Hamstring Strength

Incidence of Hamstring Injuries

The right-hand side of the discussion emphasizes that muscle injuries, particularly hamstring injuries (HSIs), represent a significant problem in sports, boasting one of the highest incidence rates among non-contact injuries. These injuries are prevalent across various sports, including soccer, rugby, and track & field, often leading to considerable time loss and re-injury rates.

The left-hand side quantifies the burden of these injuries by focusing on the number of days lost from educational or training activities. It highlights that while some injuries might be frequent, their impact on time lost varies significantly based on severity and recovery duration.

Example given: Contusions, or bruises, are quite frequent in sports but generally result in minimal time lost, typically only a few days to a week, due to their lower severity and faster healing process compared to structural muscle tears.

In stark contrast, Anterior Cruciate Ligament (ACL) injuries occur less frequently, perhaps only affecting a few athletes per season in a team setting, but they lead to drastically extended recovery times, often requiring 66 to 1212 months of rehabilitation and significant medical intervention, underscoring their severe impact.

Connection Between Hamstring Strength and ACL

The importance of eccentric hamstring muscle strength is highly emphasized as a critical factor in both preventing hamstring injuries and reducing the risk of ACL tears. This is due to its crucial role in controlling knee joint dynamics.

Eccentric strength refers to the muscle's ability to lengthen under tension while resisting an external force. In activities like running or cutting, the hamstrings eccentrically contract to decelerate the tibia (shin bone) relative to the femur (thigh bone), preventing excessive anterior tibial translation, which is a primary mechanism for ACL injury.

A deficit in eccentric hamstring strength compromises this crucial control, potentially allowing the tibia to move too far forward during rapid deceleration or landing, thereby increasing the stress on the ACL and heightening the risk of tears.

Common Causes of Hamstring Injuries

High-speed running is identified as the single most significant risk factor for acute hamstring injuries, accounting for a large proportion of these incidents in sports. Activities involving maximum or near-maximum sprinting velocities place immense stress on the hamstring muscle group.

Injuries typically occur during activities that necessitate extremely high muscular force and strain, especially when the hamstrings are performing eccentric work at long muscle lengths (e.g., during the late swing phase of sprinting).

Mechanisms of Injury

Evidence indicates that a large percentage of hamstring injuries occur while running, particularly during unresisted sprinting, or during specific interactions such as duels with opponents where sudden changes in direction or speed are required.

Injuries often initiate from a state of maximal velocity, where the hamstrings are undergoing peak eccentric loading, or during abrupt directional changes that involve rapid deceleration followed by acceleration.

Breakdown of injury mechanisms:

  1. High-speed running:

These injuries typically involve acceleration, deceleration, or rapid force generation during sprinting. The peak strain on the biceps femoris often occurs during the late swing phase, just before foot contact, when the muscle is maximally lengthened and rapidly contracting eccentrically to decelerate knee extension.

  1. Stretch-type injuries:

These injuries occur during actions that impose exceptionally long stretches on the muscle, often combined with high muscular force. Examples include kicking a ball with maximal effort, specific passing motions in sports, or executing splits, where the muscle is stretched to its extreme range of motion.

Key Study Findings on Hamstring Injuries

Recent video analysis of hamstring injuries in female athletes has provided deeper insights into the specific contexts of injury:

Approximately 80%80% of hamstring injuries arise under non-contact scenarios, meaning they occur without direct physical contact with an opponent. These usually involve actions like running at maximum velocity, rapid acceleration or deceleration, or changing direction without external force.

Injury splitting: The analysis further categorizes running-related injuries into two main types: those occurring during high-speed phases of sprinting (e.g., late swing phase) and those occurring during extreme stretching actions, which are also commonly observed during dynamic movements like kicking.

Anatomy of Hamstrings During Sprinting

A detailed description of a four-stage sprint cycle includes the following phases: toe-off, mid-swing, late swing, and terminal swing. Each phase places unique demands on the hamstring muscles.

The focus is on the intricate relationship between hip flexion and knee extension/flexion, which maximally stresses the hamstrings during various points of the running motion, particularly during late swing phase where the hip is flexed and the knee is extending.

Notes on the biceps femoris as the most commonly injured muscle during sprint activities, accounting for up to 80%80% of HSIs. Its unique structure, with a long head originating from the ischial tuberosity and attaching to the fibular head, gives it a significant anatomical leverage. This structure makes it particularly susceptible to injuries during high-speed eccentric contractions, especially when the muscle is simultaneously lengthening at the hip (hip flexion) and resisting knee extension.

High levels of strain occur as the biceps femoris undergoes rapid lengthening and then shortening during the high-speed running cycle, especially during the transition from late swing to early stance.

Injury Phases During Running

Emphasis on the importance of specific phases in the sprint cycle where injuries frequently occur, based on biomechanical analysis and observed incidents:

  1. High-speed running phases: Characterized by significant lengthening of the hamstrings (especially the biceps femoris long head) under very high eccentric loads near maximum hip flexion and knee extension. This combination creates peak stress, making this phase a high-risk window for injury. The anatomical leverage of the biceps femoris further contributes to its susceptibility in this phase.

  2. Early stance phase: This phase, occurring immediately after foot placement, is characterized by very high ground reaction forces and intense muscular activity to absorb impact and propel the body forward. The transition from eccentric to concentric contraction under high load and specific joint angles (e.g., significant knee flexion) can also be a critical time for injury risk, particularly influenced by dynamic stability, torque generation, and center of mass shifts.

  3. Late swing phase: While forces are still high, the activation patterns of different hamstring muscles (e.g., semitendinosus and semimembranosus) can vary. These muscles might be activated differently to stabilize the knee and control sagittal plane movement. Their specific activation patterns and muscle-tendon unit lengths during this phase influence their individual susceptibility to injury compared to the biceps femoris.

Preventing Hamstring Injuries

Suggestion that higher hamstring strength, particularly eccentric strength, can directly correlate with a significantly decreased risk of injury and demonstrable performance enhancement:

Higher hamstring strength is shown to reduce ACL loading by providing dynamic knee joint stability. Stronger hamstrings can effectively counteract anterior tibial translation, thus intrinsically contributing to ACL injury prevention.

Improvement in hamstring strength leads to better knee function, enhanced power production, and improved braking capabilities during high-speed running, ultimately improving overall athleticism, speed, and safety in competitive sports.

Testing Hamstring Strength

The discussion touches upon various modes of testing hamstring strength, specifically focusing on objective and practical assessments:

  1. Isokinetic Testing:

These are device-based tests (using isokinetic dynamometers) that measure muscle strength (torque) at controlled angular velocities across a range of motion. They can assess both concentric and eccentric strength, providing detailed force-velocity profiles.

Issues identified include: limited accessibility due to the specialized equipment required, high procurement and maintenance costs, and a significant dependency on highly trained staff for proper test execution and accurate data interpretation, making them less practical for widespread field use.

  1. Nordic Hamstring Exercise Testing:

Highlights its significant practicality, high accessibility (requiring minimal equipment), and proven reliability for assessing eccentric hamstring strength in the field or clinic setting. It involves an individual kneeling while an assistant steadies their ankles, and then lowering their body towards the ground as slowly as possible by eccentrically resisting the fall with their hamstrings.

The Nordic hamstring exercise provides significant insight into eccentric strength capacity and is adaptable for individual assessments, allowing for easy normative comparisons and progress tracking due to its strong correlation with HSI risk and performance metrics.

Eccentric vs Isometric Testing

Eccentric Testing

  1. Recognized for its superior reliability and specificity in assessing muscle strength, particularly in predicting injury risk and athletic performance in dynamic sports. This is largely because most sports-related injuries occur during eccentric muscle actions.

  2. It is considered critically important in understanding muscle actions occurring during sports, as it mimics the high-load lengthening contractions frequently experienced during deceleration, cutting, and sprinting.

Isometric Testing

  1. Questions are raised regarding its effectiveness and ecological validity due to a relative lack of evidence showing significant isometric actions in high-speed sprinting, where the hamstrings are predominantly acting eccentrically.

  2. Potential variances in results depending on different populations (e.g., elite athletes vs. recreational) and test angles explored (e.g., the difference between testing at 90o−30o90o−30o of knee flexion vs. 90o−90o90o−90o settings, where the first angle might isolate more eccentric capacity while the second focuses on static maximal voluntary contraction at a specific joint angle).

Validity in Testing and Measures

Importance of understanding the distinct differences between correlation versus agreement in testing methodologies. Correlation indicates a relationship between two variables, while agreement measures how closely two methods provide the same result, a critical distinction when swapping tests.

Recognizes that not all tests, even if they show a strong correlation, can be interchanged without impact on results due to differing units, measurement protocols, or specific sets of measurement (e.g., peak force in Newtons vs. work in Joules). For instance, a test measuring peak torque might correlate with a test measuring average power, but they are not interchangeable for clinical decision-making without specific agreement analysis.

Review of Normative Values in Hamstring Strength Testing

Studies have indicated specific normative force thresholds for hamstring strength that can serve as benchmarks to prevent injuries. These values are often derived from large athlete cohorts and help identify individuals at higher risk.

Values such as below 260−280260−280 Newtons (N) of eccentric force in specific tests (e.g., Nordic hamstring test) are frequently cited as showing an increased risk for hamstring injuries. This threshold signifies a potential eccentric strength deficit that needs addressing.

There is a clear necessity to normalize results based on body mass (e.g., Newtons per kilogram of body mass) to derive accurate and fair comparisons in competitive settings across athletes of different sizes and to better identify relative strength deficits.

Body Composition Considerations

Discussion of research illustrating that as athletes mature, their body composition significantly affects the absolute muscle strength produced during testing. Increases in lean muscle mass and overall body weight contribute to higher absolute force outputs.

Considerations must be made for absolute strength (total force produced) versus relative strength (force produced per unit of body mass) in youth populations. The impacts of changing body composition on test results mean that raw strength scores in adolescents might increase simply due to growth, without necessarily indicating an improvement in relative strength or injury resilience.

Conclusions on Injury Prevention and Management

Reinforcement that no single test can accurately predict injury by itself; instead, it's a multi-faceted approach incorporating various measures, individual physiological conditions, historical injury data, and sport-specific demands. A comprehensive risk assessment involves a battery of tests rather than a single metric.

Final takeaway: Careful selection of appropriate tests, a thorough understanding of physiological responses to training, and a holistic view of the athlete's overall health and biomechanics are crucial for developing effective and personalized injury management and prevention strategies in athletes.