For several years I have been concerned about the loss of length of my stride that had become increasing marked since my early sixties. At peak sprinting speed, my step length is less than 1 metre. To achieve even a modest pace of 5 min/Km, I am forced to increase cadence to over 200 steps/min. At paces in the vicinity of 5 min/Km, efficiency tends to increase as cadence increases from 180 to 200 steps per minute because the energy consumed in getting airborne and overcoming braking decreases as cadence increases up to 200 steps /min (as demonstrated my post of 6th Feb 2012). However, the energy cost of repositioning the legs during the swing phase increases with increasing cadence, as discussed in my post of 27th Feb 2012, and in my calculations performed on 5th April 2012. Therefore, at paces in the range 4 to 5 min/Km, efficiency falls as cadence increases substantially above 200 steps per min.
Initially I considered that loss of leg muscle strength was the cause of my short stride. So a year ago I embarked on a program of weight training, mainly employing squats and deadlifts. I was delighted that I was able to recover my lost strength, but unfortunately, it made little difference to my stride length. I had intended to follow the initial weight sessions with some plyometrics, in the expectation that plyometrics would help me harness the increased strength and allow me to capture more elastic energy to drive powerfully off stance, but a minor relapse of arthritis confounded my plan. By the time the arthritis had settled it was time to direct my energy towards re-building aerobic fitness for the Robin Hood half marathon in September. I increased weekly training volume fairly rapidly but only managed a rather mediocre half marathon.
After recovering from the half-marathon, it was time re-consider my former plan to introduce plyometrics. However, I was a little alarmed by continuous aching in my legs, especially at the attachment of peroneus longus to the upper part of the fibula in both legs. In addition there was a generalised aching of the connective tissues around and below both knees. This had built up gradually during the summer and did not resolve even after I cut back the amount of training quite drastically. By late October I was reluctant to put off the plyometrics any longer, though it was clear that I would need to be fairly cautious.
What intensity of plyometrics is required?
What evidence is there that a modest program of plyometrics would lead to a worthwhile gain in running efficiency? A study by Turner and colleagues had assessed the change in running efficiency produced by 6 weeks of fairly gentle plyometrics in a group of moderately trained, young adult runners. The program involved adding three plyometric session per week to the runners’ usual training. Each plyometric session involved six exercises starting with sub-maximal double-leg vertical jumps at 50% effort as a warm-up, and then proceeding to various forms of double-leg and single-leg jumps. For example, one of the exercises was submaximal double-leg repetitive vertical jumps of 6–8 in., using minimal knee and hip action while emphasizing the calf action. In the first week, each session included 60 foot-contacts, increasing to 140 foot-contacts per session by six weeks. The outcome was a significant increases in running efficiency of 2-3% at paces in the range 5 – 6 min/Km. A control group who continued with training as usual showed no increase in running efficiency. Neither group exhibited increase in VO2max, or a significant increase in counter-move jump (CMJ) height. The lack of significant increase in CMJ height was perhaps surprising, though in fact the group undergoing the plyometrics did exhibit a mean increase from 36 to 38 cm. This was not statistically significant, but the study probably lacked enough statistical power to detect the magnitude of change that might reasonably be expected. Nonetheless, it was encouraging to see a small but significant and worthwhile improvement in running efficiency from a relatively modest plyometric program.
A more demanding program
Spurrs and colleagues employed a slightly more demanding 6 week plyometric program in more experienced athletes. In the first three weeks, there were two plyometric session per week and then three sessions per week for the remaining three weeks. The majority of the exercises were hops (single or double-leg), all performed at maximal effort. Depth jumps were introduced in the fourth week. The number of foot contacts was 60 per session in the first week and increased gradually up to 180 per session by the final week. The gains were substantial. Running efficiency increased by 6.5% at 5 min/Km and by 4% at 3.75 min/Km. CMJ height increased significantly from 38cm to 43 cm and musculo-tendonous stiffness increased significantly by about 10% in each leg. 3Km time trial performance improved significantly by 2.7 % from 10:17 min to 10:10. There were no changes in VO2max or lactate threshold. A control group who continued with training as usual showed no significant changes in any measures.
My cautious program
Overall, the prospect for gain in efficiency and in race pace from a 6 week plyometric program looked promising. However in light of my age and aching legs, it was clear that I should be cautious. I decided that in contrast to the approach employed by Turner, who placed emphasis on the muscles acting around the ankle (especially gastrocnemius and soleus), I would allow more flexion of hips and knees, since the large muscles (quads, hams and glutes) acting at these joints play a key role in running. I therefore anticipated that I would need to employ somewhat greater jump heights. A cautious introductory session with some hopping over 30cm high hurdles and drop jumps from 16 cm did not exacerbate the aches. In fact, at that time, running was somewhat more painful than the plyometrics, so I decided that I would proceed with the plyometrics while cutting back the amount of running to around 10 Km per week. I allowed three days recovery after each plyometric session, giving a total of three sessions every two weeks. I interleaved a mildly demanding weight lifting session between plyometric sessions. To prevent complete loss of aerobic fitness, I replaced the some of the running with sessions on the elliptical cross trainer.
In the first plyometric session, after a gentle warm up that included body-weight squats, hip swings, calf-raises and line-jumps I did 5 sets of 2 double-leg hops over 30cm hurdles and 5 sets of 5 drop jumps from 16 cm, rebounding to 16cm (a total of 35 foot contacts in the session). This modest session left me with barely perceptible DOMS the next day. In subsequent sessions I increased the numbers of hurdle hops; the depth of the drop jumps; and added single-leg hurdle hops. By the end of the six weeks, each session included 5 sets of 7 double-leg hurdle hops (over 30cm hurdles); 5 sets of 5 single-leg hurdle hops on each leg (over 15 cm hurdles); and 5 sets of 5 drop jumps (from 30 cm rebounding to 30 cm (total of 110 foot contacts in the session). Although exact comparison with Turner’s program is not possible, I estimate that the early sessions in my program were less demanding than Turner’s, but the later sessions were roughly equivalent. However, whereas Turner’s athletes performed 18 sessions, I performed only 10 sessions over the six week period. My CMJ height increased from 30 to 33 cm. My other outcome measurement was horizontal distance covered in 5 consecutive double-leg hops. This increased from 8.63 m to 9.08 m.
Unfortunately, there seemed little point in assessing the impact on my running performance. Having done relatively little running in the 12 weeks since the half marathon, my aerobic fitness had deteriorated quite markedly, despite the elliptical sessions. It was clear that my fitness at the end of September had been built on a very narrow base, and by mid-December, it had melted away. However, one pleasing observation was that the persistent aches in my legs had almost entirely disappeared.
Overall, the six weeks of quite modest plyometrics produced a definite increase in my jumping ability – comparable as far as can reasonably be estimated, with the gains exhibited in the study by Turner, though somewhat less than achieved in the more demanding program employed by Spurrs. Although I have no direct evidence of improved running efficiency or pace, the findings of both Turner and of Spurrs suggest that the improvement in jumping ability would probably have been enough to produce a worthwhile improvement in running efficiency, had I not lost aerobic fitness due to a drastic reduction in my volume of running.
At present I find myself in an ambiguous position. I am somewhat dismayed by the severe and persistent aching in my legs that had developed in the summer during my preparation for the half marathon. If I am to succeed in my plan to run a full marathon next year, I will have to build up the volume of running more gradually than had been feasible this year. I will probably also include a higher proportion of elliptical cross-training. However, it is pleasing to have demonstrated that I can achieve gains in jumping performance from a relatively modest program of plyometrics. The gains appear comparable to those achieved by the young adults in the study by Turner, and perhaps even comparable to those achieved in the study by Spurrs, after allowing for the differences in volume and intensity of the plyometrics. Furthermore, it will be interesting to see whether or not this moderate amount of plyometrics makes me more resistant to aching legs, in the long term.