(42) THE EFFECT OF A BOUT OF ISOKINETIC EXERCISE ON MOLECULAR BIOMARKERS OF MYOGENESIS AND PROTEOLYSIS IN YOUNG FEMALES FOLLOWING ACL RECONSTRUCTION

Anterior cruciate ligament (ACL) injury results in considerable short-term disability, has a high risk of recurrence, and has an increased potential for development of post-traumatic osteoarthritis (OA). Despite surgical reconstruction and formalized rehabilitation, persistent quadriceps weakness is common at return to activity and increases the risk for secondary injury. Evaluating the activity of local myogenic and proteolytic pathways can provide insight into underlying mechanisms that contribute to quadriceps weakness and inform treatment pathways to better address dysfunction.

Purpose: To determine intra- and interlimb differences of myogenic and proteolytic gene expression of the vastus lateralis (VL) muscle at baseline and 4-hours following a fatiguing bout of isokinetic resistance exercise.

Methods: Nine women (24 ± 4 years) were recruited to participate in the study. Participants were at least one year out from ACLR (3.4 ± 1.9 years) and currently participating in physical activity without restrictions. Participants reported to the laboratory in the morning in a fasted state. Following 30-min of rest, microbiopsies of the VL of the involved (ACLR) and uninvolved (CON) limb were taken. Participants completed a bout of isokinetic knee extension (6 sets x 10 reps at 60°/s; 2-min rest between sets) with both limbs, rested 4-hours, and then microbiopsies of the VL were repeated.Muscle mRNA of specific genes was determined using quantitative real-time reverse transcriptase polymerase chain reaction (qRT-PCR). Quantification of mRNA for genes of interest was calculated using the 2−∆∆ct method relative to stable reference genes (1.0-fold change). The geometric mean of the following three reference genes was used as the stable reference point: B2M, RPS18, and GAPDH for each participant. Results are expressed as intralimb fold change from pre-exercise. Paired samples t-tests were used to determine within limb differences between pre- and post-exercise and differences between limbs post-exercise.

Results: Both quadriceps fatigued during the exercise bout (%Decline in peak torque: ACLR = -13.5 ± 16.4%; CON = -17.7 ± 13.8%) suggesting the stimulus was adequate to induce a change in the expression of myogenic regulatory factors. No significant differences were found within or between limbs in MYOD, MYOG, Myf6, MAFbx, MST (CON), Myf5 (CON), or FOXO3a (ACLR). ACLR expression of MST (0.48 ± 0.21-fold change, P < 0.01) and Myf5 (0.60 ± 0.36-fold change, P = 0.01), and CON expression of FOXO3a (0.71 ± 0.26, P = 0.01-fold change) were significantly downregulated and MuRF1 was significantly upregulated in both limbs (ACLR = 2.34 ± 1.40-fold change, P = 0.02; CON = 2.50 ± 1.85-fold change, P = 0.04) following exercise.

Conclusion: The myogenic and proteolytic gene expression measured in both the involved and uninvolved limbs of the participating females generated atypical signaling responses. This post-exercise change in expression was lower in magnitude compared to established values for healthy young adults, though they followed expected trends. Interestingly, ACLR had heightened post-exercise expression of myogenic regulatory factors compared to CON. PRACTICAL APPLICATIONS: For clinicians working with athletes following an ACL injury, it may be critical to focus on equalizing bilateral strength prior to and following return to sport to reduce their risk of developing OA.

 


Acknowledgements: None