating or exacerbating an existing lcs . in our project with the rf and hip flexion , we used 16 female volunteers from a university undergraduate population .
in this project , the biodex chair controlled hip angles and the removable casts controlled the knee angles . six hip angles ( 85 , 70 , 55 , 40 , 25 , and 10 ) were crossed with four knee angles ( 0 the anatomical position , 30 , 60 , and 90 ) to create 24 joint combinations . at the hip , the 85 ( upright ) and the 10 ( reclined ) positions represented the limits of the biodex chair . 
 figure 3 displays the mm of the hip flexion function of the rf across the various joint combinations .
the knee produced the only significant effect on the hip flexion moment of the rf .
as knee flexion increased from 0 to 90 , the hip flexion moment rose from 10.44 to 15.3 nm .
an intriguing result of this project was the contribution of the pm to the mm . as can be seen in figure 4 , a large portion of the mm was produced by the pm .
consequently , the stimulation , as shown by the sm line in figure 4 contributed little to the mm .
the hip is so thoroughly supported by various muscle and connective tissue components that in a passive state simply extending the hip can create a substantial hip flexion torque . moving the hip to the 10 position used in landin et al  
may not have produced an optimal length   for the rf during stimulation , but clearly created tension in the other soft tissue structures surrounding the joint .
these data show that contraction of the rf accounts for approximately 25% of the total hip flexion torque , indicating that chronic shortening of the phasic muscles and related fascial components can , even without strengthening of the rf , disrupt the postural / phasic muscle balance .
one avenue of future work would be investigating the effects stretching routines , either chronic or acute , have on the pm . shortening of the myofascial components surrounding the hip has been discussed in the literature for some time , and findings from over 20 years ago show that elongating these tissue elements can improve gait .
furthermore , single bouts of stretching have a deleterious effect on power production , so there may be an effect on a pm , but it would not necessarily be detrimental regarding lcs issues .
mean hip flexion sm , pm , mm values for all knee angles at each hip angle .  
a recent study of muscle activation patterns during gait showed that the rf displays high activity at the start of the swing phase and at stance phase initiation .
our results   do not contradict these , but rather indicate that , at the hip angles we used , contraction of the rf does not contribute much to hip flexion .
motion analysis of 14 elite runners performing a treadmill run at 21 km / h showed an average hip extension of -11.7 , with a range of about 20. due to limits of the biodex chair we were unable to extend the hip beyond + 10 , which is just shy of the anatomical position . a second direction for future work should include hip extension angles that approximate those reported by schadel et al .
it may well be that , since the 10/90 hip and knee position used in our project may have elongated the rf beyond its optimal length , it seems unlikely that greater lengthening would be beneficial .
 the extremities of the human body contain several bi - articular muscles . 
 the actions produced by muscles at the joints they cross are greatly influenced by joint moment arms and muscle length . 
 these factors are dynamic and subject to change as joint angles are altered . 
 therefore , to more completely understand the actions of such muscles , the angles of both joints must be manipulated . 
 this report reviews investigations , which have explored the actions of two bi - articular muscles of the lower extremities ( gastrocnemius and rectus femoris ) as the joints they cross are moved into various combinations of angles . 
 the findings have both clinical and physical performance ramifications . 
Introduction
GA
RF