Knee flexion exercises - friend or foe?

Introduction

Various renowned sprint coaches completely abstain from any kind of hamstring curls in their strength training programs. Their recommendations and beliefs are founded on biomechanical studies of running technique and empirical evidence of some track athletes, which injuries were caused by hamstring fatigue due to leg curls.

During running, inertia causes the leg to curl up under the thigh, not the contraction of the hamstrings [1, 2]. Once the hip has extended during the terminal stance phase, the hip flexors forcefully contract to bring the leg forward again. The combined action of hip extension quickly followed by hip flexion causes the leg to curl up under the thigh. EMG studies during sprinting have shown that the hamstrings are silent during the early swing phase [1, 2]. From this point of view strengthening the knee flexion movement will not result in enhanced sprinting speed.

Completely abstaining from any kind of knee flexion is a flaw for various reasons however.

The hamstrings play an important role in stabilising the knee.

First of all, these recommendations do not take into account the differences between various sports. While track and field does not require quick changes of direction, the ability to quickly decelerate and change direction is essential to be successful in team sports. Explosive stopping, cutting and turning actions require a high amount of strength and neuromuscular control from the hamstrings to stabilise the knee and decrease ACL loading [3, 4, 5].

Peak ACL tensile force occurs around 20° of knee flexion [6, 7, 8, 9, 10, 11, 12]. Around this knee angle a contraction of the semitendinosus and biceps femoris (long and short head) provide the greatest decrease of ACL loading [13].

The biceps femoris is an important dynamic stabiliser of the knee [3]. Its tendon consists of three layers that insert on the crural fascia, the knee capsule and the head of the fibula. The middle part of the tendon also surrounds the lateral collateral ligament. The short head of the biceps femoris is mono-articular with a deep tendon insertion, which makes it perfectly suitable to stabilise the knee. Because the tendon inserts into the posterolateral capsule, rotary and anterioposterior stress on the knee joint will trigger a contraction of the biceps femoris [14].

At the medial side of the knee, the conjoined tendon of the sartorius, gracilis and semitendinosus lies superficial to the medial collateral ligament and fuses with the crural fascia. These muscles, tendons and membranes play a significant role as stabilisers of the medial side of the knee joint in the upright posture [15].

It is clear that the multidirectional athlete requires strong and functional biceps femoris, semitendinosus, gracilis and sartorius muscles to provide additional valgus and varus stability to the knee and help protect the ligamentous structures of the knee. Both hamstring and ACL injuries have been related to strength deficits and fatigue of the hamstring muscles [16, 17, 18, 19, 20]. The hamstring muscles work in pair with the quads. Because the hamstrings are weaker than the quads and mainly constitute out of fast-twitch fibres they are more prone to fatigue. Establishing and maintaining adequate strength ratios between the hamstring and quadriceps (hamstr. ecc./quad. conc. ratio > 1.0; hamstr. con./quad. con ratio > 0.6) muscles plays an important role in the prevention of injuries [21, 22]. Leg curl exercises have been shown to provide an excellent training stimulus to strengthen the short head of the biceps femoris, semitendinosus and gracilis muscles [23, 24]. The short head of the biceps femoris, which plays an important role to stabilise the lateral side of the knee, is mono-articular and can only be strengthened through knee flexion exercises.

The Nordic hamstring exercise and leg curl variations induce architectural changes

Another reason not to completely exclude leg curl exercises from your training routine is their potential to increase the end-range hamstring strength and reduce hamstring strains.

Most strain-type hamstring injuries occur during the terminal swing phase [25, 26]. During this phase the hamstrings contract eccentrically to brake the knee extension of the swing leg while the hip is in flexion [25]. Just before foot-strike the hamstrings reach peak force and peak lengths [27, 28, 29, 30, 31, 32]. At high speeds the EMG activity of the hamstring muscles during the terminal swing phase has been shown to exceed the activity of a maximal voluntary contraction [33]. This suggests that eccentric knee flexion training, like the Nordic hamstring exercise, contributes to the prevention of hamstring exercises.


Nordic hamstring exercise

The Nordic hamstring exercise has its criticisms including being bilateral, which potentially promotes side-to-side strength imbalances and that it is predominantly mono-articular, whereas hamstrings are bi-articular and do not work in isolation. The forces in the Nordic hamstring exercise are not applied to the ground nor does the exercise work the hamstrings near the contraction speeds of sprinting. Coaches that reject the value of the Nordic hamstring exercise emphasise specificity without considering the architectural changes that occur when Nordics are implemented into the training routine. Nordic hamstring exercises have been shown to shift the optimal length of the hamstrings towards longer muscle lengths and increase the end-range hamstring strength [34, 35, 36, 37]. Eccentric training increases muscle fascicle length, by promoting the addition of sarcomeres in series, so the hamstrings can produce greater forces at longer muscle lengths [34, 35, 36, 37]. The hamstrings function at long lengths during sprinting [29, 30, 31]. Shifting the length-tension curve and increasing the end range strength of the hamstrings prevents fibres from reaching a length where they are susceptible to tearing [38]. A Nordic hamstring exercise program has been shown to substantially reduce the incidence of new (60%) and recurrent (85%) hamstring injuries of football players [39]. Also other leg curl exercises with a strong eccentric component like the Yoyo flywheel, slide-board or stability ball leg curl induce a greater stretch of the hamstring muscle fibres and have been shown to promote the addition of sarcomeres in series [40].

SB hip extension & leg curl
SB hip extension & leg curl

Conclusion

All criticisms posed in the literature by coaches and fitness experts that are opposed to leg curls are correct. Leg curl exercises work the hamstrings solely over the knee joint in one plane of motion and forces are not applied directly to the ground, nor do they work the hamstrings near the contraction speeds of sports skills. Very few exercises meet these criteria however. Throughout history, successful training and periodisation models have used a variety of methods and means from general to specific. A good balance between general and specific exercises is important. Specific exercises provide an overload on the central nervous system, while more general exercises are applied to load the muscles (central-peripheral model). Very few specific hamstring exercises provide a sufficient muscle overload. Constant exposure to the same specific exercises will also compromise adaptation and performance over time due to training monotony. General exercises are emphasized during the preparatory period and become auxiliary as the competitive season approaches. The optimal scheduling of exercises and dose are paramount. Not leg curl exercises but the wrong application of these exercises in terms of volume and intensity cause fatigue and increase the risk of injury.

Because forward acceleration in sprinting is merely produced by hip extension, the majority of hamstring performance enhancement exercises should be hip dominant leg exercises [41, 42, 43]. The hamstrings also fulfil an important function during the swing phase of running. The majority of hamstring injuries occur just before foot-strike when the hamstrings reach peak force and peak lengths [25, 26, 27, 28, 29, 30, 31, 32]. Eccentric knee dominant hamstring exercises have been shown to enhance the ability of the hamstrings to absorb the energy of the swing leg and reduce the risk of strain-type hamstring injuries.

A complete elimination of leg curl exercises is like throwing out the baby with the bathwater. A combination of leg curl exercises for strengthening purposes and exercises that require more neuromuscular control and enhance inter-muscular coordination prevent knee and hamstring injuries [44].

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