A Conceptual Exploration of Hamstring Muscle-Tendon Functioning during the Late-Swing Phase of Sprinting: The Importance of Evidence-Based Hamstring Training Frameworks

Abstract

An eccentrically lengthening, energy-absorbing, brake-driven model of hamstring function during the late-swing phase of sprinting has been widely touted within the existing literature. In contrast, an isometrically contracting, spring-driven model of hamstring function has recently been proposed. This theory has gained substantial traction within the applied sporting world, influencing understandings of hamstring function while sprinting, as well as the development and adoption of certain types of hamstring-specific exercises. Across the animal kingdom, both spring- and motor-driven muscle-tendon unit (MTU) functioning are frequently observed, with both models of locomotive functioning commonly utilising some degree of active muscle lengthening to draw upon force enhancement mechanisms. However, a method to accurately assess hamstring muscle-tendon functioning when sprinting does not exist. Accordingly, the aims of this review article are three-fold: (1) to comprehensively explore current terminology, theories and models surrounding muscle-tendon functioning during locomotion, (2) to relate these models to potential hamstring function when sprinting by examining a variety of hamstring-specific research and (3) to highlight the importance of developing and utilising evidence-based frameworks to guide hamstring training in athletes required to sprint. Due to the intensity of movement, large musculotendinous stretches and high mechanical loads experienced in the hamstrings when sprinting, it is anticipated that the hamstring MTUs adopt a model of functioning that has some reliance upon active muscle lengthening and muscle actuators during this particular task. However, each individual hamstring MTU is expected to adopt various combinations of spring-, brake- and motor-driven functioning when sprinting, in accordance with their architectural arrangement and activation patterns. Muscle function is intricate and dependent upon complex interactions between musculoskeletal kinematics and kinetics, muscle activation patterns and the neuromechanical regulation of tensions and stiffness, and loads applied by the environment, among other important variables. Accordingly, hamstring function when sprinting is anticipated to be unique to this particular activity. It is therefore proposed that the adoption of hamstring-specific exercises should not be founded on unvalidated claims of replicating hamstring function when sprinting, as has been suggested in the literature. Adaptive benefits may potentially be derived from a range of hamstring-specific exercises that vary in the stimuli they provide. Therefore, a more rigorous approach is to select hamstring-specific exercises based on thoroughly constructed evidence-based frameworks surrounding the specific stimulus provided by the exercise, the accompanying adaptations elicited by the exercise, and the effects of these adaptations on hamstring functioning and injury risk mitigation when sprinting.

Fig. 1

The phases of the sprint cycle [12]. Mid- and late-swing, where knee joint extension occurs at a high velocity prior to experiencing a rapid deceleration thereafter, expose the hamstring to high forces and are considered a key phase for the occurrence of hamstring injury

Fig. 2

Three-dimensional depiction of a generalised relationship between sarcomere length, force and velocity within a single sarcomere. A similar relationship profile presents at both the muscle fibre and whole muscle levels. Adapted from Fridèn and Lieber [49] with permission

Fig. 3

A three-element representation of the mechanical behaviour of muscle conceived from AV Hill’s force–velocity relationship derived from experiments using tetanised muscle contractions [67], and further developed by Felix Zajac [68]. Adapted from Wilson and Flanagan [69] with permission. CE, contractile element; SEE, series elastic element; PEE, parallel elastic element. See Table 1 for further explanations of the various components

Fig. 4

The classic force–velocity relationship. Po, isometric tension; %Vmax, percentage of maximal shortening velocity. Adapted from Fridèn and Lieber [49] with permission

Fig. 5

A diagram displaying two scenarios explored by Holt et al. [114]. In scenario A, a compliant tendon allows the muscle to operate isometrically as the tendon stretches to store mechanical energy (A-1), and then recoils to return it (A-2). In scenario B, the tendon is more rigid and all of the cyclic work must be performed by the muscle; whereby the muscle actively lengthens to absorb mechanical energy (B-1) prior to shortening to produce it (B-2)

Fig. 6

Mean (black) and standard deviation (grey) of the normalised electromyography (EMG) signals of the hamstrings during maximum-speed sprinting. Adapted from Higashihara et al. [177] with permission

Fig. 7

Joint angular velocity profile of a full gait cycle of the knee and hip joints. Standard deviation is represented in grey. Data taken from ten international and national level sprinters with an average peak sprint velocity of 10.23 m/s. TD, touchdown; TO, toe off. Adapted from Sides [12] with permission