HAMSTRING MUSCLE ENDURANCE IN SUBJECTS WITH PRIOR KNEE INJURIES

1 HAMSTRING MUSCLE ENDURANCE IN SUBJECTS WITH PRIOR KNEE INJURIES 2 3 JAMIE FAXON1, ADEOLA SANNI1*, KEVIN MCCULLY1 4 5 6 1 University of Georgia, Department of Kinesiology, 330 River Road, Athens, GA, USA, 30602 7 8 9 10 11 *Corresponding author 12 Adeola Sanni, 13 330 River Road 14 Athens, GA 30602 15 Telephone: 4045635794 16 E-mail: aas56767@uga.edu 17 18 19 Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 9 October 2018 doi:10.20944/preprints201810.0186.v1


Introduction 39
The knee joint is a common site for injury among younger people, and muscle weakness 40 commonly persists in individuals who return to activity following anterior cruciate ligament 41 reconstruction 1, 2 . People are more likely to suffer from an injury because they are spending their 42 leisure time practicing sporting activities. A 10-year study documented 17,397 patients with 43 19,350 sport injured, and other knee injuries, the anterior cruciate ligament (ACL) was damaged 44 in 45.4% of the cases of internal knee injuries 3 . Moreover, female athletes report anterior cruciate 45 ligament injuries at a 4-to-6-fold greater rate than male athletes 4 . Most athletes, however, do not 46 successfully return to their pre-injury sport despite reaching the acceptable requirements for 47 muscle function 5 . 48 Rigorous rehabilitation after anterior cruciate ligament reconstruction is necessary for a 49 successful outcome 6 . However, rehabilitation is largely based on a recovery period centered 50 around sports, so the primary focus of therapy is on accelerated muscle strength gains 6 . Therefore, 51 the underlying body of research that physical therapy is based off emphasizes the importance of 52 muscle strength [7][8][9] . Muscle endurance, in contrast, is not a focus of physical therapy following a 53 knee injury, and impaired muscle endurance could lead to reduced muscle strength during 54 activities. If the hamstring muscle has reduced endurance compared to the quadriceps muscle, 55 muscle weakness and imbalance between muscles could develop while performing sustained 56 exercise. This unsuccessful recovery following a knee injury may be associated with inadequate 57 rehabilitation and muscle imbalance, particularly of the hamstring muscle. 58 The purpose of this study was to measure the muscle endurance in the hamstring and 59 quadriceps muscles following reconstructive surgery for a knee injury. A non-invasive measure of 60 muscle specific endurance has been developed to allow testing of muscle fatigue in the hamstring 61 and quadriceps muscle independent from motor activation 10-12 . It was hypothesized that subjects 62 who have undergone reconstructive surgery and physical therapy will have impaired hamstring 63 muscle endurance in their affected leg compared to their non-affected leg. 64 65

Materials and Methods 66
Study Population. Eight female subjects were tested. The subjects reported a knee injury 67 and completed physical therapy at least 12 months prior to testing. All subjects had resumed a 68 recreationally active lifestyle. This study was approved by University of Georgia, Athens 69 Institutional Review Board, and each subject signed an informed consent form before participating 70 in the study.

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Experimental Protocol. This study is a one group design where both hamstring and both 72 quadriceps muscles were tested in each subject. Each subject was tested in a single day. The 73 subjects were asked to report their recent activity level or what sports or activities they perform on 74 a weekly basis. During testing, each subject participated in an endurance test of the four muscles.

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Following the endurance test, the subjects were tested for muscle strength of their hamstring and 76 quadriceps of the affected and non-affected leg with an isokinetic ergometer (Biodex). 77 Endurance test. A muscle specific endurance test was performed as reported previously 78 10 . The subjects were positioned prone on a padded table, and the accelerometer was attached the 79 subject's skin with double-sided tape and placed on the belly of the muscle. Two electrodes 4 x 5 80 cm were placed 2-3 cm proximal and distal to the accelerometer. The muscles were stimulated 81 using current levels between 30 and 50 mA. The level of current was based on the visibility of 82 muscle twitch and the pain tolerance or comfortability of the subject. Previous studies have shown 83 the endurance index measurements to be independent of current level 10 . The triaxial 84 accelerometer was set to collect data at 400 Hz (WAX-3, Axivity UK). Stimulation consisted of 85 2, 4, and 6 Hertz stimulations for three minutes each with ten seconds of rest in-between and a 30 86 second baseline established before and after each interval of study. Declines in the acceleration of 87 the muscle twitch contractions was used to calculate muscle endurance. 88 Muscle strength. A Biodex System 4 Quick Set was used to measure the strength of the 89 subject's hamstrings and quadriceps 13 . The subject sat in the chair, and one leg was strapped into 90 a lever that measured the torque (Newton-Meters) with which the subject was kicking out, 91 measuring the strength of the quadriceps, or pulling back, measuring the strength of the hamstring.

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A total of six measurements are collected, three for the quadriceps and three for the hamstring, and 93 the average of all three values is taken to calculate a single strength value for each muscle. This 94 was completed on both legs of each individual. 95 Data analysis. Data from the accelerometer was transferred to Microsoft Excel, and a 96 resultant vector was calculated. Further analysis was done in MATLAB R2017b (Mathworks inc., 97 USA) using a customized written analysis program, where percentage decline in acceleration for 98 the 2, 4, and 6 Hz frequencies was calculated. Endurance Index data was calculated as the percent 99 of acceleration at the end of each stimulation frequency in relation to its peak value (12). 100 Statistical analysis. A two-way between and within subject Analysis of Variance 101 (ANOVA) was used to evaluate the difference between the injured and non-injured leg and the 102 effect of the three frequencies of stimulation (2, 4, and 6), this was done for both Hamstrings and 103 Quadriceps muscles. A two-way ANOVA was also used to evaluate the difference between the 104 Hamstring muscle and the Quadriceps muscle, and the effect of the stimulation frequency. A 105 follow up pairwise comparison was done for each pair of the stimulation frequency. A 2x2 106 ANOVA was conducted to evaluate the difference between strength of the Non-injured and the 107 injured leg, and the Hamstring and the Quadriceps. 108 109

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Data was completed on eight females with prior knee injury (average duration 54 months, 111 range was 29-94 months). Table 1 shows the descriptive statistics of the participants. Figure 1a  112 (Hamstring) and 1b (Quadriceps) shows the representative figure of the endurance graph of one of 113 the participants.  The main finding of this study is that the hamstring endurance in the affected leg was 201 reduced in individuals with prior knee injuries. We were not able to find many studies that 202 evaluated muscle endurance after knee surgery. One study found quadriceps muscle endurance to 203 be reduced compared to controls 18 months after surgery, with no change in hamstring muscle 204 endurance at the same time 14 . However, the endurance protocol used was not described other than 205 it was performed with an isokinetic ergometer at various muscle speeds. That study did not make 206 comparisons to the non-affected leg. Another study found no differences in hamstring or 207 quadriceps muscle endurances 26 months following injury 15 . The endurance test consisted of the 208 total knee flexion work performed in 45 seconds using an isokinetic ergometer. The endurance 209 protocol in this study, however, was different because it did not rely on voluntary muscle rehabilitation of the trunk muscles was the reason for the deficit in trunk muscle endurance 16 . 216 In our study, we found that hamstring and quadriceps muscle isometric strength were 217 similar in the affected and non-affected legs. Previous studies have reported strength deficits in 218 the affected leg for both the hamstring and quadriceps muscles 17 . However, some of these deficits 219 are not apparent 2-3 years post injury 18 , consistent with our results. We found the ratio of 220 quadriceps to hamstring muscle strength to be within the range expected of healthy uninjured 221 people, suggesting our participants did recover their strength even if the hamstring muscle 222 endurance did not recover.

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Our study has a number of limitations. We made our measurements an average of 4.5 years 224 after the injury. This is longer than most studies that look at muscle function post injury. Our 225 study does not allow us to make any conclusions on the potential time course of changes in muscle 226 endurance, relative to changes in muscle strength. In this study, we did not correct by type of 227 surgery to repair the injured knee. Literature addresses that among all of the different grafts used 228 in ACL reconstruction, bone-patella tendon-bone, hamstrings, allograft and synthetic grafts, that 229 there is not a "best" graft, but that there are clear differences between the different surgical options 230 19 . These different methods of repair may have an effect on the recovery of muscle endurance and 231 strength, and this could be a topic of interest for further research. This study only focused on female 232 subjects, in part due to the available research subject pool 15 . As previously stated, females are 233 more likely than males to suffer from a knee injury, most specifically a torn anterior cruciate 234 ligament 20-23 . The sex of the subject may also play a role in their recovery following reconstructive 235 surgery, but more research needs to be done in this area 24 . We also did not look specifically at 236 rehabilitation because we tested recreational athletes where rehabilitation was up to the individual, 237 but on average, the subjects still expressed a deficit in muscle endurance years post-surgery. This 238 study was not prospective, so we cannot conclude that weakness leads to injury. However, athletes 239 with hamstrings to quadriceps ratios below the normal ranges are more likely to sustain an overuse 240 injury, so balancing muscle endurance may work to reduce this likelihood of injury or re-injury 25 . 241 This research is relevant because following the ankle, the knee joint is the second most 242 commonly injured body part, and knee injuries are the leading cause of sport-related surgeries 26 . 243 The effectiveness of rehabilitation and full recovery following a sustain knee injury are largely 244 based on literature 27 . Therefore, these finding have the potential to transform rehabilitation 245 following anterior cruciate ligament reconstructive surgery by including an emphasis on 246 rehabilitation of muscle endurance along with rehabilitation for strength. 247 248

Conclusion 249
Muscle endurance is reduced in the hamstring muscles at least one-year post injury, while 250 hamstring strength is not. The hamstring muscle of the affected leg of the individual does not 251 appear to fully recover following rehabilitation of an ACL knee injury. The quadriceps endurance, 252 the hamstring and quadriceps strength does appear to recover. Reduced hamstring muscle 253 endurance could be a result of lack of focus on muscle endurance during rehabilitation after injury 254 and may contribute to re-injury in the particular muscle even in people who have recovered muscle 255 strength. Additional studies, and perhaps a focus on muscle endurance during rehabilitation are 256 needed to improve physical therapy and rehabilitation following knee injuries.