The need to consider functional, not just isolated, strength

Strength is the ability to produce force and is influenced by morphological, biomechanical and neural factors; the contributions of each depend on the strength task. Functional strength is the ability to produce force in situations in which muscles are commonly used (and injured). Isolated strength tasks (ie, isokinetic testing or Nordic hamstring curls) minimise the requirements for neural control to develop the muscle’s ‘capacity’ to produce force. They do not mimic the way in which the muscles function.5 The dominant injury mechanism for HMI is sprint running,6 which requires high levels of fine motor control. There are numerous biomechanical and neural differences between isolated force development and force production during sprint running. Although isolated strength serves as a ‘capacity’ to produce force, certain factors may limit the transfer of isolated strength to functional situations.

The stabiliser muscle system is becoming more recognised as an important contributor to dynamic strength,7 with some evidence that core stability can influence HMI risk.8 9 Poor recruitment of stabiliser muscles may limit the ability of biarticular agonists to produce maximal force and increase the reliance of these muscles to act as global stabilisers.7 Compensatory demands placed on the muscle will result in elevated activation for a given exercise intensity7 and further compromise the ability to produce force when challenged maximally as a primary agonist (ie, reduced eccentric force at terminal swing phase at maximum running speed). Core stability training has been shown to result in reduced hamstring stiffness, likely due to reduced requirements for the hamstring muscles to compensate as global stabilisers.8

Altered ‘pelvic balance’ may also contribute to HMI. For example, increased hip flexor tension and reduced hip flexor range of motion (potentially secondary to decreased anterior pelvic stability and compensation) results in both an anterior tilted pelvis and altered intermuscular coordination. Reduced hip flexor range of motion has been associated with reciprocal inhibition of gluteus maximus and synergistic dominance of the hamstrings.10 Furthermore, an anterior tilted pelvis elicits a stretch on the hamstring muscle resulting in altered length–tension relationship, thereby reducing the ability of the muscle to produce force at longer muscle lengths, typically indicative of injury. This situation would be exacerbated by weakness in hip extension, as well as sacroiliac joint dysfunction.11

The ability to develop force rapidly and sufficient muscle preactivation are both important to produce force during rapid sporting movements. Sprinting involves ground contact times significantly shorter than the 250–300 ms required to develop maximal force. As such reduced muscle preactivation or poor explosive force characteristics would limit the ability to use the available force capacity explosively. Deficits in eccentric rate of force development of 40% was reported in previously injured hamstring muscles on return to sport.6

Players need to be conditioned to meet the ever-increasing demands of elite football

Underpreparedness for training and match play determined as a high acute:chronic workload, particularly when there has been limited sprint exposure in training, represents a major risk factor for injury.12 When athletes are exposed to unaccustomed volumes of high-speed running, there is an increased probability of sustaining a HMI.13 Therefore, most elite clubs now use GPS to monitor high-intensity running distances and peak running speeds during on-field rehabilitation and regular training; one goal being to ensure adequate acute:chronic workloads, which is now recommended as a key component in HMI prevention.12–14

Do not forget the influence of fatigue

Fatigue likely plays a key role in HMI,11 although the exact pathophysiological basis of this is poorly understood. Contributing factors could be the individual’s muscle fibre type, biomechanical and neural factors and metabolic fitness coupled with acute to chronic workload imbalance and insufficient periods of recovery.

Summary

We speculate (but acknowledge we have no supporting epidemiological evidence) that holistic HMI prevention programmes represent the best plan of attack for a progressive medicine and science department.2 Developing the muscle capacity to produce force is undoubtedly an important component of HMI prevention, and we do not refute the importance of this in HMI prevention. However, this needs to be considered alongside the factors which limit the muscles capacity to be expressed functionally, as well as factors which increase the reliance of the muscles to compensate.