Hamstring strain injury: objective assessment tools and exercise-specific progression criteria during pain-threshold rehabilitation

Hamstring strain injury (HSI) is a persistent cause of time lost in sports that involve high speed running. Clinicians working with sports injuries are therefore often faced with the task of HSI rehabilitation, with the simultaneous aims of minimising time to return to play (RTP) and risk of re-injury. Following rehabilitation and RTP clearance, previously injured hamstrings display elevated risk of re-injury likely, in part, due to persistent deficits in eccentric knee flexor strength and biceps femoris long head (BFlh) fascicle length. Elevated re-injury risk and persistent deficits in hamstring muscle structure and function suggest inadequacies in current rehabilitation practices. The overarching aim of this program of research was to attempt to improve HSI rehabilitation practices.

The aim of chapter 3 was to systematically review criteria used to guide HSI rehabilitation progression and determine RTP clearance. The review identified a wide variety of criteria, which were used to progress HSI rehabilitation across the nine included studies. By far the most common guideline was to only perform and progress HSI rehabilitation in the complete absence of pain, despite the fact that such recommendations have never been compared to an alternative approach. Objective criteria were rarely implemented as part of rehabilitation progression and RTP decision making, especially where knee flexor strength was concerned. Three of the nine studies did implement isokinetic dynamometry as an objective measure of knee flexor strength, which is a lab-based methodology rarely available to clinicians dealing with HSI rehabilitation. As such, the need to develop objective measures of knee flexor strength, which could be implemented by clinicians during HSI rehabilitation, was identified.

The primary aim of chapter 4 was to establish the test re-test reliability of a novel apparatus capable of objectively measuring knee flexor strength during a range of hamstring exercises commonly implemented during HSI rehabilitation. Secondary to this, chapter 4 aimed to investigate whether this apparatus could identify between-leg deficits in previously injured hamstrings during these exercises. The apparatus displayed moderate to high test re-test reliability for isometric knee flexor strength (ICC = 0.87 to 0.92), peak rate of force development (RFD) during isometric contraction (ICC = 0.87 to 0.95) and mean force impulse during the eccentric slider exercise (ICC = 0.83 to 0.90). Previously injured hamstrings displayed large deficits (d range = -0.88 to -1.09) in mean force impulse during the unilateral eccentric slider isometric knee flexor strength and peak RFD. The novel apparatus provides clinicians with an objective tool to monitor knee flexor strength during exercises commonly implemented throughout rehabilitation.

Further to improving objective measures of knee flexor strength, chapter 5 aimed to provide clinicians with exercise-specific guidelines for the progression of a HSI rehabilitation protocol with an emphasis on early eccentric loading. It is commonly recommended that eccentric loading be delayed until the alleviation of pain and/or between-leg deficits in isometric knee flexor strength during HSI rehabilitation. Using exercise-specific progression criteria, eccentric loading was introduced during early HSI rehabilitation and was well tolerated by participants despite concurrent pain and/or between-leg deficits in isometric knee flexor strength. As such, chapter 5 showed that delaying the introduction of eccentric loading until alleviation of pain and/or isometric strength deficits may be unnecessary during HSI rehabilitation.

Chapter 6 investigated RTP clearance time, rates of re-injury and hamstring muscle structure and function following either pain-free or pain-threshold HSI rehabilitation. The median number of days from HSI to RTP clearance was 15 (95% CI = 13 to 17) in the pain-free group and 17 (95% CI = 11 to 24) in the pain-threshold group, which was not significantly different (p = 0.37). Both groups significantly increased BFlh fascicle length from initial clinical assessment to RTP clearance, although these improvements at two-month follow-up, were on average 0.91cm (95% CI = 0.34 to 1.48) greater in the pain-threshold group. The pain-threshold group achieved greater improvements in isometric knee flexor strength at 90/90 degrees of hip/knee flexion compared to the pain-free group at RTP clearance by an average of 15% (95%CI = 1 to 28) and two-month follow-up by an average of 15% (95%CI = 1 to 29). In the six months following RTP clearance, two re-injuries occurred in the both the pain-free (12%) & pain-threshold (10%) group.

This program of research has contributed knew knowledge to the HSI rehabilitation evidence base, specifically by 1) highlighting the large emphasis on subjective criteria for rehabilitation progression and RTP decision making; 2) developing a reliable objective tool used to measure knee flexor strength during various hamstring exercises commonly employed during rehabilitation; 3) describing a HSI rehabilitation protocol with exercise-specific progression criteria, which safely accelerates the introduction of eccentric loading and 4) showing that performing and progressing exercise up to a pain-threshold results in similar RTP clearance time and re-injury rates compared to pain-free rehabilitation, whilst eliciting greater isometric knee flexor strength improvements and greater long-term improvements in BFlh fascicle length. It is anticipated that this new knowledge will improve the clinician’s ability to rehabilitate HSI, whilst concurrently minimising RTP times and re-injury risk.