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Columns: Exercise Technique

The Sliding Leg Curl

Taberner, Matt CSCS; O'keefe, Jason BSc; Cohen, Daniel D. PhD, CSCS

Editor(s): Dawes, Jay PhD, CSCS*D, NSCA-CPT*D, FNSCA

Author Information
Strength and Conditioning Journal: June 2016 - Volume 38 - Issue 3 - p 117-121
doi: 10.1519/SSC.0000000000000214
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Abstract

BACKGROUND

Hamstring strain injuries (HSIs) are among the most common lower limb injuries in both track and team-sport athletes (4,48). HSIs are characterized by posterior thigh pain, predominantly in the bicep femoris (BF) (45), which range in severity from a partial loss of function (grade I) to a complete tear of the muscle fibers and immobility (grade III) (6). The particular vulnerability of the BF to injury is hypothesized to be related to its biarticular structure, such that the muscle is stretched both proximally and distally in a position of concurrent hip flexion and knee extension during both high-speed running and explosive actions, including sprinting, change of direction, and jump/landing (10).

HSI risk is multifactorial (27), influenced by modifiable and nonmodifiable predisposing factors. Nonmodifiable risk factors include previous injury (3,10,20), age (19,46), muscle fiber type and distribution (33), and ethnicity (48). A number of modifiable factors have also been reported, such as inadequate eccentric strength and strength imbalances (bilateral and hamstrings/quadriceps ratio) (14,18,35), core stability (37), and flexibility (3,8,47). In soccer players, interventions aimed at developing eccentric strength show HSI risk reduction (2,31). Furthermore, it is suggested that not only peak strength but also strength at a longer muscle length (9,22,32,36) and the strength/power endurance capacity of the hamstrings (15,23,48) are also neuromuscular factors relevant to HSI risk reduction.

During rapid acceleration in the later stage of swing phase, the hamstrings work eccentrically to counteract torque generated by the quadriceps to decelerate the limb and control extension at the knee (5). During this rapid changeover from eccentric deceleration of knee extension and hip flexion to concentric function as an active hip extensor, the hamstrings are highly vulnerable to injury (3).

Exercise selection should therefore not only target eccentric muscle action to help increase the muscle's safe zone of operating but also the rapid changeover into concentric functioning; providing both a proximal and distal stimulus. Nordic hamstring curls (NHCs) provide an eccentric overload without the need for specialized equipment and have been shown to increase concentric and eccentric strength, shift peak torque to a longer length (11), and reduce HSI incidence (2), they have become a popular component of many injury prevention and conditioning programs.

Despite the beneficial outcomes reported after NHC training, it is noted that athletes who do not possess a sufficient strength base may have difficulty performing the exercise with correct technique through the full range of motion (24,28), potentially reducing the stimulus for neuromuscular adaptations in a more extended position, a range believed to be important in protecting the hamstrings against injury (22,36). Unless using the Nordbord, a new device with braces which hold the ankle in position, the NHC requires the assistance of a partner to hold down the heels of the athlete to perform the exercise, or if the assisted version is performed, a further partner to hold the band to provide the assistance to the movement (24), presenting a potential drawback if training alone or accessibility to resistance bands is limited. Also of note, NHC training is performed bilaterally, such that bilateral strength or strength endurance deficits may not be corrected (29). Indeed, increased angle of peak torque asymmetry is reported after a 4-week NHC training intervention (11).

In view of these potential limitations, and according to the principle of varying the loading stimulus, whereby nonlinear periodization of training, with varied volume and intensity is significantly more effective for strength development as opposed to a linearly structured program (39), alternative eccentric emphasis hamstring exercises are needed. One such exercise is the sliding leg curl (SLC), shown to generate relatively high BF and semitendinosus activity particularly during the eccentric phase, and to increase hamstring strength in a 4-week intervention study (29). The SLC can be performed without a partner and provides an alternative to the NHC, which targets the same stretched position hip extension/knee flexion moments, but in an anterior-posterior load vector; with the direction of load front to back in comparison with back to front. Once competence is gained in the basic exercise form progression to single limb and other variations which progress the stimulus for overload and facilitate the targeting of interlimb strength deficits.

The SLC also differs from other forms of performing the leg curl exercise. The machine leg curl is an open chain exercise, with the distal end of the limb free, whereas the SLC is closed chain with the limb in contact with the ground, thereby offering a higher degree of neuromuscular/mechanical specificity and as McGill (25) stated “stiffening the torso proximal to the shoulders and hips transfers the full force and movement of muscles to the distal side of these ball and socket joints resulting in greater limb strength and speed.” The leg curl can also be performed using a suspension device, but as a form of unstable training with a high proprioceptive demand, it can be described as an “imperfect method of strength training” (38) because strength is most effectively improved from a stable base of support.

SLCs usually involve the use of a slide board and slide pads. However, a towel can also be used on a wooden floor making it possible to perform the exercise and its variations when access to equipment is limited.

COACHING POINTS—SLIDING LEG CURL

  • The athlete begins in a supine position with heels positioned on sliding pads, or towel; in contact with slide board, or other suitable surface.
  • The athlete then lifts hips into extension with the shoulders, hips, and knees aligned, and the ankles in a dorsiflexed position; creating stiffness throughout the posterior chain (Figure 1).
  • The athlete should have their elbows tucked in to their sides to help create a stable base of support to perform the movement.
  • The athlete should aim to maintain active hip extension by “squeezing the glutes” while simultaneously controlling the eccentric portion of the lift as the degree of knee extension increases (Figures 2 and 3).
  • As the angle of knee extension increases, so does the tension on the hamstrings, and the intensity of the exercise also increases. While maintaining stability about the hip, the athlete initiates the rapid changeover from eccentric to concentric by pulling the heels back toward starting position; as the hamstrings are recruited to flex the knee (Figure 4).
  • The athlete should aim to maintain a “ribs down” position during exercise maintaining a neutral spine position. The ribs down position provides a coaching cue to engage the anterior core musculature and maintain zone of apposition to allow the diaphragm to expand during inhalation (7).
F2-14
Figure 1:
Starting position of the sliding leg curl. The hips are lifted into extension, with the shoulders, hips, and knees aligned.
F3-14
Figure 2:
The athlete should maintain active hip extension by squeezing the glutes during the eccentric portion of the lift.
F4-14
Figure 3:
Control of the eccentric portion must be maintained throughout. As the angle of knee extension increases, so does the tension on the hamstrings.
F5-14
Figure 4:
While maintaining stability at the hip, the heels are pulled back to starting position to begin next repetition.

VARIATIONS

The SLC offers a great deal of flexibility for the strength and conditioning coach as the exercise can be easily regressed or progressed dependent on the strength level of the athlete or adapted to emphasize a specific part of the range of motion.

ECCENTRICS

Athlete performs the eccentric portion of the lift only; resetting once full knee extension has been achieved. The tempo of eccentrics is 2–5 seconds, according to the strength level of the athlete.

PROXIMAL OVERLOAD

Increased overload to specifically challenge the hip extension component of the SLC may be generated by placing chains or a barbell across the athlete's hips. The athlete must focus on squeezing the glutes to maintain hip extension while controlling knee extension during the eccentric portion of the exercise and knee flexion, if the concentric portion is performed.

DISTAL OVERLOAD

The addition of external load to the distal segment may increase the challenge to the knee flexor component of the SLC. Weight discs, ankle weights, or cable resistance are all viable options (Figure 4). This option can also be used to overload the eccentric only version of the exercise. Providing a training stimulus to the distal insertion aims to address the role of eccentric hamstring contraction in decreasing anterior tibial translation and increasing stability at the knee (13).

SINGLE LEG

Both eccentric only and eccentric: concentric versions of the SLC can be performed unilaterally to target interlimb strength or strength endurance asymmetries. While performing the exercise unilaterally, the contralateral limb should be positioned in isometric hip flexion and knee flexed/extended depending on the athlete's preference.

VARIABLE RESISTANCE

An important principle guiding any training program is to progressively increase the specificity of movement in terms of recruitment pattern velocity (44). Variable resistance in the form of elastic bands can provide an overload stimulus that aligns with the force capabilities of the muscle throughout a given range of motion (26). Elastic bands aim to challenge the ascending strength curve by providing greater resistance near full extension; the position of peak force capabilities (1,12). Additional loading may be required to develop strength at longer lengths and promote a shift in the angle of peak torque towards a more extended position, as a short-term intervention that included the unloaded SLC, did not produce these adaptations despite the relative high electromyography signal (EMG) noted during the eccentric phase of the exercise (29). Elastic bands can be attached to increase tension at either proximal or distal segments during the exercise. For proximal tension, the band should be placed around the athlete's ankles with the band stretched in the starting position of the exercise; hips extended and knees flexed. The athlete is then required to control deceleration of knee extension at a higher velocity during the eccentric portion of the exercise; tension on the band unloads as the knees move toward full extension. The athlete then resets if the eccentric version of the lift is being performed, or if the eccentric-concentric version is performed, they drive the heels back through the slide pads into knee flexion; while maintain hip extension. The level of resistance from the bands increases in the concentric phase as the degree of knee flexion increases. For increasing distal tension, the band should be elongated when the knees are fully extended, placing an overload at this point of the exercise.

OCCLUSION TRAINING

Within a team sport environment, most hamstring strains occur toward the end of matches or training sessions (48). In simulated match-play protocols, fatigue is associated with reduced hamstrings eccentric peak torque, shifts in the angle of torque to a shorter length, specific changes in the eccentric hamstring to concentric quadriceps ratio close to full knee extension (12), a reduced rate of hamstrings torque development (23), earlier activation of the BF and semitendinosus during the swing phase of a maximal sprint cycle (30), and alterations in the biomechanics of the sprint cycle (40). These changes are believed to increase the vulnerability of the muscle to injury during the sprint cycle (40,46). Taking into consideration these findings, and the independence of a muscle's fatigue resistance and its maximal strength (17,34), this suggests the need to not only develop hamstring strength but also implement specific conditioning to enhance the muscle's tolerance of metabolic by-products and minimize the impact of high-intensity intermittent activity on neuromuscular function. In accordance with this distinction, Small et al. (41) observed improved hamstring fatigue resistance following an NHC intervention performed after on-field training and thus in a prefatigued state, but not when the intervention was performed in a fresh state prior to on-field training.

However, the classic approach to develop muscle fatigue resistance; involving high repetitions, short rest periods, and a high volume of work (16), may present a challenge in the team sport conditioning scenario. In this context, occlusion training, which has been shown to result in local tissue hypoxia and suppression of lactate clearance within the muscle subject to occlusion (42) represents a potential means to enhance the fatigue resistance training effect of a smaller volume of work. Performing occluded SLCs has the potential to create conditions which require the hamstrings to operate in a state of fatigue (21). Elastic knee wraps should first be wrapped around the proximal part of the muscle, and either eccentric or eccentric-concentric SLCs are performed with higher rep ranges until fatigue. Depending on the capabilities of the athlete, 3–5 sets should be performed, using interset rest periods between 30 seconds and 1 minute during which occlusion is maintained (43).

In conclusion, the SLC and its variations provide an alternative or complement to the Nordic hamstring exercise, allowing specific muscle function, lengths, and fatigue conditions to be trained at which injury occurs without the assistance of a partner while also providing an opportunity to target interlimb strength deficits.

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