Abstract
Objective:
Low energy availability (LEA) is a prevalent health concern in female athletes and is characterized by a mismatch between nutritional energy intake and exercise energy expenditure. Low energy availability threatens musculoskeletal health and may impair strength adaptations in response to physical training. Neuromuscular training programs are implemented following anterior cruciate ligament reconstruction (ACLR) injuries in athletes to regain strength and function for successful return to sport, however, low energy availability may threaten efficacy of such programs. Further, young female athletes are at particular risk for secondary ACL injury, which may be associated with lower extremity strength profiles and lower energy availability. The purpose of this study was to assess the influence of low energy availability status on 1) muscle strength in response to a neuromuscular training program in the uninjured knee post-ACLR, and 2) subsequent contralateral ACL injury. We hypothesized that adolescent female athletes with low energy availability would demonstrate impaired strength gains post-ACLR in their uninjured, normal knee compared to those without LEA, during standardized 6- and 9-month post-operative testing.
Methods:
This was a retrospective case control study of females (age 13-18 years) post-ACLR who completed a standardized 10-session neuromuscular training program starting 6 months post-ACLR. For the purpose of this assessment, isometric strength measurements for the hamstrings and quadriceps in the uninjured limb were included at 6- and 9-months post-ACLR. Low energy availability (LEA) was defined as presence of menstrual dysfunction (amenorrhea, oligomenorrhea) and/or diagnosed eating disorder. Separate repeated measures analyses of variance were used to compare uninjured limb hamstrings and quadriceps mass-normalized isometric strength, and hamstring:quadriceps (H:Q) ratios between female athletes with and without LEA at 6- and 9-months post-ACLR. Subsequent contralateral ACL injuries were quantified by group.
Results:
There were 76 female athletes post-ACLR that met inclusion criteria. There were 18 (23.7%) that were classified as having LEA, and this cohort was age-matched to 18 females without LEA (Table 1). There were no significant differences in body mass index (BMI) or ACL graft type used in the operative extremity between groups (Table 1). In the uninjured knee, quadriceps strength did not significantly change in response to the 10-session neuromuscular training program among those with LEA (Δstrength: 0.19Nm/kg, p= 0.235) but did in the non-LEA-group (Δstrength: 0.29Nm/kg, p= 0.028). Hamstring strength was significantly lower at both timepoints for the LEA group compared to non-LEA (Mean difference [MD] range: -1.72 to -2.15Nm/kg, p<0.001). The LEA group had significantly lower H:Q ratio at both timepoints compared to the non-LEA group (MD range: -52% to -31%; p<0.001). The LEA group H:Q symmetry did not significantly change over time (ΔH:Q: +0.55, p=0.77), while the non-LEA group had slightly decreased symmetry over time driven by increased quadriceps strength over time (ΔH:Q: -0.16, p=0.04), though this symmetry was still higher than the LEA group (Figure 1). The LEA group had three (16.6%) contralateral ACL injuries (Median [Interquartile Range]: 38 [23, 54] months post-ACLR), while the non-LEA group had none during this timeframe.
Conclusions:
This is the first study to investigate the impact of low energy availability on neuromuscular training outcomes in adolescent female athletes. In the uninjured knee, low energy availability was associated with significantly impaired strength, strength gains, and lower H:Q ratios driven by muscle weakness during a post-ACLR neuromuscular training program, compared to those without low energy availability. This is concerning as reduced response to intervention may prolong recovery and lead to worse physical performance outcomes. Furthermore, this study provides preliminary evidence of a possible association between low energy availability and subsequent ACL injury. The impact of low energy availability on performance, postoperative recovery, and secondary ACL risk deserves further study in this high-risk female athlete population.
