Table 1

Selected trends in countermovement jump (CMJ) bilateral, involved and uninvolved limb outputs, and interlimb asymmetries considered during the rehabilitation process: interpretation and influence on decision-making

PhaseEarly rehabilitation (4–6 months)
Gym-based physical preparation
Return to participation (6–9.5 months)
On-pitch reconditioning and gym-based physical preparation
CMJ TestsCMJ SUB-MAXCMJ SUB-MAX→CMJ MAX1CMJ MAX1→CMJ MAX2CMJ MAX2→CMJ MAX4CMJ MAX5RTS CRITERIA
What were the key trends we observed and how we interpreted them75% decrease in jump height, 58% decrease eccentric peak velocity and 27% decrease in countermovement depth relative to pre-injury.

Indicative of a combination of capacity deficits and strategies (decreased landing impact, deceleration demands and eccentric loading in deeper knee flexion) which serve to limit impact and eccentric loads.32
Increases in eccentric peak velocity and countermovement depth, with increased total eccentric deceleration RFD and a larger increase in involved limb than the uninvolved limb output.

Increase in peak landing force (30.2 →61.6 N/kg) and decrease in peak landing force asymmetry from 46%→2% higher on involved limb.

Trends indicative of an increased capacity and willingness to eccentrically load in deeper flexion without involved limb deceleration load avoidance and similar absence of avoidance strategy on landing despite increase in total peak landing force.
Increase in eccentric peak velocity (−0.72→−0.97 m/s), increase in countermovement depth (19.7→22.9 cm) without an increase in eccentric deceleration RFD asymmetry.

Trends suggest increased confidence/capacity in loading knee at high velocity and in deeper flexion.

Increase in flight time:contraction time, eccentric peak power and total eccentric deceleration RFD with similar eccentric deceleration RFD trends on both limbs.

Indicative of positive adaptations in both involved and uninvolved limbs to increased load and velocity plyometric loading activity, 28 concurrent with anti-gravity treadmill running progression.

Increase in jump height (5 cm) alongside decrease in indices of total peak landing force: peak landing force/body weight (→ 48.2 N/kg) and/jump height (→106 N/cm) and an increase in peak landing force asymmetry (from 2% higher on involved to 15% higher on uninvolved limb) due to a larger decrease on the involved limb.

Overall, increases in capacity to load on landing, greater knee flexion in countermovement and in estimated knee flexion on landing; total peak landing force indices now below pre-injury values with some load avoidance evident on the involved limb.
Max3 and Max4 tests were performed following a week of new loading demands (with the expectation of residual fatigue affecting values); we therefore considered Max2→Max3 and Max3→Max4 separately to the overall trends between Max1→Max2→Max5.

Between Max2 and Max3, large (15–18%) decreases in force at zero velocity, eccentric deceleration RFD, concentric impulse-100 (selected Max3 and Max4 data shown in online supplemental appendix). Increased asymmetries due to a larger decrease in uninvolved than involved limb performance: for example, force at zero velocity Involved: 555→533 N; uninvolved: 527→453 N; eccentric deceleration RFD involved: 1532→1360 N/s; uninvolved: 1575→1166 N/s.

The total force at zero velocity and eccentric deceleration RFD trends suggest neuromuscular fatigue 25 and followed a week with 3× on-pitch sessions with total distance: 8040 m (0.8× gameload), first exposure to FWB running. While the individual limb trends in the eccentric phase indicate that fatigue was asymmetrical. The larger magnitude of fatigue on the uninvolved limb suggesting preferential loading of that limb/relative unloading of the involved limb during on-pitch running activity.

At Max4 despite the test performed following a further increment in running load; 6× on-pitch sessions (total distance: 20 148 m (1.9× gameload), there was some recovery of force at zero velocity and concentric impulse-100 but remained below Max2 outputs.

This suggested that the player was becoming accustomed to FWB running, adapting to the stimulus and potentially loading less asymmetrically in on-pitch training.
Relative to Max2, bilateral performance and total peak landing force indices were relatively stable, recovering after the declines highlighted between Max2 and Max3

Decreased total eccentric deceleration RFD with a larger decrease on the uninvolved than the involved limb, such that involved limb eccentric deceleration RFD now 10% > uninvolved.

Increase on involved limb peak landing force (1423→1650 N) and decrease in uninvolved limb (1702→1633 N) leading to an asymmetry shift from 16% higher on uninvolved to 1% higher on involved limb.

Absolute and trends and % asymmetry indicate a greater contribution of the involved limb during deceleration and landing load acceptance tasks

Trends in total concentric impulse show small increase, with both limbs increasing and a small decline in concentric impulse asymmetry to 9%.
While pre-injury ‘benchmark’' data were available for the player, we did not consider eccentric deceleration RFD asymmetry: 17% (involved > uninvolved) and peak landing force asymmetry:17% (uninvolved > involved) sensible benchmarks to return the player to.

Instead, to qualify her status and progression, we were guided by data from male English professional players without lower limb injury in the prior season — eccentric deceleration RFD asymmetry: ~10%, peak landing force asymmetry: ~9%,20 21 and her 8-month asymmetries and involved limb progress between 6 months and 8 months in the context of values in professional male players post-ACLR (eccentric deceleration RFD asymmetry: ~20%, peak landing force asymmetry: ~17%).31

We also considered the recovery of her ability/willingness to eccentrically load at velocity (eccentric peak velocity only 1% less than pre-injury).

We prioritised recovery of eccentric deceleration and landing load acceptance, based on evidence of persistence of deficits/avoidance strategies in these phases17 19 21 31 and associated poor knee function.17 19 However, we were also interested in the progression of overall performance and reducing concentric impulse asymmetry which at RTS, at 9% was slightly above the mean+1SD of healthy uninjured male professional players, but lower than the ~13% asymmetry in that phase reported in male professionals at 8 months.31

Relative to pre-injury values, bilateral performance deficits for CMJ-TYP variables were below 10%; jump height: 8%, concentric peak power: 3%, concentric peak velocity: 1%. Deficits were larger for CMJ-ALT: flight time: contraction time: 27%, concentric impulse-100: 23% aligning with evidence that these deficits persist after RTC after CMJ-TYP recovered21 and at least partly driven by a substantial change in countermovement depth.
If and how this information influenced decisionsRather than informing decisions, data from this timepoint was used principally as a post-surgery baseline to guide gym-based exercise prescription: 1) emphasising involved limb hypertrophy and strength development alongside their maintenace on the uninvolved limb 2) progressive integration of jump-landing activities to improve deceleration and control of impact forces.Alongside other strength and power diagnostics, CMJ trends in both overall performance particularly in eccentric qualities suggestive of a positive response to initial off-pitch conditioning. Furthermore, these responses were without manifesting the large avoidance strategies/injured limb capacity deficits reported in professional male players 6-months post-ACLR asymmetries of: 20% in eccentric deceleration RFD, 25% in peak landing force while her concentric impulse asymmetry was comparable to the 18% reported at this timepoint.31 These trends gave us confidence to progress dynamic strength training and jump-landing activities and to initiate anti-gravity treadmill running.In the context of the positive response to anti-gravity treadmill running, dynamic strength training and increases in decelerative loading in plyometric activities represented by the substantial improvements in bilateral performance across eccentric, concentric and landing phases, and the stability of eccentric deceleration RFD asymmetry. This was despite increases in eccentric peak velocity and countermovement depth, and the lack of adverse response (pain or joint effusion) — indicative of the joint ‘coping’' with loading demands, therefore we did not consider the peak landing force asymmetry increase alone as a trend which warranted delay in progression to a return to on-pitch running.In the context of returning to on-pitch running (high control) alongside the continued off-pitch physical preparation and absence of adverse joint response, we considered the declines in specific outputs acceptable and representative of residual fatigue related to the large increment in loading demands rather than a setback to the process. Furthermore, these variables showed recovery at Max4 despite further increases in running load (→20 000 m)(Please refer to online supplemental appendix for further discussion on fatigue-recovery monitoring in rehabilitation). Our clinical risk assessment was that these data did not warrant delaying transition to the moderate control phase. At this stage with RTT (2 months) and RTC (3.5 months) away, ensuring adequate exposure to loading was prioritised over peak performance.The continued improvement in indicators of total deceleration capacity/loading without the emergence of involved limb avoidance strategies in combination with the absence of pain/joint effusion indicated a positive response to the associated loading demands of early football-specific activities (control to chaos phase) and achieving on-pitch running load targets. This gave us confidence to progress her to moderate/high chaos phases and expose the player to the associated increases in volume/intensity.Key CMJ RTS considerations included bilateral performance progress, particularly in deceleration capacity without excessive avoidance strategies. We also considered the greater flexion during the countermovment and a more compliant landing a favourable shift in strategy/capability in a female athlete post-ACLR .33

As a result, while we expected further increases in total eccentric deceleration RFD and other eccentric/SSC variables during RTT and RTC, we speculated that outputs for these variables may not return to pre-injury levels and might be considered as new baselines for monitoring. Figures 4 and 5 do however show the further recovery of these variables following RTC and a return to performance.
  • Bilateral performance=combined limb output variables. Those considered ‘typical’ (CMJ-TYP) (jump height, concentric peak power, concentric peak velocity), and those considered ‘alternative’ (CMJ-ALT). CMJ-ALT variables help further and understand neuromechanical trends by quantifying changes in ‘strategy’ denoting duration of phases (ie, flight time:contraction time, eccentric/concentric duration) and in other time-constrained mechanical outputs across specific phases and subphases (ie, eccentric deceleration RFD) as well as descriptors of movement strategies (ie, eccentric peak velocity, countermovement depth).20 25 26 29 31 Landing indices highlighted are body weight and jump height relative peak landing force. Several of these variables and components are shown in figure 3. Please refer to Figure 4 for complete data for pre-injury, Sub-max, Max1, Max2, Max5 and 1-yr RTS. See online supplemental appendix for Max3 and Max4.

  • Total refers to combined limb values for a variable for which individual limb outputs are also given, for example, total peak landing force = left+right peak landing force.

  • FWB, full weight-bearing (running); eccentric deceleration RFD, eccentric deceleration rate of force development; (N/kg, Newton per kilogram body weight, N/cm, Newton per centimetre of jump height); concentric impulse-100=(Net) concentric impulse from the start of 100 ms of concentric impulse; RTC, return to competition; RTS, return to sport; RTT, return to training; SSC, stretch-shortening cycle.