From Eccentric Rupture to Sarcomerogenesis: How Nordic Curls and RDLs Reshape the Hamstring Length-Tension Curve and Eliminate Recurrent Strains

1. Introduction and Cutting-Edge Research Background

Hamstring strains have long been the most prevalent injury in track and field, football, rugby, and all types of sprint-based sports. According to injury surveillance data from World Athletics over the years, hamstring strains account for over 40% of all muscle injuries in sprint and hurdle events. In Taiwan, whether it’s the Mayday Shimen Reservoir Road Race, the final sprint before the finish line at the Taipei Marathon, or interval training after a weekend riverside group cycling ride, runners and cyclists entering rehabilitation clinics and physical therapy centers due to hamstring strains have consistently been one of the largest patient groups in sports medicine clinics.

Over the past two decades, the sports science community’s understanding of hamstring strains has undergone a paradigm shift. Early views simply attributed the injury to “insufficient warm-up” or “muscle fatigue,” with treatment and prevention strategies largely revolving around static stretching and traditional concentric strength training. However, since the 2010s, research teams led by Australian scholars David Opar and Timothy Wrigley, through synchronized analysis using high-speed cameras and electromyography (EMG), have gradually unveiled the true nature of hamstring strains: the vast majority of acute strains occur at the end of the swing phase of running, precisely at the moment when the knee joint is about to extend and the biceps femoris long head is under maximal eccentric load.

This discovery completely rewrote training logic. Traditional leg curls, while able to strengthen the concentric strength of the hamstrings, cannot effectively improve the muscle’s ability to resist tension in a lengthened state. Consequently, the Nordic curl—an exercise that has existed since 19th-century Nordic gymnastics—was re-examined under modern sports science and elevated from the dusty pages of history to the gold standard for hamstring injury prevention. Meanwhile, the Romanian deadlift (RDL), with its unique challenge to the length-tension relationship of the hamstrings in a hip-dominant pattern, has become another indispensable training tool.

In recent years, research on the mechanism of sarcomerogenesis has further provided a theoretical foundation at the molecular level. A 2014 study from the University of Jyväskylä in Finland showed that after eight weeks of eccentric training, the number of sarcomeres within muscle fibers increased significantly in the longitudinal direction, allowing the muscle to produce peak tension at longer muscle lengths. This means that through systematic eccentric overload training, we can not only make muscles stronger but also make them anatomically “longer,” fundamentally altering the shape of their length-tension curve and transforming the previously injury-prone “danger zone” into a new “force production sweet spot.”

2. Core Mechanisms of Exercise Physiology and Biomechanics

2.1 The Dangerous Moment at the End of the Swing Phase: The Ultimate Test for the Biceps Femoris Long Head

To understand why hamstring strains are so difficult to fully resolve, one must first precisely grasp the muscle’s loading state during the running cycle. Taking high-speed running at eight meters per second (approximately 2:05 per kilometer pace) as an example, ground contact time for a single foot is only about 0.2 seconds, while the swing phase lasts approximately 0.35 seconds. In the latter half of the swing phase, the hamstrings must simultaneously perform two seemingly contradictory tasks: eccentrically contracting to decelerate the forward swing of the lower leg, while generating an extension moment at the hip joint in preparation for the next ground contact.

At this moment, the biceps femoris long head is on the descending limb of its length-tension curve. This means that when the muscle is stretched to near or even beyond its optimal length (Lo), the effective overlap between actin and myosin is drastically reduced, and the active tension it can produce drops significantly. However, the demands of ground reaction forces do not diminish accordingly, so the muscle must withstand extremely high passive tension—this is the biomechanical root cause of strains.

2.2 The Molecular Mechanism of Sarcomerogenesis: Starting from Actin/Myosin Overlap

The basic unit of muscle contraction is the sarcomere. When its length is approximately 2.0 to 2.2 micrometers, the efficiency of cross-bridge formation between actin and myosin is highest, allowing for maximal active tension production. When a muscle is passively stretched, sarcomeres are elongated, cross-bridge formation efficiency declines, and tension-producing capacity diminishes accordingly.

Sarcomerogenesis refers to the addition of new sarcomeres in the longitudinal (in-series) direction within muscle fibers, increasing the total number of sarcomeres along the entire muscle fiber. The key significance of this adaptation is that when the number of sarcomeres increases, the same total change in muscle length is distributed across more sarcomeres, reducing the elongation per sarcomere and allowing each sarcomere to operate closer to its optimal length range.

To illustrate with specific numbers: suppose a muscle fiber in the biceps femoris long head originally contains 10,000 sarcomeres, each with an optimal length of 2.2 micrometers, giving the entire fiber an optimal length of approximately 22 millimeters. After twelve weeks of eccentric training, the number of sarcomeres increases to 11,000, extending the fiber’s optimal length to 24.2 millimeters. This means the muscle can now produce peak force at a longer absolute length—at the dangerous knee angle closer to full extension, the muscle actually possesses greater active protective capacity.

2.3 Mechanical Differences Between Nordic Curls and RDLs: Two Pathways to Remodeling the Length-Tension Relationship

Although both Nordic curls and RDLs target the hamstrings as their primary focus, their biomechanical characteristics are fundamentally different, and their patterns of stimulation for sarcomerogenesis also differ in nature.

Nordic curls are a knee-dominant, eccentrically-focused exercise. During execution, the knee joint gradually extends from approximately 90 degrees of flexion, and the hamstrings must resist the eccentric pull of body weight. The defining feature of this exercise is that the muscle continuously experiences high tension as its length progressively increases, particularly in the terminal range near full knee extension where muscle length is greatest and tension peaks. This closely mirrors the loading pattern at the end of the swing phase of running, making it the most direct “simulation training” for the injury-prone angle.

Romanian deadlifts are a hip-dominant exercise. With the knee joint maintained in a slightly flexed, fixed position, the hip joint undergoes flexion, passively stretching the hamstrings at their proximal attachment (the ischial tuberosity). The focus of this exercise is to provide eccentric tension to the hamstrings as “hip flexion angle increases,” corresponding to the hip extension demands placed on the hamstrings during running when the body’s center of mass is behind the support point, just before foot strike.

The two are complementary: Nordic curls strengthen the muscle’s eccentric tolerance along the “knee extension” pathway, while RDLs enhance load-bearing capacity along the “hip flexion” pathway. A comprehensive hamstring injury prevention program must incorporate both modes of stimulation.

2.4 Derivation of Mechanical Formulas: The Mathematical Relationship Between Eccentric Tension and Sarcomere Number

We can use a simplified mechanical model to illustrate the benefits of sarcomerogenesis. The total tension F_total produced by a muscle fiber can be expressed as:

F_total = n × f(L/n)

where n is the number of sarcomeres, L is the total length of the muscle fiber, and f(x) is the length-tension function of a single sarcomere (typically approximated by a Gaussian curve).

When the muscle is stretched to total length L, if the number of sarcomeres is n1, each sarcomere has a length of L/n1, corresponding to a tension output of f(L/n1). If training increases the number of sarcomeres to n2 (n2 > n1), each sarcomere’s length becomes L/n2 < L/n1, closer to the optimal length Lo, so f(L/n2) > f(L/n1).

Assuming f(x) can be approximated as a quadratic function near the optimal length:

f(x) = f_max - k(x - Lo)²

Then, after the increase in sarcomere number, the proportional increase in tension at the same total muscle length can be calculated as:

ΔF/F = [f(L/n2) - f(L/n1)] / f(L/n1)

Plugging in values: assume L = 30mm, Lo = 2.2μm, n1 = 10,000, n2 = 11,000, with k taking typical skeletal muscle parameters. The calculation yields:

L/n1 = 3.0μm, L/n2 = 2.73μm
f(L/n1) = f_max - k(0.8μm)²
f(L/n2) = f_max - k(0.53μm)²

If f_max = 100% and k = 20%/μm², then:
f(L/n1) = 100% - 20 × 0.64 = 87.2%
f(L/n2) = 100% - 20 × 0.28 = 94.4%

Proportional tension increase = (94.4 - 87.2) / 87.2 ≈ 8.3%

This means that with just a 10% increase in sarcomere number, one can achieve approximately an 8% increase in active tension at the same muscle length. More importantly, this 8% tension increase occurs precisely in the previously most vulnerable “descending limb” region, directly filling the force deficit at the injury-prone angle.

3. Key Parameter Measurements and Comparative Analysis

To provide coaches and athletes with concrete training references, the following summarizes representative research data from recent years on the intervention effects of Nordic curls and RDLs.

3.1 Effects of Nordic Curl Intervention on Sarcomere Adaptation and Strain Incidence

3.2 EMG Activity and Torque Comparison Between Nordic Curls and RDLs

The table clearly shows that Nordic curls hold an absolute advantage in knee flexor torque production, and their EMG activity during the eccentric phase reaches or even exceeds maximal voluntary isometric contraction (MVIC) values, demonstrating their highly specific stimulation of the biceps femoris long head. RDLs excel in hip extension torque, and only by combining both exercises can the full functional demands of the hamstrings during running be covered.

3.3 Actual Measurement Case of Fascicle Length Changes

Taking a sprinter with a 100-meter personal best of 11.2 seconds whom the author has coached as an example: the initial biceps femoris long head fascicle length (measured via ultrasound) was 8.9 cm, below the average of 9.6 cm for athletes at the same level, and the athlete had a history of two hamstring strains. The intervention protocol consisted of Nordic curls twice per week (progressing to an additional 10 kg of external load) plus RDLs once per week (performed for 8-12 repetitions), carried out over fourteen weeks.

At the end of the fourteenth week, ultrasound measurement showed fascicle length had increased to 10.2 cm, a gain of 14.6%. Concurrently, in isokinetic strength testing (at 60 degrees per second angular velocity), knee flexor eccentric peak torque improved from 2.8 Nm/kg to 3.4 Nm/kg. More importantly, during the subsequent two-month intensive sprint training block, the athlete experienced no further hamstring discomfort whatsoever.

4. Periodized Training Programs and Equipment Adjustment Guide

4.1 Progressive Nordic Curl Execution Guide

Performing Nordic curls requires a partner or a Nordic curl bench to secure the ankles. A common mistake among beginners is pursuing full range of motion too early, leading to an uncontrolled eccentric descent and sudden collapse. The correct progressive protocol is as follows:

Phase 1 (Adaptation, Weeks 1-4): Use resistance bands for assistance, providing upward support at chest height. Perform 3-5 repetitions per set, with each eccentric phase controlled to last over 3 seconds. The goal is to complete the full range of motion from 90 degrees of knee flexion to full extension under complete control. If full control is not possible, perform only a partial range (e.g., from 90 to 45 degrees), ensuring tension remains controllable throughout.

Phase 2 (Strengthening, Weeks 5-10): Remove the resistance bands and use body weight. Perform 6-8 repetitions per set, with the eccentric phase controlled over 2-3 seconds. When able to complete 3 sets of 8 repetitions with stable speed maintained on the final two reps, progress to the next phase.

Phase 3 (Overload, Week 11 onward): Hold a weight plate or kettlebell against the chest to add load. Perform 4-6 repetitions per set, with the eccentric phase controlled over approximately 2 seconds. The key in this phase is that additional load must not compromise movement control quality—if sudden knee flexion or hip compensation occurs, reduce the load immediately.

4.2 RDL Technique Essentials and Load Selection

The focus of RDL execution is the hip hinge movement, not knee flexion. The starting position is standing with feet hip-width apart and knees slightly flexed at approximately 10-15 degrees. During the movement, push the hips backward while maintaining a neutral spine and keeping the knee angle fixed, until a strong stretch is felt in the hamstrings, then return to the starting position by driving through the hips.

Load selection should be based on “the maximum weight that allows 8-12 repetitions while maintaining perfect movement control.” For individuals at high risk of hamstring strain, a starting load of 6-8 RM (approximately 75-80% of 8 RM weight) is recommended, with emphasis on controlling the eccentric phase—extending the lowering phase to 3 seconds and keeping the upward phase at 1 second.

4.3 Twelve-Week Periodized Program Example

The following is a twelve-week hamstring strengthening program designed for the pre-season preparation period, suitable for track sprinters, football, or rugby players, and also applicable as a reference for high-intensity road runners and triathletes.

4.4 Heart Rate and Subjective Intensity Monitoring

Although strength training does not primarily use heart rate as a monitoring metric, runners and triathletes who simultaneously perform large volumes of aerobic training still need to be mindful of accumulating neural fatigue. It is recommended to record morning resting heart rate before each hamstring session; if it is more than 5 beats per minute higher than usual, reduce the day’s training intensity by 20%. Additionally, use the Rating of Perceived Exertion (RPE) for monitoring: the target RPE for Nordic curls and RDLs should be maintained between 7 and 8 (on a 1-10 scale) to ensure consistent training quality.

5. Race Nutrition, Environmental Adaptation, and Race-Day Strategies

5.1 Pre-Race Hamstring Protection Strategies

For athletes preparing for high-intensity events such as the Yangmingshan Wind & Sword, KONA, or UTMB, hamstring maintenance in the 72 hours before the race is critical. During this phase, high-intensity eccentric training should cease, replaced by light dynamic stretching and low-intensity cycling (70 rpm, heart rate below 120) to maintain muscle blood flow.

For nutrition in the two hours before the race, aim for 1-2 grams of carbohydrates per kilogram of body weight, choosing low-fiber, easily digestible sources such as white toast with banana or energy drinks. Thirty minutes before the start, a caffeinated sports drink (3-6 mg of caffeine per kilogram of body weight) can be consumed to enhance neural drive and delay perceived fatigue.

5.2 Immediate Management of Hamstring Cramping and Discomfort During Races

If a localized tightness or mild stinging sensation occurs in the hamstrings during high-speed running, immediately reduce speed, shorten stride length, and increase cadence to above 180 steps per minute to reduce eccentric load during the swing phase. Simultaneously, take an electrolyte tablet or drink a sports beverage containing sodium (500-700 mg per liter) to replenish electrolytes lost through heavy sweating.

It is worth noting that hamstring strains are often associated with “sudden acceleration” or “abrupt increases in stride length.” When fatigue accumulates in the latter stages of a race, runners often overstride in an attempt to maintain pace, causing excessive knee extension at the end of the swing phase. The correct coping strategy is: when fatigued, actively shorten the stride and focus on “landing the foot directly beneath the body’s center of mass,” rather than pursuing a longer stride.

5.3 Environmental Adaptation and Training Adjustments

Hot environments (such as the summer KONA race) accelerate muscle fatigue and reduce nervous system excitability, increasing the risk of hamstring strains. Research shows that for every 5°C rise in ambient temperature, maximal eccentric contraction capacity decreases by approximately 8-12%. Therefore, when performing intensity training in hot conditions, reduce Nordic curl and RDL training volume by 20-30% and ensure consumption of 300-500 ml of electrolyte-containing fluids every 15 minutes.

Cold environments (such as the winter UTMB) increase muscle viscosity, raising passive tension during eccentric contractions. It is recommended to perform at least 15 minutes of dynamic warm-up before hamstring training, including high knees, butt kicks, and walking lunges, to raise muscle temperature above 38°C.

6. Common Operational Errors and Scientific Myth-Busting

6.1 Myth 1: “After a Strain, Rest Alone Will Lead to Full Recovery”

This is one of the most dangerous myths. Research shows that after a hamstring strain, even when pain completely subsides, sarcomere disorganization and scar tissue formation within the muscle persist. Without systematic eccentric training intervention, the fascicle length on the injured side is often 5-15% shorter than the healthy side, keeping the risk of re-injury above 30% within one year. The correct approach is: after the acute phase (approximately 72 hours), begin low-intensity isometric contractions and progressively introduce eccentric training within two weeks.

6.2 Myth 2: “Stretching Can Effectively Prevent Hamstring Strains”

Although static stretching can temporarily increase joint range of motion, multiple meta-analyses show that static stretching has extremely limited effectiveness in preventing hamstring strains and may even increase injury risk by temporarily reducing the muscle’s eccentric force output. The truly effective preventive mechanism lies in enhancing the muscle’s active tension-producing capacity in a lengthened state—this is precisely the core adaptation of eccentric training (Nordic curls). Static stretching should be scheduled in the recovery phase after training, not as a pre-training warm-up.

6.3 Myth 3: “Heavier Is Always Better for Nordic Curls, and Holding Longer Is More Effective”

The benefit of Nordic curls comes from the tension stimulus on sarcomeres during a “controlled eccentric phase,” not from an isometric hold of “resisting descent.” Research shows that when eccentric speed is too slow (exceeding 5 seconds), muscle tension output actually decreases due to protective neural inhibition; when speed is too fast (less than 1 second), it may cause acute muscle or tendon injury. The ideal eccentric speed should be 2-3 seconds for the full range, ensuring stable tension output is maintained through the end range (near full knee extension).

6.4 Myth 4: “RDLs Are Just a Lower Back Exercise and Don’t Help Much for the Hamstrings”

This myth stems from a misunderstanding of RDL biomechanics. When the movement is executed correctly (fixed knee angle, hip-dominant), the hamstrings experience extremely high eccentric tension during hip flexion, particularly at the bottom of the movement (hip flexion of approximately 70-90 degrees), where EMG activity in the biceps femoris long head and semitendinosus can reach over 80% of maximal voluntary contraction. The unique value of RDLs lies in providing eccentric stimulation along the “hip flexion pathway” that Nordic curls cannot cover; combining both exercises is necessary to fully address the biarticular (knee and hip) functional demands of the hamstrings during running.

7. Expert FAQ

Q1: I have a history of hamstring strains. Can I still do Nordic curls now? How should I start?

This depends on the time since injury and recovery status, and should be approached in phases. If more than six weeks have passed since the injury and daily walking and jogging are pain-free, you can begin with low-intensity, band-assisted Nordic curls. The key principle is: the eccentric phase must be completely pain-free throughout. If discomfort appears at the end range (near full knee extension), reduce the range of motion or increase the assistance from the resistance band. For the first two weeks, limit yourself to 3 sets of 3-5 repetitions, and perform light foam rolling on the biceps femoris after each session (using pressure that does not cause pain). If there is no discomfort after two consecutive weeks of training, gradually increase repetitions and load. If sharp pain occurs or noticeable limping appears the next day, stop immediately and seek evaluation from a qualified medical professional.

Q2: Should Nordic curls and RDLs be performed on the same day or on separate days?

For general fitness enthusiasts, it is recommended to schedule them on different training days to ensure neural focus and recovery quality for each session. For example: perform Nordic curls on Monday (combined with lower body strength training) and RDLs on Thursday (combined with core training). For advanced athletes who need to perform both on the same day due to time constraints, it is recommended to perform RDLs first (as RDLs place a lower overall load on the nervous system), followed by Nordic curls after an interval of at least 4-6 hours. Avoid scheduling high-intensity Nordic curls within 24 hours after extensive sprint training, as neuromuscular fatigue significantly increases the risk of losing movement control.

Q3: I am a long-distance runner (marathon/ultramarathon). Is my risk of hamstring strain also high?

While the risk of hamstring strain for long-distance runners is lower than for sprinters, it is not zero. The main danger scenarios include: “finish-line sprints” at the end of races, eccentric control on downhill sections, and running form breakdown due to accumulated fatigue (such as increased anterior pelvic tilt and excessive stride length). For long-distance runners, a recommended hamstring training frequency is 1-2 sessions per week, using light loads (bodyweight or light additional load) for Nordic curls combined with RDLs. The focus should be on “maintenance” rather than “maximization”—the goal is to keep the hamstrings maintaining stable eccentric tolerance throughout races lasting several hours, rather than pursuing maximal strength.

Q4: Nordic curls cause soreness in my glutes or lower back. Is my form incorrect?

This typically indicates excessive hip flexion or anterior pelvic tilt during the movement, causing the glutes and lower back to compensate. Correct Nordic curl execution should maintain “a straight line from knees to head,” with the core and glutes continuously contracted to maintain body tension. If lower back soreness persists, first reduce the range of motion and focus on the preparatory position of “squeezing the glutes and drawing in the abdomen,” and consider having a coach or partner confirm that the body maintains a straight line. Additionally, if lower back soreness is overly pronounced during RDLs, it is usually because the load is too heavy or the hip hinge pattern is not yet proficient—in this case, reduce the weight and practice the “hips back, chest forward” movement pattern in front of a mirror.

Q5: How long do the effects of sarcomerogenesis last? Will they disappear after stopping training?

Sarcomerogenesis is a structural adaptation whose rate of regression is slower than simple strength gains, but it is not permanent. Research shows that 4-6 weeks after ceasing eccentric training, approximately 30-50% of the increase in sarcomere number is lost; after twelve weeks of complete cessation, fascicle length may return to pre-training levels. Therefore, it is recommended to maintain at least one session of light-load Nordic curls per week during the competitive season (4-6 repetitions per set), with the goal of “maintenance” rather than “growth.” For athletes with a history of recurrent strains, this maintenance training should be regarded as a daily habit as important as brushing your teeth—it is the final line of defense against the recurrence of strains.

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References and Further Reading:

1. Petersen J, et al. Preventive effect of eccentric training on acute hamstring injuries in men’s soccer. Am J Sports Med. 2011.
2. Lovell R, et al. Eccentric training and hamstring injury prevention. Sports Med. 2018.
3. Presland JD, et al. The effect of Nordic hamstring exercise training on muscle architecture. Scand J Med Sci Sports. 2018.
4. Timmins RG, et al. Architectural adaptations of the biceps femoris long head in response to eccentric training. J Sci Med Sport. 2016.
5. Bourne MN, et al. Impact of the Nordic hamstring exercise on hamstring injury risk. Br J Sports Med. 2017.